We explore the relationship between the spectral shape of the Ly{\alpha} emission and the UV morphology of the host galaxy using a sample of 304 Ly{\alpha}-emitting BV i-dropouts at 3 < z < 7 in the GOODS and COSMOS fields. Using our extensive reservoir of high-quality Keck DEIMOS spectra combined with HST WFC3 data, we measure the Ly{\alpha} line asymmetries for individual galaxies and compare them to axial ratios measured from observed J- and H-band (restframe UV) images. We find that the Ly{\alpha} skewness exhibits a large scatter at small elongation (a/b < 2), and this scatter decreases as axial ratio increases. Comparison of this trend to radiative transfer models and various results from literature suggests that these high-redshift Ly{\alpha} emitters are not likely to be intrinsically round and symmetric disks, but they probably host galactic outflows traced by Ly{\alpha} emitting clouds. The ionizing sources are centrally located, with the optical depth a good indicator of the absorption and scattering events on the escape path of Ly{\alpha} photons from the source. Our results find no evidence for evolution in Ly{\alpha} asymmetry or axial ratio with look-back time.
An understanding of the mass build-up in galaxies over time necessitates tracing the evolution of cold gas (molecular and atomic) in galaxies. To that end, we have conducted a pilot study called CO Observations with the LMT of the Blind Ultra-Deep H I Environment Survey (COOL BUDHIES). We have observed 23 galaxies in and around the two clusters Abell 2192 (z = 0.188) and Abell 963 (z = 0.206), where 12 are cluster members and 11 are slightly in the foreground or background, using about 28 total hours on the Redshift Search Receiver (RSR) on the Large Millimeter Telescope (LMT) to measure the $^{12}$CO J = 1 --> 0 emission line and obtain molecular gas masses. These new observations provide a unique opportunity to probe both the molecular and atomic components of galaxies as a function of environment beyond the local Universe. For our sample of 23 galaxies, nine have reliable detections (S/N$\geq$3.6) of the $^{12}$CO line, and another six have marginal detections (2.0 < S/N < 3.6). For the remaining eight targets we can place upper limits on molecular gas masses roughly between $10^9$ and $10^{10} M_\odot$. Comparing our results to other studies of molecular gas, we find that our sample is significantly more abundant in molecular gas overall, when compared to the stellar and the atomic gas component, and our median molecular gas fraction lies about $1\sigma$ above the upper limits of proposed redshift evolution in earlier studies. We discuss possible reasons for this discrepancy, with the most likely conclusion being target selection and Eddington bias.
Based on data from the ongoing OGLE Galaxy Variability Survey (OGLE GVS) we have verified observed properties of stars detected by the near-infrared VVV survey in a direction near the Galactic plane at longitude l~-27 deg and recently tentatively classified as classical Cepheids belonging to a, hence claimed, dwarf galaxy at a distance of about 90 kpc from the Galactic Center. Three of four stars are detected in the OGLE GVS I-band images. We show that two of the objects are not variable at all and the third one with a period of 5.695 d and a nearly sinusoidal light curve of an amplitude of 0.5 mag cannot be a classical Cepheid and is very likely a spotted object. These results together with a very unusual shape of the Ks-band light curve of the fourth star indicate that very likely none of them is a Cepheid and, thus, there is no evidence for a background dwarf galaxy. Our observations show that a great care must be taken when classifying objects by their low-amplitude close-to-sinusoidal near-infrared light curves, especially with a small number of measurements. We also provide a sample of high-amplitude spotted stars with periods of a few days that can mimick pulsations and even eclipses.
The role of gravitational instability-driven turbulence in determining the structure and evolution of disk galaxies, and the extent to which gravity rather than feedback can explain galaxy properties, remains an open question. To address it, we present high resolution adaptive mesh refinement simulations of Milky Way-like isolated disk galaxies, including realistic heating and cooling rates and a physically motivated prescription for star formation, but no form of star formation feedback. After an initial transient, our galaxies reach a state of fully-nonlinear gravitational instability. In this state, gravity drives turbulence and radial inflow. Despite the lack of feedback, the gas in our galaxy models shows substantial turbulent velocity dispersions, indicating that gravitational instability alone may be able to power the velocity dispersions observed in nearby disk galaxies on 100 pc scales. Moreover, the rate of mass transport produced by this turbulence approaches $\sim 1$ $M_\odot$ yr$^{-1}$ for Milky Way-like conditions, sufficient to fully fuel star formation in the inner disks of galaxies. In a companion paper we add feedback to our models, and use the comparison between the two cases to understand what galaxy properties depend sensitively on feedback, and which can be understood as the product of gravity alone. All of the code, initial conditions, and simulation data for our model are publicly available.
The most frequently proposed model for the origin of quasars holds that the high accretion rates seen in luminous active galactic nuclei are primarily triggered during major mergers between gas-rich galaxies. While plausible for decades, this model has only begun to be tested with statistical rigor in the past few years. Here we report on a Hubble Space Telescope study to test the merger-triggering hypothesis for $z=2$ quasars with high super-massive black hole masses ($M_\mathrm{BH}=10^9-10^{10}~M_\odot{}$), which dominate cosmic black hole growth at this redshift. We compare Wide Field Camera 3 $F160W$ (rest-frame $V$-band) imaging of 19 point source-subtracted quasar hosts to a matched sample of 84 inactive galaxies, testing whether the quasar hosts have a statistically higher fraction of strong gravitational interaction signatures. We recover strong distortion fractions of $f_\mathrm{m,qso}=0.39\pm{}0.11$ for the quasar hosts and $f_\mathrm{m,gal}=0.30\pm{}0.05$ for the inactive galaxies (distribution modes, 68\% confidence intervals), with both measurements subjected to the same observational conditions and limitations. We definitively rule out both extreme cases (all mergers, no mergers) for the quasar host population. The slight observed enhancement in merger signatures for quasar hosts over inactive galaxies is not statistically significant, with a probability that the quasar fraction is higher of $P(f_\mathrm{m,qso}>f_\mathrm{m,gal}) = 0.78$ ($0.78\,\sigma$), in line with results for lower mass and lower $z$ AGN. We thus find no evidence that major mergers are the primary triggering mechanism for the massive active galactic nuclei that dominate accretion at the peak of cosmic quasar activity.
We present an analysis of the predictions made by the Galform semi-analytic galaxy formation model for the evolution of the relationship between stellar mass and halo mass. We show that for the standard implementations of supernova feedback and gas reincorporation used in semi-analytic models, this relationship is predicted to evolve weakly over the redshift range 0<z<4. Modest evolution in the median stellar mass versus halo mass (SHM) relationship implicitly requires that, at fixed halo mass, the efficiency of stellar mass assembly must be almost constant with cosmic time. We show that in our model, this behaviour can be understood in simple terms as a result of a constant efficiency of gas reincorporation, and an efficiency of SNe feedback that is, on average, constant at fixed halo mass. We present a simple explanation of how feedback from active galactic nuclei (AGN) acts in our model to introduce a break in the SHM relation whose location is predicted to evolve only modestly. Finally, we show that if modifications are introduced into the model such that, for example, the gas reincorporation efficiency is no longer constant, the median SHM relation is predicted to evolve significantly over 0<z<4. Specifically, we consider modifications that allow the model to better reproduce either the evolution of the stellar mass function or the evolution of average star formation rates inferred from observations.
In the last few years, it became possible to observationally resolve galaxies with two distinct nuclei in their centre. For separations smaller than 10kpc, dual and offset active galactic nuclei (AGN) are distinguished: in dual AGN, both nuclei are active, whereas in offset AGN only one nucleus is active. To theoretically study the origin of such AGN pairs, we employ a cosmological, hydrodynamic simulation with a large volume of (182 Mpc)^3 from the set of Magneticum Pathfinder Simulations. The simulation self-consistently produces 35 resolved black hole (BH) pairs at redshift z=2, with a comoving distance smaller than 10kpc. 14 of them are offset AGN and nine are dual AGN, resulting in a fraction of (1.2 \pm 0.3)% AGN pairs with respect to the total number of AGN. In this paper, we discuss fundamental differences between the BH and galaxy properties of dual AGN, offset AGN and inactive BH pairs and investigate their different triggering mechanisms. We find that in dual AGN, the corresponding BH from the less massive progenitor galaxy always accretes with a higher Eddington ratio and that dual AGN have similar BH masses. In contrast, in offset AGN, the active BH is typically more massive than its non-active counterpart. Furthermore, dual AGN in general accrete more gas from the intergalactic medium than offset AGN and non-active BH pairs. This highlights that merger events, particularly minor mergers, do not necessarily lead to strong gas inflows and thus, do not always drive strong nuclear activity.
Pulsar timing arrays (PTAs) are placing increasingly stringent constraints on the strain amplitude of continuous gravitational waves emitted by supermassive black hole binaries on subparsec scales. In this paper, we incorporate independent measurements of the dynamical masses $M_{\rm bh}$ of supermassive black holes in specific galaxies at known distances and leverage this additional information to further constrain whether or not those galaxies could host a detectable supermassive black hole binary. We estimate the strain amplitudes from individual binaries as a function of binary mass ratio for two samples of nearby galaxies: (1) those with direct dynamical measurements of $M_{\rm bh}$ in the literature, and (2) the 116 most massive early-type galaxies (and thus likely hosts of the most massive black holes) within 108 Mpc from the MASSIVE Survey. Our exploratory analysis shows that the current PTA upper limits on continuous waves can already constrain the mass ratios of hypothetical black hole binaries in a dozen galaxies in our samples. The constraints are stronger for galaxies with larger $M_{\rm bh}$ and at smaller distances. For the black holes with $M_{\rm bh} \gtrsim 5\times 10^9 M_\odot$ at the centers of NGC 4889, NGC 4486 (M87) and NGC 4649 (M60), any binary companion in orbit within the PTA frequency bands would have to have a mass ratio of less than about 1:10.
We analyze the resolved stellar populations of the faint stellar system, Crater, based on deep optical imaging taken with the Hubble Space Telescope. The HST/ACS-based color-magnitude diagram (CMD) of Crater extends $\sim$4 magnitudes below the oldest main sequence turnoff, providing excellent leverage on Crater's physical properties. Structurally, Crater has a half-light radius of $\sim$20 pc and shows no evidence for tidal distortions. Crater is well-described by a simple stellar population with an age of $\sim$7.5 Gyr, [M/H]$\sim-1.65$, a M$_{\star}\sim10^4$ M$_{\odot}$, M$_{\rm V}\sim -5.3$, located at a distance of (d$_{\odot}$, d$_{\rm GC}$) $\sim$ (145, 110) kpc, with modest uncertainties in these properties due to differences in the underlying stellar evolution models. The sparse sampling of stars above the turnoff and sub-giant branch are likely to be 1.0-1.4 M$_{\odot}$ binary star systems (blue stragglers) and their evolved descendants, as opposed to intermediate age main sequence stars. Confusion of these populations highlights a substantial challenge in accurately characterizing sparsely populated stellar systems. Our analysis shows that Crater is not a dwarf galaxy, but instead is an unusually young cluster given its location in the Milky Way's very outer stellar halo. Crater is similar to SMC cluster Lindsay 38, and its position and velocity are in good agreement with observations and models of the Magellanic stream debris, suggesting it may have accreted from the Magellanic Clouds. However, its age and metallicity are also in agreement with the age-metallicity relationships of lower mass dwarf galaxies such as Leo I or Carina. Despite uncertainty over its progenitor system, Crater appears to have been incorporated into the Galaxy more recently than $z\sim1$ (8 Gyr ago), providing an important new constraint on the accretion history of the Milky Way. [abridged]
The study of supernova remnants (SNRs) is fundamental to understanding the chemical enrichment and magnetism in galaxies, including our own Milky Way. In an effort to understand the connection between the morphology of SNRs and the Galactic magnetic field (GMF), we have examined the radio images of all known SNRs in our Galaxy and compiled a large sample that have an "axisymmetric" morphology, which we define to mean SNRs with a "bilateral" or "barrel"-shaped morphology, in addition to one-sided shells. We selected the cleanest examples and model each of these at their appropriate Galactic position using two GMF models, those of Jansson & Farrar (2012a), which includes a vertical halo component, and Sun et al. (2008) that is oriented entirely parallel to the plane. Since the magnitude and relative orientation of the magnetic field changes with distance from the sun, we analyse a range of distances, from 0.5 to 10 kpc in each case. Using a physically motivated model of a SNR expanding into the ambient GMF, we find the models using Jansson & Farrar (2012a) are able to reproduce observed morphologies of many SNRs in our sample. These results strongly support the presence of an off-plane, vertical component to the GMF, and the importance of the Galactic field on SNR morphology. Our approach also provides a potential new method for determining distances to SNRs, or conversely, distances to features in the large-scale GMF if SNR distances are known.
The collapse of the primordial gas in the density regime $\sim 10^{8}\hbox{--}10^{10}$ cm$^{-3}$ is controlled by the three-body $\rm H_2$ formation process, in which the gas can cool faster than free-fall time $\hbox{--}$ a condition proposed as the chemothermal instability. We investigate how the heating and cooling rates are affected during the rapid transformation of atomic to molecular hydrogen. With a detailed study of the heating and cooling balance in a 3D simulation of Pop~III collapse, we follow the chemical and thermal evolution of the primordial gas in two dark matter minihaloes. The inclusion of sink particles in modified Gadget-2 smoothed particle hydrodynamics code allows us to investigate the long term evolution of the disk that fragments into several clumps. We find that the sum of all the cooling rates is less than the total heating rate after including the contribution from the compressional heating ($pdV$). The increasing cooling rate during the rapid increase of the molecular fraction is offset by the unavoidable heating due to gas contraction. We conclude that fragmentation occurs because $\rm H_2$ cooling, the heating due to $\rm H_2$ formation and compressional heating together set a density and temperature structure in the disk that favors fragmentation, not the chemothermal instability.
Ultra luminous X-ray sources (ULXs) are usually believed to be black holes with mass about 10^{2--3}M_{sun}. However, the recent discovery of ULX NuSTAR J095551+6940.8 in M82 with the spin period P=1.37s and period derivation P_{dot}=-2*10^{-10} ss^{-1} provides a strong evidence that some ULXs are accreting neutron stars (NSs). To investigate such a particular accreting neutron star, we ascribe it as an evolved magnetar in the accretion binary system. By means of the model of accretion induced the NS magnetic evolution and standard spinup torque, we calculate the magnetic field decay and spin-up of M82 X-2, and show that its magnetic field is now 4.5*10^{12} G, which is evolved from a magnetar in a high mass Xray binary system (HMXB) with the initial values of magnetic field B~10^{14.5} G and spin period P~100 s by accreting ~10^{-3}M_{sun}, while the mass accretion rate for spin-up is set as 5.0*10^{18} gs^{-1}. The evolutionary track of magnetic field and spin period of M82 X-2 is simulated and plotted in the B-P diagram, with which we compare the observed pulsars, and find that several pulsars are consistent with the B-P track of M82 X-2. Since the birth rate of magnetar is about ten percent of the normal NSs, it is inferred that a couple of ULXs should also be the similar cases like M82 X-2. Furthermore, we argue that the existence of the local super-strong magnetic multipole structure of M82 X-2 destroys the spherical accretion condition of Eddington critical luminosity, which arises the ULX M82 X-2 to be different from the usual NS in HMXBs with the luminosity no more than the Eddington limit ......
Theoretical studies have revealed that dust grains are usually moving fast through the turbulent interstellar gas, which could have significant effects upon molecular cloud chemistry by modifying grain accretion. This effect is investigated in this work on the basis of numerical gas-grain chemical modeling. Major features of the grain motion effect in the typical environment of dark clouds (DC) can be summarised as follows: 1) decrease of gas-phase (both neutral and ionic) abundances and increase of surface abundances by up to 2-3 orders of magnitude; 2) shifts of the existing chemical jumps to earlier evolution ages for gas-phase species and to later ages for surface species by factors of about ten; 3) a few exceptional cases in which some species turn out to be insensitive to this effect and some other species can show opposite behaviors too. These effects usually begin to emerge from a typical DC model age of about 10^5 yr. The grain motion in a typical cold neutral medium (CNM) can help overcome the Coulomb repulsive barrier to enable effective accretion of cations onto positively charged grains. As a result, the grain motion greatly enhances the abundances of some gas-phase and surface species by factors up to 2-6 or more orders of magnitude in the CNM model. The grain motion effect in a typical molecular cloud (MC) is intermediate between that of the DC and CNM models, but with weaker strength. The grain motion is found to be important to consider in chemical simulations of typical interstellar medium.
We have used the Arecibo L-band Feed Array to map three regions, each of 5 square degrees, around the isolated galaxies NGC 1156, UGC 2082, and NGC 5523. In the vicinity of these galaxies we have detected two dwarf companions: one near UGC 2082, previously discovered by ALFALFA, and one near NGC 1156, discovered by this project and reported in an earlier paper. This is significantly fewer than the 15.4 $^{+1.7}_{-1.5}$ that would be expected from the field HI mass function from ALFALFA or the 8.9 $\pm$ 1.2 expected if the HI mass function from the Local Group applied in these regions. The number of dwarf companions detected is, however, consistent with a flat or declining HI mass function as seen by a previous, shallower, HI search for companions to isolated galaxies.We attribute this difference in Hi mass functions to the different environments in which they are measured. This agrees with the general observation that lower ratios of dwarf to giant galaxies are found in lower density environments.
With high resolution (0"25 x 0"18) ALMA CO 3-2 observations of the nearby
(D=21 Mpc), extremely radio quiet galaxy NGC1377, we have discovered a high
velocity, very collimated molecular jet with a projected length of $\pm$160 pc.
Along the jet axis we find strong velocity reversals swinging from -180 to +180
km/s. A simple model of a precessing molecular jet can reproduce the
observations. The launch region is inside a radius r<10 pc and the velocity of
the outflowing gas lies between 250 and 600 km/s. The CO emission is clumpy and
the jet molecular mass ranges between 2e6 Msun (light jet) and 2e7 Msun
(massive jet).
We suggest that the driving mechanism of the molecular jet is either a
(fading) radio jet or an accretion disk-wind similar to those found towards
protostars. It seems unlikely that a massive jet could have been driven out by
the current level of nuclear activity which should then have undergone rapid
quenching. In contrast, a light jet would have expelled only 10% of the nuclear
gas and may facilitate nuclear activity instead of suppressing it. The
precession can be powered by a binary supermassive black hole (SMBH) or by gas
of misaligned angular momentum flowing onto a warped accretion disk. Large
columns of H2 in the nucleus of NGC1377 suggest a high rate of recent gas
infall. The current IR emission of NGC1377 may be powered by a SMBH accreting
at a rate of about 10% Eddington. There is tentative evidence that the
molecular gas in the jet is decelerating and that the gas in the outflow
therefore can return and fuel future nuclear growth. Further studies are
required to determine the age and mass of the molecular jet and the role it
plays in the nuclear growth of NGC1377. There is also a broad, cone-like
structure of CO emission in NGC1377 which seems to be a slower, wide-angle
molecular outflow with an estimated molecular mass of approximately 1e8 Msun.
In high-resolution X-ray observations of the hot plasma in clusters of galaxies significant structures caused by AGN feedback, mergers, and turbulence can be detected. Many clusters have been observed by Chandra in great depth and at high resolution. Using archival data taken with the Chandra ACIS instrument the aim was to study thermodynamic perturbations of the X-ray emitting plasma and to apply this to better understand the thermodynamic and dynamic state of the intra cluster medium (ICM). We analysed deep observations for a sample of 33 clusters with more than 100 ks of Chandra exposure each at distances between redshift 0.025 and 0.45. The combined exposure of the sample is 8 Ms. Fitting emission models to different regions of the extended X-ray emission we searched for perturbations in density, temperature, pressure, and entropy of the hot plasma. For individual clusters we mapped the thermodynamic properties of the ICM and measured their spread in circular concentric annuli. Comparing the spread of different gas quantities to high-resolution 3D hydrodynamic simulations, we constrain the average Mach number regime of the sample to Mach1D ~ 0.16 +- 0.07. In addition we found a tight correlation between metallicity, temperature and redshift with an average metallicity of Z ~ 0.3 +- 0.1 Z(solar). This study provides detailed perturbation measurements for a large sample of clusters which can be used to study turbulence and make predictions for future X-ray observatories like eROSITA, Astro-H, and Athena.
We analyse the environment of the supermassive black hole (SMBH) in the centre of a massive elliptical galaxy NGC 1275 in the Perseus cluster, hosting the radio source 3C 84. We focus on the young radio lobe observed inside the estimated Bondi accretion radius. We discuss the momentum balance between the jet associated with the lobe and the surrounding gas. The results are compared with the proper motion of the radio lobe obtained with the VLBI. We find that under assumption of a high-density environment >~ 100 cm^-3), the jet power must be comparable to the Eddington luminosity --- this is clearly inconsistent with the current moderate activity of 3C 84, which indicates instead that the jet is expanding in a very low density region (<~1 cm^-3), along the rotation axis of the accretion flow. The power required for the jet to expand in the low-density environment is comparable to the past average jet power estimated from the X-ray observations. We estimate the classical Bondi accretion rate, assuming that (1) gas accretion is spherically symmetric, (2) accretion is associated with the jet environment, and (3) the medium surrounding the jet is representative of the properties of the dominant accreting gas. We find that Bondi accretion is inconsistent with the estimated jet power. This means that either accretion of the cold gas in the NGC 1275 is more efficient than that of the hot gas, or the jets are powered by the SMBH spin.
Most star clusters at an intermediate age (1-2 Gyr) in the Large and Small Magellanic Clouds show a puzzling feature in their color-magnitude diagrams (CMD) that is not in agreement with a simple stellar population. The main sequence turn-off of these clusters is much broader than would be expected from photometric uncertainties. One interpretation of this feature is that age spreads of the order 200-500 Myr exist within individual clusters, although this interpretation is highly debated. Such large age spreads should affect other parts of the CMD, which are sensitive to age, as well. In this study, we analyze the CMDs of a sample of 12 intermediate-age clusters in the Large Magellanic Cloud that all show an extended turn-off using archival optical data taken with the Hubble Space Telescope. We fit the star formation history of the turn-off region and the red clump region independently with two different theoretical isochrone models. We find that in most of the cases, the age spreads inferred from the red clumps are smaller than the ones resulting from the turn-off region. However, the age ranges resulting from the red clump region are broader than would be expected for a single age. Only two out of 12 clusters in our sample show a red clump which seems to be consistent with a single age. As our results are not unambiguous, we can not ultimately tell if the extended main sequence turn-off feature is due to an age spread, or not, by fitting the star formation histories to the red clump regions. However, we find that the width of the extended main sequence turn-off feature is correlated with the age of the clusters in a way which would be unexplained in the "age spread" interpretation, but which may be expected if stellar rotation is the cause of the spread at the turn-off.
We present Ks-band light curves for 299 new Cepheids in the Small Magellanic Cloud (SMC) that were identified using multi-epoch near-infrared photometry obtained by the VISTA survey of the Magellanic Clouds system (VMC). The new Cepheids have periods in the range from 0.38 to 13.15 days and cover the magnitude interval 12.35 < Ks < 17.6 mag. Our method was developed using variable stars previously identified by the optical microlensing survey OGLE. We focus on searching new Cepheids in external regions of the SMC for which complete VMC Ks-band observations are available and no comprehensive identification of different types of variable stars from other surveys exists yet.
Links to: arXiv, form interface, find, astro-ph, recent, 1511, contact, help (Access key information)
Accretion disks around supermassive black holes (SMBHs) in active galactic nuclei contain stars, stellar mass black holes, and other stellar remnants, which perturb the disk gas gravitationally. The resulting density perturbations in turn exert torques on the embedded masses causing them to migrate through the disk in a manner analogous to the behavior of planets in protoplanetary disks. We determine the strength and direction of these torques using an empirical analytic description dependent on local disk gradients, applied to two different analytic, steady-state disk models of SMBH accretion disks. We find that there are radii in such disks where the gas torque changes sign, trapping migrating objects. Our analysis shows that major migration traps generally occur where the disk surface density gradient changes sign from positive to negative, around 20--300$R_{\rm g}$, where $R_{\rm g}=2GM/c^{2}$ is the Schwarzschild radius. At these traps, massive objects in the AGN disk can accumulate, collide, scatter, and accrete. Intermediate mass black hole formation is likely in these disk locations, which may lead to preferential gap and cavity creation at these radii. Our model thus has significant implications for SMBH growth as well as gravitational wave source populations.
We present a study of spatial variations in the metallicity of old red giant branch stars in the Andromeda galaxy. Photometric metallicity estimates are derived by interpolating isochrones for over seven million stars in the Panchromatic Hubble Andromeda Treasury (PHAT) survey. This is the first systematic study of stellar metallicities over the inner 20 kpc of Andromeda's galactic disk. We see a clear metallicity gradient of $-0.020\pm0.004$ dex/kpc from $\sim4-20$ kpc assuming a constant RGB age. This metallicity gradient is derived after correcting for the effects of photometric bias and completeness and dust extinction and is quite insensitive to these effects. The unknown age gradient in M31's disk creates the dominant systematic uncertainty in our derived metallicity gradient. However, spectroscopic analyses of galaxies similar to M31 show that they typically have small age gradients that make this systematic error comparable to the 1$\sigma$ error on our metallicity gradient measurement. In addition to the metallicity gradient, we observe an asymmetric local enhancement in metallicity at radii of 3-6 kpc that appears to be associated with Andromeda's elongated bar. This same region also appears to have an enhanced stellar density and velocity dispersion.
We present the first results from the KMOS AGN Survey at High redshift (KASHz), a VLT/KMOS integral-field spectroscopic survey of z>0.6 AGN. We present galaxy-integrated spectra of 89 X-ray AGN (Lx=10^42-10^45 erg/s), for which we observed [O III] (z=1.1-1.7) or Halpha emission (z=0.6-1.1). The targets have X-ray luminosities representative of the parent AGN population and we explore the emission-line luminosities as a function of X-ray luminosity. For the [O III] targets, ~50 per cent have ionised gas velocities indicative of gas that is dominated by outflows and/or highly turbulent material (i.e., overall line-widths >~600 km/s). The most luminous half (i.e., Lx>6x10^43 erg/s) have a >~2 times higher incidence of such velocities. On the basis of our results, we find no evidence that X-ray obscured AGN are more likely to host extreme kinematics than unobscured AGN. Our KASHz sample has a distribution of gas velocities that is consistent with a luminosity-matched sample of z<0.4 AGN. This implies little evolution in the prevalence of ionised outflows, for a fixed AGN luminosity, despite an order-of-magnitude decrease in average star-formation rates over this redshift range. Furthermore, we compare our Halpha targets to a redshift-matched sample of star-forming galaxies and despite a similar distribution of Halpha luminosities and likely star-formation rates, we find extreme ionised gas velocities are up to ~10x more prevalent in the AGN-host galaxies. Our results reveal a high prevalence of extreme ionised gas velocities in high-luminosity X-ray AGN and imply that the most powerful ionised outflows in high-redshift galaxies are driven by AGN activity.
Recent hydrodynamic (HD) simulations have shown that galactic disks evolve to reach well-defined statistical equilibrium states. The star formation rate (SFR) self-regulates until energy injection by star formation feedback balances dissipation and cooling in the interstellar medium (ISM), and provides vertical pressure support to balance gravity. In this paper, we extend our previous models to allow for a range of initial magnetic field strengths and configurations, utilizing three-dimensional, magnetohydrodynamic (MHD) simulations. We show that a quasi-steady equilibrium state is established as rapidly for MHD as for HD models unless the initial magnetic field is very strong or very weak, which requires more time to reach saturation. Remarkably, models with initial magnetic energy varying by two orders of magnitude approach the same asymptotic state. In the fully saturated state of the fiducial model, the integrated energy proportions E_kin:E_th:E_mag,t:E_mag,o are 0.35:0.39:0.15:0.11, while the proportions of midplane support P_turb:P_th:\Pi_mag,t:\Pi_mag,o are 0.49:0.18:0.18:0.15. Vertical profiles of total effective pressure satisfy vertical dynamical equilibrium with the total gas weight at all heights. We measure the "feedback yields" \eta_c=P_c/\Sigma_SFR (in suitable units) for each pressure component, finding that \eta_turb~4 and \eta_th~1 are the same for MHD as in previous HD simulations, and \eta_mag,t~1. These yields can be used to predict the equilibrium SFR for a local region in a galaxy based on its observed gas and stellar surface densities and velocity dispersions. As the ISM weight (or dynamical equilibrium pressure) is fixed, an increase in $\eta$ from turbulent magnetic fields reduces the predicted \Sigma_SFR by ~25% relative to the HD case.
Cosmic reionization by starlight from early galaxies affected their evolution, thereby impacting reionization, itself. Star formation suppression, for example, may explain the observed underabundance of Local Group dwarfs relative to N-body predictions for Cold Dark Matter. Reionization modelling requires simulating volumes large enough ~(100 Mpc)^3 to sample reionization "patchiness", while resolving millions of galaxy sources above ~10^8 Msun, combining gravitational and gas dynamics with radiative transfer. Modelling the Local Group requires initial cosmological density fluctuations pre-selected to form the well-known structures of the local universe today. Cosmic Dawn ("CoDa") is the first such fully-coupled, radiation-hydrodynamics simulation of reionization of the local universe. Our new hybrid CPU-GPU code, RAMSES-CUDATON, performs hundreds of radiative transfer and ionization rate-solver timesteps on the GPUs for each hydro-gravity timestep on the CPUs. CoDa simulated (91 Mpc)^3 with 4096^3 particles and cells, to redshift 4.23, on ORNL supercomputer Titan, utilizing 8192 cores and 8192 GPUs. Global reionization ended slightly later than observed. However, a simple temporal rescaling which brings the evolution of ionized fraction into agreement with observations also reconciles ionizing flux density, cosmic star formation history, CMB electron scattering optical depth and galaxy UV luminosity function with their observed values. Haloes below ~3 x 10^9 Msun were severely affected by the rising UV background: photoionization heating suppressed their star formation. For most of reionization, star formation was dominated by haloes between 10^10 - 10^11Msun. Intergalactic filaments display sheathed structures, with hot envelopes surrounding cooler cores, but do not self-shield, unlike regions denser than 100 rho_average.
Radio emission from radio-quiet quasars may be due to star formation in the quasar host galaxy, to a jet launched by the supermassive black hole, or to relativistic particles accelerated in a wide-angle radiatively-driven outflow. In this paper we examine whether radio emission from radio-quiet quasars is a byproduct of star formation in their hosts. To this end we use infrared spectroscopy and photometry from Spitzer and Herschel to estimate or place upper limits on star formation rates in hosts of ~300 obscured and unobscured quasars at z<1. We find that low-ionization forbidden emission lines such as [NeII] and [NeIII] are likely dominated by quasar ionization and do not provide reliable star formation diagnostics in quasar hosts, while PAH emission features may be suppressed due to the destruction of PAH molecules by the quasar radiation field. While the bolometric luminosities of our sources are dominated by the quasars, the 160 micron fluxes are likely dominated by star formation, but they too should be used with caution. We estimate median star formation rates to be 6-29 Msun/year, with obscured quasars at the high end of this range. This star formation rate is insufficient to explain the observed radio emission from quasars by an order of magnitude, with log(L_radio, observed/L_radio, SF)=0.6-1.3 depending on quasar type and star formation estimator. Although radio-quiet quasars in our sample lie close to the 8-1000 micron infrared / radio correlation characteristic of the star-forming galaxies, both their infrared emission and their radio emission are dominated by the quasar activity, not by the host galaxy.
We use three-dimensional magnetohydrodynamic (MHD) simulations to investigate the quasi-equilibrium states of galactic disks regulated by star formation feedback. We incorporate effects from massive-star feedback via time-varying heating rates and supernova (SN) explosions. We find that the disks in our simulations rapidly approach a quasi-steady state that satisfies vertical dynamical equilibrium. The star formation rate (SFR) surface density self-adjusts to provide the total momentum flux (pressure) in the vertical direction that matches the weight of the gas. We quantify feedback efficiency by measuring feedback yields, \eta_c\equiv P_c/\Sigma_SFR (in suitable units), for each pressure component. The turbulent and thermal feedback yields are the same for HD and MHD simulations, \eta_th~1 and \eta_ turb~4, consistent with the theoretical expectations. In MHD simulations, turbulent magnetic fields are rapidly generated by turbulence, and saturate at a level corresponding to \eta_mag,t~1. The presence of magnetic fields enhances the total feedback yield and therefore reduces the SFR, since the same vertical support can be supplied at a smaller SFR. We suggest further numerical calibrations and observational tests in terms of the feedback yields.
We investigate the observational signatures and physical origin of
ram-pressure stripping (RPS) in 63 massive galaxy clusters at $z=0.3-0.7$,
based on images obtained with the Hubble Space Telescope. Using a training set
of a dozen "jellyfish" galaxies identified earlier in the same imaging data, we
define morphological criteria to select 211 additional, less obvious cases of
RPS. Spectroscopic follow-up observations of 124 candidates so far confirmed 53
as cluster members. For the brightest and most favourably aligned systems we
visually derive estimates of the projected direction of motion based on the
orientation of apparent compression shocks and debris trails.
Our findings suggest that the onset of these events occurs primarily at large
distances from the cluster core ($>400$ kpc), and that the trajectories of the
affected galaxies feature high impact parameters. Simple models show that such
trajectories are highly improbable for galaxy infall along filaments but common
for infall at high velocities, even after observational biases are accounted
for, provided the duration of the resulting RPS events is $\lesssim$500 Myr. We
thus tentatively conclude that extreme RPS events are preferentially triggered
by cluster mergers, an interpretation that is supported by the disturbed
dynamical state of many of the host clusters. This hypothesis implies that
extreme RPS might occur also near the cores of merging poor clusters or even
merging groups of galaxies.
Finally, we present nine additional "jellyfish" galaxies at z$>$0.3
discovered by us, thereby doubling the number of such systems known at
intermediate redshift.
We present the first evidence of clear signatures of tidal distortions in the density distribution of the fascinating open cluster NGC~6791. We find that the 2D density map shows a clear elongation and an irregular distribution starting from $\sim 300^{\prime\prime}$ from the cluster center and two tails extending in opposite directions beyond the tidal radius. These features are aligned to both the absolute proper motion and to the Galactic centre directions. Accordingly we find that both the surface brightness and star count density profiles reveal a departure from a King model starting from $\sim600^{\prime\prime}$. These observational evidences suggest that NGC~6791 is currently undergoing mass-loss likely due to gravitational shocking and interactions with the tidal field of the Milky Way. We derive the expected mass-loss due to stellar evolution and tidal interactions and we estimate the initial cluster mass to be $M_{ini} = (1.5-4.0 ) \times 10^5 M_{\odot}$.
The thermodynamical evolution of gas during the collapse of the primordial star-forming cloud depends significantly on the initial degree of rotation. However, there is no clear understanding of how the initial rotation can affect the heating and cooling process and hence the temperature that leads to the fragmentation of the gas during Population III star formation. We report the results from three\hbox{-}dimensional, smoothed-particle hydrodynamics (SPH) simulations of a rotating self-gravitating primordial gas cloud with a modified version of the Gadget-2 code, in which the initial ratio of the rotational to the gravitational energy ($\beta_0$) is varied over two orders of magnitude. We find that despite the lack of any initial turbulence and magnetic fields in the clouds, the angular momentum distribution leads to the formation and build-up of a disk that fragments into several clumps. We further examine the behavior of the protostars that form in both idealized as well as more realistic minihalos from the cosmological simulations. The thermodynamical evolution and the fragmentation behavior of the cosmological minihalos are similar to that of the artificial cases, especially in those with a similar $\beta_0$-parameter. Protostars with a higher rotation support exhibit spiral-arm-like structures on several scales, and have lower accretion rates. These type of clouds tend to fragment more, while some of the protostars escape from the cluster with the possibility of surviving until the present day. They also take much longer to form compared to their slowly rotating counterparts. We conclude that the use of appropriate initial conditions of the gas in minihalos is a pivotal and decisive quantity to study the evolution and final fate of the primordial stars.
Substantial evidence points to dusty, geometrically thick tori obscuring the central engines of active galactic nuclei, but so far no mechanism satisfactorily explains why cool dust in the torus remains in a puffy geometry. Infrared (IR) radiation pressure on dust can play a significant role in shaping the torus, yet the separation of hydrodynamic evolution from radiative transfer (RT) in previous work on radiation-supported tori precluded a self-consistent picture. Here we present radiative hydrodynamics simulations of an initially smooth torus; we solve the hydrodynamics equations, the time-dependent multi-angle group IR RT equation, and the time-independent ultraviolet (UV) RT equation. IR radiation is highly anisotropic, leaving primarily through the central hole of the torus. The inner edge of the torus exhibits a break in axisymmetry under the influence of radiation and differential rotation. In addition, UV radiation pressure on dust launches a strong wind along the inner edge of the torus with speed $\sim 5.3\times10^3 (M/10^7 M_\odot)^{1/4} [L_\mathrm{UV}/(0.1 L_\mathrm E)]^{1/4} \mathrm{km}\,\mathrm s^{-1}$ and mass loss rate $\sim 0.12 (M/10^7 M_\odot)^{3/4} [L_\mathrm{UV}/(0.1 L_\mathrm E)]^{3/4} M_\odot\,\mathrm{yr}^{-1}$, where $M$, $L_\mathrm{UV}$, and $L_\mathrm E$ are the mass, UV luminosity, and Eddington luminosity of the central object respectively; these values are comparable to those inferred from observations.
We report a new sample of obscured active galactic nuclei (AGNs) selected from the XMM serendipitous source and AKARI point-source catalogs. We match X-ray sources with infrared (18 and 90 micron) sources located at |b|>10 deg to create a sample consisting of 173 objects. Their optical classifications and absorption column densities measured by X-ray spectra are compiled and study efficient selection criteria to find obscured AGNs. We apply the criteria (1) X-ray hardness ratio defined by using the 2-4.5 keV and 4.5-12 keV bands >-0.1 and (2) EPIC-PN count rate (CR) in the 0.2-12 keV to infrared flux ratio CR/F90<0.1 or CR/F18<1, where F18 and F90 are infrared fluxes at 18 and 90 micron in Jy, respectively, to search for obscured AGNs. X-ray spectra of 48 candidates, for which no X-ray results have been published, are analyzed and X-ray evidence for the presence of obscured AGNs such as a convex shape X-ray spectrum indicative of absorption of NH~10^{22-24} cm^-2, a very flat continuum, or a strong Fe-K emission line with an equivalent width of >700 eV is found in 26 objects. Six among them are classified as Compton-thick AGNs, and four are represented by either Compton-thin or Compton-thick spectral models. The success rate of finding obscured AGNs combining our analysis and the literature is 92% if the 18 micron condition is used. Of the 26 objects, 4 are optically classified as an HII nucleus and are new "elusive AGNs" in which star formation activity likely overwhelms AGN emission in the optical and infrared bands.
Compact steep spectrum (CSS) and GHz-peaked spectrum (GPS) radio sources represent a large fraction of the extragalactic objects in flux density-limited samples. They are compact, powerful radio sources whose synchrotron peak frequency ranges between a few hundred MHz to several GHz. CSS and GPS radio sources are currently interpreted as objects in which the radio emission is in an early evolutionary stage. In this contribution I review the radio properties and the physical characteristics of this class of radio sources, and the interplay between their radio emission and the ambient medium of the host galaxy.
The last comprehensive catalogue of high-mass X-ray binaries in the Small Magellanic Cloud (SMC) was published about 10 years ago. Since then new such systems were discovered, mainly by X-ray observations with Chandra and XMM-Newton. For the majority of the proposed HMXBs in the SMC no X-ray pulsations were discovered yet and unless other properties of the X-ray source and/or the optical counterpart confirm their HMXB nature, they remain only candidate HMXBs. From a literature search we collect a catalogue of 148 confirmed and candidate HMXBs in the SMC and investigate their properties to shed light on their real nature. Based on the sample of well established HMXBs (the pulsars), we investigate which observed properties are most appropriate for a reliable classification. Using spectral and temporal characteristics of the X-ray sources and colour-magnitude diagrams from the optical to the infrared of their likely counterparts and taking into account the uncertainty in the X-ray position we define different levels of confidence for being a genuine HMXB. From the lack of an infrared excess of the proposed counterpart, mainly for X-ray sources with large positional uncertainty, and using additional information obtained from more recent observations, we identify 27 objects as likely mis-identifications. This results in a catalogue of 121 relatively high-confidence HMXBs (the vast majority with Be companion stars) with about half of the objects showing X-ray pulsations while for the rest no pulsations are known yet. A comparison of the two subsamples suggests that long pulse periods in excess of a few 100 s are expected for the "non-pulsars", which are likely undetected because of aperiodic variability on similar time scales and insufficiently long X-ray observations. (abbreviated)
[abridged] How does a star cluster of more than few 10,000 solar masses form? We present the case of the cluster NGC 346 in the Small Magellanic Cloud, and its star-forming region N66, and we propose a scenario for its formation, based on observations of the rich stellar populations in the region. Young massive clusters (YMCs) host a high fraction of early-type stars, indicating an extremely high star formation efficiency. The Magellanic Clouds host a wide range of such clusters with the youngest being still embedded in their giant HII regions. Hubble Space Telescope imaging of such star-forming complexes allows the detailed study of star formation at scales typical for molecular clouds. Our cluster analysis of newly-born stars in N66 shows that star formation in the region proceeds in a clumpy hierarchical fashion, leading to the formation of both a dominant YMC, hosting about half of the observed pre--main-sequence population, and a dispersed self-similar distribution of the remaining stars. We investigate the correlation between star formation rate derived from star-counts and molecular gas surface density in order to unravel the physical conditions that gave birth to NGC 346. We find a steep correlation between these two parameters with a considerable scatter. The fraction of mass in stars is found to be systematically higher within the central 15 pc (where the YMC is located) than outside, which suggests variations in the star formation efficiency within the same star-forming complex. This trend possibly reflects a change of star formation efficiency in N66 between clustered and non-clustered star formation. Our findings suggest that the formation of NGC 346 is the combined result of star formation regulated by turbulence and of early dynamical evolution induced by the gravitational potential of the dense interstellar medium.
Aims: By utilising spectra of early-type stellar probes of known distances in the same region of the sky, the large and small-scale (pc) structure of the Galactic ISM can be investigated. This paper determines the variation in line strength of CaII at 3933.661 A, as a function of probe separation for a sample of stars, including many sightlines in the Magellanic Clouds. Methods: FLAMES-GIRAFFE data taken with the VLT towards early-type stars in 3 Galactic & 4 Magellanic open clusters in CaII are used to obtain the velocity, EW, column density and line width of IS Galactic Ca for a total of 657 stars, of which 443 are Magellanic sightlines. In each cluster there are 43-110 stars observed. Additionally, FEROS and UVES CaII & NaI spectra of 21 Galactic & 154 Magellanic early-type stars are presented and combined with data from the literature to study the Ca column density/parallax relationship. Results: For the four Magellanic clusters studied with FLAMES, the strength of the Galactic IS CaII K EW over transverse scales from 0.05-9 pc is found to vary by factors of 1.8-3.0, corresponding to column density variations of 0.3-0.5 dex in the optically-thin approximation. Using FLAMES, FEROS and UVES archive spectra, the min and max reduced EW for MW gas is found to lie in the range 35-125 mA & 30-160 mA for CaII K and NaI D, respectively. The range is consistent with a simple model of the ISM published by van Loon et al. (2009) consisting of spherical cloudlets of filling factor 0.3, although other geometries are not ruled out. Finally, the derived functional form for parallax and CaII column density is found to be pi(mas)=1/(2.39e-13 x N(CaII)(cm-2)+0.11). Our derived parallax is 25 per cent lower than predicted by Megier et al. (2009) at a distance of 100 pc and 15% lower at a distance of 200 pc, reflecting inhomogeneity in the CaII distribution in the different sightlines studied.
AGN feedback from supermassive black holes (SMBHs) at the center of early type galaxies is commonly invoked as the explanation for the quenching of star formation in these systems. The situation is complicated by the significant amount of mass injected in the galaxy by the evolving stellar population over cosmological times. In absence of feedback, this mass would lead to unobserved galactic cooling flows, and to SMBHs two orders of magnitude more massive than observed. By using high-resolution 2D hydrodynamical simulations with radiative transport and star formation in state-of-the-art galaxy models, we show how the intermittent AGN feedback is highly structured on spatial and temporal scales, and how its effects are not only negative (shutting down the recurrent cooling episodes of the ISM), but also positive, inducing star formation in the inner regions of the host galaxy.
Early-type galaxies (ETGs) host a hot ISM produced mainly by stellar winds, and heated by Type Ia supernovae and the thermalization of stellar motions. High resolution 2D hydrodynamical simulations showed that ordered rotation in the stellar component results in the formation of a centrifugally supported cold equatorial disc. In a recent numerical investigation we found that subsequent generations of stars are formed in this cold disc; this process consumes most of the cold gas, leaving at the present epoch cold masses comparable to those observed. Most of the new stellar mass formed a few Gyrs ago, and resides in a disc.
We performed the first spectral-line survey at 82--106 GHz and 335--355 GHz toward the outflow-shocked region, OMC 2-FIR 4, the outflow driving source, FIR 3, and the northern outflow lobe, FIR 3N. We detected 120 lines of 20 molecular species. The line profiles are found to be classifiable into two types: one is a single Gaussian component with a narrow ($<$ 3 km s$^{-1}$) width and another is two Gaussian components with narrow and wide ($>$ 3km s$^{-1}$) widths. The narrow components for the most of the lines are detected at all positions, suggesting that they trace the ambient dense gas. For CO, CS, HCN, and HCO$^{+}$, the wide components are detected at all positions, suggesting the outflow origin. The wide components of C$^{34}$S, SO, SiO, H$^{13}$CN, HC$^{15}$N, H$_2^{13}$CO, H$_2$CS, HC$_3$N, and CH$_3$OH are detected only at FIR 4, suggesting the outflow-shocked gas origin. The rotation diagram analysis revealed that the narrow components of C$_2$H and H$^{13}$CO$^+$ show low temperatures of 12.5$\pm$1.4 K, while the wide components show high temperatures of 20--70 K. This supports our interpretation that the wide components trace the outflow and/or outflow-shocked gas. We compared observed molecular abundances relative to H$^{13}$CO$^+$ with those of the outflow-shocked region, L1157 B1, and the hot corino, IRAS 16293-2422. Although we cannot exclude a possibility that the chemical enrichment in FIR 4 is caused by the hot core chemistry, the chemical compositions in FIR 4 are more similar to those in L1157 B1 than those in IRAS 16293-2422.
We present a five-band Herschel study (100-500um) of three galaxy clusters at z~1.2 from the Spitzer Adaptation of the Red-Sequence Cluster Survey (SpARCS). With a sample of 120 spectroscopically-confirmed cluster members, we investigate the role of environment on galaxy properties utilizing the projected cluster phase space (line-of-sight velocity versus clustercentric radius), which probes the time-averaged galaxy density to which a galaxy has been exposed. We divide cluster galaxies into phase-space bins of (r/r200) x (v/sigma_v), tracing a sequence of accretion histories in phase space. Stacking optically star-forming cluster members on the Herschel maps, we measure average infrared star formation rates, and, for the first time in high-redshift galaxy clusters, dust temperatures for dynamically distinct galaxy populations---namely, recent infalls and those that were accreted onto the cluster at an earlier epoch. Proceeding from the infalling to virialized (central) regions of phase space, we find a steady decrease in the specific star formation rate and increase in the stellar age of star-forming cluster galaxies. We perform a probability analysis to investigate all acceptable infrared spectral energy distributions within the full parameter space and measure a ~4 sigma drop in the average dust temperature of cluster galaxies in an intermediate phase-space bin, compared to an otherwise flat trend with phase space. We suggest one plausible quenching mechanism which may be consistent with these trends, invoking ram-pressure stripping of the warmer dust for galaxies within this intermediate accretion phase.
We derive the mean wavelength dependence of stellar attenuation in a sample of 239 high redshift (1.90 < z < 2.35) galaxies selected via Hubble Space Telescope (HST) WFC3 IR grism observations of their rest-frame optical emission lines. Our analysis indicates that the average reddening law follows a form similar to that derived by Calzetti et al. for local starburst galaxies. However, over the mass range 7.2 < log M/Msolar < 10.2, the slope of the attenuation law in the UV is shallower than that seen locally, and the UV slope steepens as the mass increases. These trends are in qualitative agreement with Kriek & Conroy, who found that the wavelength dependence of attenuation varies with galaxy spectral type. However, we find no evidence of an extinction "bump" at 2175 A in any of the three stellar mass bins, or in the sample as a whole. We quantify the relation between the attenuation curve and stellar mass and discuss its implications.
We report the discovery of spiral galaxies that are as optically luminous as elliptical brightest cluster galaxies, with r-band monochromatic luminosity L_r=8-14L* (4.3-7.5E44 erg/s). These super spiral galaxies are also giant and massive, with diameter D=57-134 kpc and stellar mass M_stars=0.3-3.4E11 M_sun. We find 53 super spirals out of a complete sample of 1,616 SDSS galaxies with redshift z<0.3 and L_r>8L*. The closest example is found at z=0.089. We use existing photometry to estimate their stellar masses and star formation rates (SFRs). The SDSS and WISE colors are consistent with normal star-forming spirals on the blue sequence. However, the extreme masses and rapid SFRs of 5-65 M_sun/yr place super spirals in a sparsely populated region of parameter space, above the star-forming main sequence of disk galaxies. Super spirals occupy a diverse range of environments, from isolation to cluster centers. We find four super spiral galaxy systems that are late-stage major mergers--a possible clue to their formation. We suggest that super spirals are a remnant population of unquenched, massive disk galaxies. They may eventually become massive lenticular galaxies after they are cut off from their gas supply and their disks fade.
We check the performance of the {\sl\,PARSEC} tracks in reproducing the blue loops of intermediate age and young stellar populations at very low metallicity. We compute new evolutionary {\sl\,PARSEC} tracks of intermediate- and high-mass stars from 2\Msun to 350\Msun with enhanced envelope overshooting (EO), EO=2\HP and 4\HP, for very low metallicity, Z=0.0005. The input physics, including the mass-loss rate, has been described in {\sl\,PARSEC}~V1.2 version. By comparing the synthetic color-magnitude diagrams (CMDs) obtained from the different sets of models with envelope overshooting EO=0.7\HP (the standard {\sl\,PARSEC} tracks), 2\HP and 4\HP, with deep observations of the Sagittarius dwarf irregular galaxy (SagDIG), we find an overshooting scale EO=2\HP to best reproduce the observed loops. This result is consistent with that obtained by \citet{Tang_etal14} for Z in the range 0.001-0.004. We also discuss the dependence of the blue loop extension on the adopted instability criterion and find that, contrary to what stated in literature, the Schwarzschild criterion, instead of the Ledoux criterion, favours the development of blue loops. Other factors that could affect the CMD comparisons such as differential internal extinction or the presence of binary systems are found to have negligible effects on the results. We thus confirm that, in presence of core overshooting during the H-burning phase, a large envelope overshooting is needed to reproduce the main features of the central He-burning phase of intermediate- and high-mass stars.
Multiple stellar populations in the Milky Way globular clusters manifest themselves with a large variety. Although chemical abundance variations in light elements, including He, are ubiquitous, the amount of these variations is different in different globulars. Stellar populations with distinct Fe, C+N+O and slow-neutron capture elements have been now detected in some globular clusters, whose number will likely increase. All these chemical features correspond to specific photometric patterns. I review the chemical+photometric features of the multiple stellar populations in globular clusters and discuss how the interpretation of data is being more and more challenging. Very excitingly, the origin and evolution of globular clusters is being a complex puzzle to compose.
Water probes the dynamics in young stellar objects (YSOs) effectively, especially shocks in molecular outflows. It is a key molecule for exploring whether the physical properties of low-mass protostars can be extrapolated to massive YSOs. As part of the WISH key programme, we investigate the dynamics and the excitation conditions of shocks along the outflow cavity wall as function of source luminosity. Velocity-resolved Herschel-HIFI spectra of the H2O 988, 752, 1097 GHz and 12CO J=10-9, 16-15 lines were analysed for 52 YSOs with bolometric luminosities (L_bol) ranging from <1 to >10^5 L_sun. The profiles of the H2O lines are similar, indicating that they probe the same gas. We see two main Gaussian emission components in all YSOs: a broad component associated with non-dissociative shocks in the outflow cavity wall (cavity shocks) and a narrow component associated with quiescent envelope material. More than 60% of the total integrated intensity of the H2O lines (L_H2O) comes from the cavity shock component. The H2O line widths are similar for all YSOs, whereas those of 12CO 10-9 increase slightly with L_bol. The excitation analysis of the cavity shock component, performed with the non-LTE radiative transfer code RADEX, shows stronger 752 GHz emission for high-mass YSOs, likely due to pumping by an infrared radiation field. As previously found for CO, a strong correlation with slope unity is measured between log(L_H2O) and log(L_bol), which can be extrapolated to extragalactic sources. We conclude that the broad component of H2O and high-J CO lines originate in shocks in the outflow cavity walls for all YSOs, whereas lower-J CO transitions mostly trace entrained outflow gas. The higher UV field and turbulent motions in high-mass objects compared to their low-mass counterparts may explain the slightly different kinematical properties of 12CO 10-9 and H2O lines from low- to high-mass YSOs.
The discrepancy between observed virial and baryonic mass in galaxy clusters have lead to the missing mass problem. To resolve this, a new, non-baryonic matter field, known as dark matter has been invoked. However, till date no possible constituents of the dark matter components are known. This has led to various models, by modifying gravity at large distances to explain the missing mass problem. The modification to gravity appears very naturally when effective field theory on a lower dimensional manifold, embedded in a higher dimensional spacetime is considered. It has been shown that in a scenario with two lower dimensional manifolds separated by a finite distance is capable to address the missing mass problem, which in turn determines the kinematics of the brane separation.
Links to: arXiv, form interface, find, astro-ph, recent, 1511, contact, help (Access key information)
The leakage of Lyman continuum photons from star forming galaxies is an elusive parameter. When observed, it provides a wealth of information on star formation in galaxies and the geometry of the interstellar medium, and puts constraints on the role of star forming galaxies in the reionization of the universe. H-alpha-selected galaxies at z~2 trace the highest star formation population at the peak of cosmic star formation history, providing a base for directly measuring Lyman continuum escape. Here we present this method, and highlight its benefits as well as caveats. We also use the method on 10 H-alpha emitters in the Chandra Deep Field South at z=2.2, also imaged with the Hubble Space Telescope in the ultraviolet. We find no individual Lyman continuum detections, and our stack puts a 5 sigma upper limit on the average absolute escape fraction of <24%, consistent with similar studies. With future planned observations, the sample sizes would rapidly increase and the method presented here would provide very robust constraints on the escape fraction.
We present the first study of the spatial distribution of star formation in z~0.5 cluster galaxies. The analysis is based on data taken with the Wide Field Camera 3 as part of the Grism Lens-Amplified Survey from Space (GLASS). We illustrate the methodology by focusing on two clusters (MACS0717.5+3745 and MACS1423.8+2404) with different morphologies (one relaxed and one merging) and use foreground and background galaxies as field control sample. The cluster+field sample consists of 42 galaxies with stellar masses in the range 10^8-10^11 M_sun, and star formation rates in the range 1-20 M_sun/yr. Both in clusters and in the field, H{\alpha} is more extended than the rest-frame UV continuum in 60% of the cases, consistent with diffuse star formation and inside out growth. In ~20% of the cases, the H{\alpha} emission appears more extended in cluster galaxies than in the field, pointing perhaps to ionized gas being stripped and/or star formation being enhanced at large radii. The peak of the H{\alpha} emission and that of the continuum are offset by less than 1 kpc. We investigate trends with the hot gas density as traced by the X-ray emission, and with the surface mass density as inferred from gravitational lens models and find no conclusive results. The diversity of morphologies and sizes observed in H_alpha illustrates the complexity of the environmental process that regulate star formation. Upcoming analysis of the full GLASS dataset will increase our sample size by almost an order of magnitude, verifying and strengthening the inference from this initial dataset.
We analyze the mass, temperature, metal enrichment, and OVI abundance of the circumgalactic medium (CGM) around $z\sim 0.2$ galaxies of mass $10^9 M_\odot <M_\bigstar < 10^{11.5} M_\odot$ in the Illustris simulation. Among star-forming galaxies, the mass, temperature, and metallicity of the CGM increase with stellar mass, driving an increase in the OVI column density profile of $\sim 0.5$ dex with each $0.5$ dex increase in stellar mass. Observed OVI column density profiles exhibit a weaker mass dependence than predicted: the simulated OVI abundance profiles are consistent with those observed for star-forming galaxies of mass $M_\bigstar = 10^{10.5-11.5} M_\odot$, but underpredict the observed OVI abundances by $\gtrsim 0.8$ dex for lower-mass galaxies. We suggest that this discrepancy may be alleviated with additional heating of the abundant cool gas in low-mass halos, or with increased numerical resolution capturing turbulent/conductive mixing layers between CGM phases. Quenched galaxies of mass $M_\bigstar = 10^{10.5-11.5} M_\odot$ are found to have 0.3-0.8 dex lower OVI column density profiles than star-forming galaxies of the same mass, in qualitative agreement with the observed OVI abundance bimodality. This offset is driven by AGN feedback, which quenches galaxies by heating the CGM and ejecting significant amounts of gas from the halo. Finally, we find that the inclusion of the central galaxy's radiation field may enhance the photoionization of the CGM within $\sim 50$ kpc, further increasing the predicted OVI abundance around star-forming galaxies.
Collisional debris around interacting and post-interacting galaxies often display condensations of gas and young stars that can potentially form gravitationally bound objects: Tidal Dwarf Galaxies (TDGs). We summarise recent results on TDGs, which are originally published in Lelli et al. (2015, A&A). We study a sample of six TDGs around three different interacting systems, using high-resolution HI observations from the Very Large Array. We find that the HI emission associated to TDGs can be described by rotating disc models. These discs, however, would have undergone less than one orbit since the time of the TDG formation, raising the question of whether they are in dynamical equilibrium. Assuming that TDGs are in dynamical equilibrium, we find that the ratio of dynamical mass to baryonic mass is consistent with one, implying that TDGs are devoid of dark matter. This is in line with the results of numerical simulations where tidal forces effectively segregate dark matter in the halo from baryonic matter in the disc, which ends up forming tidal tails and TDGs.
We investigate the origin of extragalactic continuum emission and its relation to the stellar population of a recently discovered peculiar z=3.344 Lyman alpha emitter. Based on an analysis of the broad-band colors and morphology we find further support for the idea that the underlying galaxy is being fed by a large-scale (L > 35 kpc) accretion stream. Archival HST images show small scale (~5 kpc) tentacular filaments converging near a hot-spot of star-formation, possibly fueled by gas falling in along the filaments. The spectral energy distribution of the tentacles is broadly compatible with either (1) non-ionizing rest-frame far-UV continuum emission from stars formed in an 60 million-year-old starburst; (2) nebular 2-photon-continuum radiation, arising from collisional excitation cooling, or (3) a recombination spectrum emitted by hydrogen fluorescing in response to ionizing radiation escaping from the galaxy. The latter possibility simultaneously accounts for the presence of asymmetric Lyman alpha emission from the large-scale gaseous filament and the nebular continuum in the smaller-scale tentacles as caused by the escape of ionizing radiation from the galaxy. Possible astrophysical explanations for the nature of the tentacles include: a galactic wind powered by the starburst; in-falling gas during cold accretion, or tails of interstellar medium dragged out of the galaxy by satellite halos that have plunged through the main halo. The possibility of detecting extragalactic 2-photon continuum emission in space-based, broad-band images suggests a tool for studying the gaseous environment of high redshift galaxies at much greater spatial detail than possible with Lyman alpha or other resonance line emission.
The photo-dissociation of H$_2$ by a nearby anisotropic source of radiation is seen as a critical component in creating an environment in which a direct collapse black hole may form. Employing radiative transfer we model the effect of multi-frequency (0.76 eV - 60 eV) radiation on a collapsing halo at high redshift. We vary both the shape of the spectrum which emits the radiation and the distance to the emitting galaxy. We use blackbody spectra with temperatures of $\rm{T = 10^4\ K}$ and $\rm{T = 10^5\ K}$ and a realistic stellar spectrum. We find that an optimal zone exists between 1 kpc and 4 kpc from the emitting galaxy. If the halo resides too close to the emitting galaxy the photo-ionising radiation creates a large HII region which effectively disrupts the collapsing halo, too far from the source and the radiation flux drops below the level of the expected background and the H$_2$ fraction remains too high. When the emitting galaxy is initially placed between 1 kpc and 2 kpc from the collapsing halo, with a spectral shape consistent with a star-forming high redshift galaxy, then a large central core forms. The mass of the central core is between 5000 and 10000 $\rm{M_{\odot}}$ at a temperature of approximately 700 K. This core is however surrounded by a reservoir of hotter gas at approximately 8000 K which leads to mass inflow rates of the order of $\sim 0.1$ $\rm{M_{\odot}}$ yr$^{-1}$. This environment has the potential to form a massive primordial star which can then lead to the formation of a direct collapse black hole.
Based on a combined analysis of SDSS imaging and CALIFA integral field spectroscopy data, we report on the detection of faint (24 < {\mu}$_r$ mag/arcsec$^2$ < 26) star-forming spiral-arm-like features in the periphery of three nearby early-type galaxies (ETGs). These features are of considerable interest because they document the still ongoing inside-out growth of some local ETGs and may add valuable observational insight into the origin and evolution of spiral structure in triaxial stellar systems. A characteristic property of the nebular component in the studied ETGs, classified i+, is a two-radial-zone structure, with the inner zone that displays faint (EW(H\alpha)$\simeq$1{\AA}) low-ionization nuclear emission-line region (LINER) properties, and the outer one (3{\AA}<EW(H\alpha)<~20{\AA}) HII-region characteristics. This spatial segregation of nebular emission in two physically distinct concentric zones calls for an examination of aperture effects in studies of type i+ ETGs with single-fiber spectroscopic data.
We assess the effectiveness of the Jeans-Anisotropic-MGE (JAM) technique with a state-of-the-art cosmological hydrodynamic simulation, the Illustris project. We perform JAM modelling on 1413 simulated galaxies with stellar mass M^* > 10^{10}M_{sun}, and construct an axisymmetric dynamical model for each galaxy. Combined with a Markov Chain Monte Carlo (MCMC) simulation, we recover the projected root-mean-square velocity (V_rms) field of the stellar component, and investigate constraints on the stellar mass-to-light ratio, M^*/L, and the fraction of dark matter f_{DM} within 2.5 effective radii (R_e). We find that the enclosed total mass within 2.5 R_e is well constrained to within 10%. However, there is a degeneracy between the dark matter and stellar components with correspondingly larger individual errors. The 1 sigma scatter in the recovered M^*/L is 30-40% of the true value. The accuracy of the recovery of M^*/L depends on the triaxial shape of a galaxy. There is no significant bias for oblate galaxies, while for prolate galaxies the JAM-recovered stellar mass is on average 18% higher than the input values. We also find that higher image resolutions alleviate the dark matter and stellar mass degeneracy and yield systematically better parameter recovery.
It is widely accepted that stars do not form in isolation but result from the fragmentation of molecular clouds, which in turn leads to star cluster formation. Over time, clusters dissolve or are destroyed by interactions with molecular clouds or tidal stripping, and their members become part of the general field population. Star clusters are thus among the basic building blocks of galaxies. In turn, star cluster populations, from young associations and open clusters to old globulars, are powerful tracers of the formation, assembly, and evolutionary history of their parent galaxies. Although their importance had been recognised for decades, major progress in this area has only become possible in recent years, both for Galactic and extragalactic cluster populations. Star clusters are the observational foundation for stellar astrophysics and evolution, provide essential tracers of galactic structure, and are unique stellar dynamical environments. Star formation, stellar structure, stellar evolution, and stellar nucleosynthesis continue to benefit and improve tremendously from the study of these systems. Additionally, fundamental quantities such as the initial mass function can be successfully derived from modelling either the H-R diagrams or the integrated velocity structures of, respectively, resolved and unresolved clusters and cluster populations. Star cluster studies thus span the fields of Galactic and extragalactic astrophysics, while heavily affecting our detailed understanding of the process of star formation in dense environments.This report highlights science results of the last decade in the major fields covered by IAU Commission 37: Star clusters and associations.
Ground-based observations at terahertz (THz) frequencies are a newly explorable area of astronomy for the next ten years. We discuss science cases for a first-generation 10-m class THz telescope, focusing on the Greenland Telescope as an example of such a facility. We propose science cases and provide quantitative estimates for each case. The largest advantage of ground-based THz telescopes is their higher angular resolution (~ 4 arcsec for a 10-m dish), as compared to space or airborne THz telescopes. Thus, high-resolution mapping is an important scientific argument. In particular, we can isolate zones of interest for Galactic and extragalactic star-forming regions. The THz windows are suitable for observations of high-excitation CO lines and [N II] 205 um lines, which are scientifically relevant tracers of star formation and stellar feedback. Those lines are the brightest lines in the THz windows, so that they are suitable for the initiation of ground-based THz observations. THz polarization of star-forming regions can also be explored since it traces the dust population contributing to the THz spectral peak. For survey-type observations, we focus on ``sub-THz'' extragalactic surveys, whose uniqueness is to detect galaxies at redshifts z ~ 1--2, where the dust emission per comoving volume is the largest in the history of the Universe. Finally we explore possibilities of flexible time scheduling, which enables us to monitor active galactic nuclei, and to target gamma-ray burst afterglows. For these objects, THz and submillimeter wavelength ranges have not yet been explored.
We employ CaII K and NaI D interstellar absorption-line spectroscopy of
early-type stars in the Large and Small Magellanic Clouds to investigate the
large- and small-scale structure in foreground Intermediate and High Velocity
Clouds (I/HVCs). These data include FLAMES-GIRAFFE CaII K observations of 403
stars in four open clusters, plus FEROS or UVES spectra of 156 stars in the LMC
and SMC. The FLAMES observations are amongst the most extensive probes to date
of CaII structures on 20 arcsec scales
From the FLAMES data within a 0.5 degree field-of-view, the CaII K equivalent
width in the I/HVC components towards three clusters varies by factors of >10.
There are no detections of molecular gas in absorption at intermediate or high
velocities, although molecular absorption is present at LMC and Galactic
velocities towards some sightlines. The sightlines show variations in EW
exceeding a factor 7 in CH+ towards NGC 1761 over scales of less than 10
arcminutes.
The FEROS/UVES data show CaII K I/HVC absorption in $\sim$60 per cent of
sightlines. No NaI D is found at non-Magellanic HVC velocities aside from a
tentative detection towards the star LHA 120-S 93. The range in the CaII/NaI
ratio in I/HVCs is from -0.45 to +1.5 dex, similar to previous measurements for
I/HVCs.
In ten sightlines we find CaII/OI ratios in I/HVC gas ranging from 0.2 to 1.5
dex below the solar value, indicating either dust or ionisation effects. In
nine sightlines I/HVC gas is detected in both HI and CaII, and shows similar
CaII/HI ratios to typical I/HVCs, and similar velocities, implying that in
these sightlines the two elements form part of the same structure.
Extinction remains one of the most reliable methods of measuring column density of nearby Galactic interstellar clouds. The current and ongoing near-infrared surveys enable the mapping of extinction over large sky areas. We produce allsky extinction maps using the 2MASS near-infrared survey. We use the NICER and NICEST methods to convert the near-infrared colour excesses to extinction estimates. The results are presented in Healpix format at the resolutions of 3.0, 4.5, and 12.0 arcmin. The main results of this study are the calculated J-band extinction maps. The comparison with earlier large-scale extinction mappings shows good correspondence but also demonstrates the presence of resolution-dependent bias. A large fraction of the bias can be corrected by using the NICEST method. For individual regions, best extinction estimates are obtained by careful analysis of the local stellar population and the use of the highest resolution afforded by the stellar density. However, the uniform allsky maps should still be useful for many global studies and as the first step into the investigation of individual clouds.
Several methods exist to convert near-infrared (NIR) stellar observations into extinction maps. We present a new method based on NIR multiband observations. The method uses a discretised version of the distribution of intrinsic stellar colours. A number of variations of the basic method are tested, and the results are compared to NICER calculations. When photometric errors are large, the results are close to those of NICER method but some advantages can be seen when the distribution of intrinsic colours cannot be described well with a single covariance matrix. A priori information about relative column density variations at sub-beam scales can result in a significant increase in accuracy. The results may be further improved by considering the magnitude dependence of the intrinsic colours. Thus, the new methods are useful mostly when photometric errors are small, the distribution of intrinsic colours is well known, or one has prior knowledge of the small-scale structures.
We present near-infrared (NIR) $H+K$-band longslit spectra of eleven galaxies
which are obtained with SOFI at the NTT (ESO). The galaxies are chosen from the
low-luminosity type-1 quasi-stellar object (LLQSO) sample which comprises the
99 closest ($z\leq 0.06$) QSOs from the Hamburg/ESO survey for bright UV-excess
QSOs. These objects are ideal targets to study the gap between local Seyfert
galaxies and high-redshift quasars, since they show much stronger AGN activity
compared to local objects but are still close enough for a detailed structural
analysis.
We fit hydrogen recombination, molecular hydrogen, and [FeII] lines after
carefully subtracting the continuum emission. From the broad Pa$\alpha$
components, we estimate black hole masses and enlarge the sample of LLQSOs that
show a deviation from the $M_\mathrm{BH}-L_\mathrm{bulge}$ relations of
inactive galaxies from 12 to 16 objects.
All objects show emission from hot dust ($T\sim 1200\,\mathrm{K}$) as well as
stellar contribution. However, the particular fractions vary a lot between the
objects. More than half of the objects show H$_2$ emission lines that are
indicating a large reservoir of molecular gas which is needed to feed the AGN
and star formation.
In the NIR diagnostic diagram all objects lie in the location of AGN
dominated objects. However, most of the objects show indications of star
formation activity, suggesting that their offset location with respect to
$M_\mathrm{BH}-L_\mathrm{bulge}$ relations of inactive galaxies may be a
consequence of overluminous bulges.
We conduct a survey of low surface brightness (LSB) satellite galaxies around the Local Volume massive spirals using long exposures with small amateur telescopes. We identified 27 low and very low surface brightness objects around the galaxies NGC,672, 891, 1156, 2683, 3344, 4258, 4618, 4631, and 5457 situated within 10 Mpc from us, and found nothing new around NGC,2903, 3239, 4214, and 5585. Assuming that the dwarf candidates are the satellites of the neighboring luminous galaxies, their absolute magnitudes are in the range of -8.6 > M_B > -13.3, their effective diameters are 0.4-4.7 kpc, and the average surface brightness is 26.1 mag/sq arcsec. The mean linear projected separation of the satellite candidates from the host galaxies is 73 kpc. Our spectroscopic observations of two LSB dwarfs with the Russian 6-meter telescope confirm their physical connection to the host galaxies NGC,891 and NGC,2683.
We derive the CO luminosity function (LF) for different rotational transitions (i.e. (1-0), (3-2), (5-4)) starting from the Herschel LF by Gruppioni et al. and using appropriate $L'_{\rm CO} - L_{\rm IR}$ conversions for different galaxy classes. Our predicted LFs fit the data so far available at $z\approx0$ and $2$. We compare our results with those obtained by semi-analytical models (SAMs): while we find a good agreement over the whole range of luminosities at $z\approx0$, at $z\approx1$ and $z\approx2$ the tension between our LFs and SAMs in the faint and bright ends increases. We finally discuss the contribution of luminous AGN ($L_{X}>10^{44}\,\rm{erg\,s^{-1}}$) to the bright end of the CO LF concluding that they are too rare to reproduce the actual CO luminosity function at $z\approx2$.
Galaxy morphologies and star-formation rates depend on environment. Galaxies in under-dense regions are generally star-forming and disky whereas galaxies in overdense regions tend to be early-type and not actively forming stars. The mechanism(s) responsible for star-formation quenching and morphological transformation remain unclear, although many processes have been proposed. We study the dependence of star-formation and morphology on X-ray luminosity for galaxies in Sloan Digital Sky Survey Data Release 7 (SDSS-DR7) groups and clusters. While controlling for stellar and halo mass dependencies, we find that galaxies in X-ray strong groups and clusters have preferentially low star-forming and disk fractions -- with the differences being strongest at low stellar masses. The trends that we observe do not change when considering only galaxies found within or outside of the X-ray radius of the host group. When considering central and satellite galaxies separately we find that this dependence on X-ray luminosity is only present for satellites, and we show that our results are consistent with "galaxy stangulation" as a mechanism for quenching these satellites. We investigate the dynamics of the groups and clusters in the sample, and find that the velocity distributions of galaxies beyond the virial radius in low X-ray luminosity halos tend to be less Gaussian in nature than the rest of the data set. This may be indicative of low X-ray luminosity groups and clusters having enhanced populations of star-forming and disk galaxies as a result of recent accretion.
We present simulations of star forming filaments incorporating - to our knowledge - the largest chemical network used to date on-the-fly in a 3D-MHD simulation. The network contains 37 chemical species and about 300 selected reaction rates. For this we use the newly developed package KROME (Grassi et al. 2014). Our results demonstrate the feasibility of using such a complex chemical network in 3D-MHD simulations on modern supercomputers. We perform simulations with different strengths of the interstellar radiation field and the cosmic ray ionisation rate and find chemical and physical results in accordance with observations and other recent numerical work.
We investigate the dynamical evolution of 40 open clusters (OCs) by means of their astrophysical parameters derived from field-decontaminated 2MASS photometry. We find a bifurcation in the planes core radius vs. age and cluster radius vs. age, in which part of the clusters appear to expand with time probably due to the presence of stellar black holes while others seem to shrink due to dynamical relaxation. Mass functions (MFs) are built for 3$/$4 of the sample (31 OCs), which are used to search for indications of mass segregation and external dynamical processes by means of relations among astrophysical, structural and evolutionary parameters. We detect a flattening of MF slopes ocurring at the evolutionary parameters $\tau_{core}\leq 32$ and $\tau_{overall}\leq 30$, respectively. Within the uncertainties involved, the overall MF slopes of 14 out of 31 OCs with $m_{overall} > 500~M_{\odot}$ are consistent with Kroupa's initial mass function, implying little or no dynamical evolution for these clusters. The remaining 17 OCs with MF slopes departing from that of Kroupa show mild/large scale mass segregation due to dynamical evolution.
We have obtained high-spatial-resolution spectrophotometric data on several nearby spiral galaxies with the Southern African Large Telescope (SALT) Fabry-P\'erot interferometer on the Robert Stobie Spectrograph (RSS) as a part of the RSS Imaging spectroscopy Nearby Galaxy Survey (RINGS). We have successfully reduced two tracks of Fabry-P\'erot data for the galaxy NGC 2280 to produce a velocity field of the H-alpha line of excited hydrogen. We have modeled these data with the DiskFit modeling software and found these models to be in excellent agreement both with previous measurements in the literature and with our lower-resolution HI velocity field of the same galaxy. Despite this good agreement, small regions exist where the difference between the H-alpha and HI velocities is larger than would be expected from typical dispersions. We investigate these regions of high velocity difference and offer possible explanations for their existence.
The "gravitational million-body problem," to model the dynamical evolution of a self-gravitating, collisional N-body system with N ~10^6 over many relaxation times, remains a major challenge in computational astrophysics. Unfortunately, current techniques to model such a system suffer from severe limitations. A direct N-body simulation with more than 10^5 particles can require months or even years to complete, while an orbit-sampling Monte Carlo approach cannot adequately treat the details of the core dynamics, particularly in the presence of many black holes. We have developed a new technique combining the precision of direct N-body codes with the speed of a Monte Carlo approach. Our Rapid And Precisely Integrated Dynamics code, the RAPID code, statistically models interactions between neighboring stars and stellar binaries while integrating directly the orbits of stars in the cluster core. This allows us to accurately simulate the dynamics of the black holes in a realistic globular cluster environment without the burdensome N^2 scaling of a full N-body integration. We compare models of idealized globular clusters created by the RAPID approach to direct N-body and Monte Carlo models. Our tests show that RAPID can reproduce the half-mass and core radii of the direct N-body models far more accurately than the Monte Carlo approach and in ~1/200th of the computing time. With this technique, it will be possible to create realistic models of Milky Way globular clusters with sufficient rapidity to explore the full parameter space of dense stellar clusters.
The complex interplay of processes at the Galactic Center is at the heart of numerous past, present, and (likely) future mysteries. We aim at a more complete understanding of how spectra extending to >10 TeV result. We first construct a simplified model to account for the peculiar energy and angular dependence of the intense central parsec photon field. This allows for calculating anisotropic inverse Compton scattering and mapping gamma-ray extinction due to gamma gamma -> e^+ e^- attenuation. Coupling these with a method for evolving electron spectra, we examine several clear and present excesses, including the diffuse hard X-rays seen by NuSTAR and GeV gamma rays by Fermi. We address further applications to cosmic rays, dark matter, neutrinos, and gamma rays from the Center and beyond.
A large number of intermediate-age (~1-2-Gyr old) globular clusters (GCs) in the Large and the Small Magellanic Cloud (MC) exhibit either bimodal or extended main-sequence (MS) turn off and dual red clump. Moreover, recent papers have shown that the MS of the young clusters NGC1844 and NGC1856 is either broadened or split. These features of the color-magnitude diagram (CMD) are not consistent with a single isochrone and suggest that star clusters in MCs have experienced a prolonged star formation, in close analogy with Milky-Way GCs with multiple stellar populations. As an alternative, stellar rotation or interacting binaries can be responsible of the CMD morphology. In the following I will summarize the observational scenario and provide constraints on the nature of the complex CMD of young and intermediate-age MC clusters from our ongoing photometric survey with the Hubble Space Telescope.
Rotation is a directly-observable stellar property, and drives magnetic field generation and activity through a magnetic dynamo. Main sequence stars with masses below approximately 0.35Msun (mid-to-late M dwarfs) are fully-convective, and are expected to have a different type of dynamo mechanism than solar-type stars. Measurements of their rotation rates provide insights into these mechanisms, but few rotation periods are available for these stars at field ages. Using photometry from the MEarth transit survey, we measure rotation periods for 391 nearby, mid-to-late M dwarfs in the Northern hemisphere, finding periods from 0.1 to 150 days. The typical detected rotator has stable, sinusoidal photometric modulations at a semi-amplitude of 0.5 to 1%. We find no period-amplitude relation for stars below 0.25Msun and an anti-correlation between period and amplitude for higher-mass M dwarfs. We highlight the existence of older, slowly-rotating stars without H{\alpha} emission that nevertheless have strong photometric variability. The Galactic kinematics of our sample is consistent with the local population of G and K dwarfs, and rotators have metallicities characteristic of the Solar Neighborhood. We use the W space velocities and established age-velocity relations to estimate that stars with P<10 days are on average <2 Gyrs, and that those with P>70 days are about 5 Gyrs. The period distribution is mass dependent: as the mass decreases, the slowest rotators at a given mass have longer periods, and the fastest rotators have shorter periods. We find a lack of stars with intermediate rotation periods. [Abridged]
The presence of megaparsec-scale radio halos in galaxy clusters has already been established by many observations over the last two decades. The emerging explanation for the formation of these giant sources of diffuse synchrotron radio emission is that they trace turbulent regions in the intracluster medium, where particles are trapped and accelerated during cluster mergers. Our current observational knowledge is, however, mainly limited to massive systems. Here we present observations of a sample of 14 mass-selected galaxy clusters, i.e. $M_{\rm 500} > 4\times10^{14}$~M${_\odot}$, in the Southern Hemisphere, aimed to study the occurrence of radio halos in low mass clusters and test the correlation between the radio halo power at 1.4 GHz $P_{\rm 1.4}$ and the cluster mass $M_{\rm 500}$. Our observations were performed with the 7-element Karoo Array Telescope at 1.86 GHz. We found three candidates to host diffuse cluster-scale emission and derived upper limits at the level of $0.6 - 1.9 \times 10^{24}$~Watt~Hz$^{-1}$ for $\sim 50\%$ of the clusters in the sample, significantly increasing the number of clusters with radio halo information in the considered mass range. Our results confirm that bright radio halos in less massive galaxy clusters are statistically rare.
Links to: arXiv, form interface, find, astro-ph, recent, 1511, contact, help (Access key information)
We investigate the Eddington ratio distribution of X-ray selected broad-line active galactic nuclei (AGN) in the redshift range 1.0<z<2.2, where the number density of AGNs peaks. Combining the optical and Subaru/FMOS near-infrared spectroscopy, we estimate black hole masses for broad-line AGNs in the Chandra Deep Field-South (CDF-S), Extended Chandra Deep Field-South (E-CDF-S), and the XMM-Newton Lockman Hole (XMM-LH) surveys. AGNs with similar black hole masses show a broad range of AGN bolometric luminosities, which are calculated from X-ray luminosities, indicating that the accretion rate of black holes is widely distributed. We find that a substantial fraction of massive black holes accreting significantly below the Eddington limit at z~2, in contrast to what is generally found for luminous AGNs at high redshift. Our analysis of observational selection biases indicates that the "AGN cosmic downsizing" phenomenon can be simply explained by the strong evolution of the co-moving number density at the bright end of the AGN luminosity function, together with the corresponding selection effects. However, it might need to consider a correlation between the AGN luminosity and the accretion rate of black holes that luminous AGNs have higher Eddington ratios than low-luminosity AGNs in order to understand the relatively small fraction of low-luminosity AGNs with high accretion rates in this epoch. Therefore, the observed downsizing trend could be interpreted as massive black holes with low accretion rates, which are relatively fainter than less massive black holes with efficient accretion.
We study the distribution and evolution of highly ionised intergalactic metals in the Evolution and Assembly of Galaxies and their Environment (EAGLE) cosmological, hydrodynamical simulations. EAGLE has been shown to reproduce a wide range of galaxy properties while its subgrid feedback was calibrated without considering gas properties. We compare the predictions for the column density distribution functions (CDDFs) and cosmic densities of SiIV, CIV, NV, OVI and NeVIII absorbers with observations at redshift z = 0 to ~ 6 and find reasonable agreement, although there are some differences. We show that the typical physical densities of the absorbing gas increase with column density and redshift, but decrease with the ionization energy of the absorbing ion. The typical metallicity increases with both column density and time. The fraction of collisionally ionized metal absorbers increases with time and ionization energy. While our results show little sensitivity to the presence or absence of AGN feedback, increasing/decreasing the efficiency of stellar feedback by a factor of two substantially decreases/increases the CDDFs and the cosmic densities of the metal ions. We show that the impact of the efficiency of stellar feedback on the CDDFs and cosmic densities is largely due to its effect on the metal production rate. However, the temperatures of the metal absorbers, particularly those of strong OVI, are directly sensitive to the strength of the feedback.
We perform a systematic Bayesian analysis of rotation vs. dispersion support ($v_{\rm rot} / \sigma$) in $40$ dwarf galaxies throughout the Local Volume (LV) over a stellar mass range $10^{3.5} M_{\rm \odot} < M_{\star} < 10^8 M_{\rm \odot}$. We find that the stars in $\sim 90\%$ of the LV dwarf galaxies studied -- both satellites and isolated systems -- are dispersion-supported. In particular, we show that $7/10$ *isolated* dwarfs in our sample have stellar populations with $v_{\rm rot} / \sigma < 0.6$. All have $v_{\rm rot} / \sigma \lesssim 2$. These results challenge the traditional view that the stars in gas-rich dwarf irregulars (dIrrs) are distributed in cold, rotationally-supported stellar disks, while gas-poor dwarf spheroidals (dSphs) are kinematically distinct in having dispersion-supported stars. We see no clear trend between $v_{\rm rot} / \sigma$ and distance to the closest $\rm L_{\star}$ galaxy, nor between $v_{\rm rot} / \sigma$ and $M_{\star}$ within our mass range. We apply the same Bayesian analysis to four FIRE hydrodynamic zoom-in simulations of isolated dwarf galaxies ($10^9 M_{\odot} < M_{\rm vir} < 10^{10} M_{\rm \odot}$) and show that the simulated *isolated* dIrr galaxies have stellar ellipticities and stellar $v_{\rm rot} / \sigma$ ratios that are consistent with the observed population of dIrrs *and* dSphs without the need to subject these dwarfs to any external perturbations or tidal forces. We posit that most dwarf galaxies form as puffy, dispersion-supported systems, rather than cold, angular momentum-supported disks. If this is the case, then transforming a dIrr into a dSph may require little more than removing its gas.
We measure the projected density profile, shape and alignment of the stellar and dark matter mass distribution in 11 strong-lens galaxies. We find that the projected dark matter density profile - under the assumption of a Chabrier stellar initial mass function - shows significant variation from galaxy to galaxy. Those with an outermost image beyond $\sim 10$ kpc are very well fit by a projected NFW profile; those with images within 10 kpc appear to be more concentrated than NFW, as expected if their dark haloes contract due to baryonic cooling. We find that over several half-light radii, the dark matter haloes of these lenses are rounder than their stellar mass distributions. While the haloes are never more elliptical than $e_{dm} = 0.2$, their stars can extend to $e_* > 0.2$. Galaxies with high dark matter ellipticity and weak external shear show strong alignment between light and dark; those with strong shear ($\gamma \gtrsim 0.1$) can be highly misaligned. This is reassuring since isolated misaligned galaxies are expected to be unstable. Our results provide a new constraint on galaxy formation models. For a given cosmology, these must explain the origin of both very round dark matter haloes and misaligned strong-lens systems.
The Local Group of galaxies offer some of the most discriminating tests of models of cosmic structure formation. For example, observations of the Milky Way (MW) and Andromeda satellite populations appear to be in disagreement with N-body simulations of the "Lambda Cold Dark Matter" ({\Lambda}CDM) model: there are far fewer satellite galaxies than substructures in cold dark matter halos (the "missing satellites" problem); dwarf galaxies seem to avoid the most massive substructures (the "too-big-to-fail" problem); and the brightest satellites appear to orbit their host galaxies on a thin plane (the "planes of satellites" problem). Here we present results from APOSTLE (A Project Of Simulating The Local Environment), a suite of cosmological hydrodynamic simulations of twelve volumes selected to match the kinematics of the Local Group (LG) members. Applying the Eagle code to the LG environment, we find that our simulations match the observed abundance of LG galaxies, including the satellite galaxies of the MW and Andromeda. Due to changes to the structure of halos and the evolution in the LG environment, the simulations reproduce the observed relation between stellar mass and velocity dispersion of individual dwarf spheroidal galaxies without necessitating the formation of cores in their dark matter profiles. Satellite systems form with a range of spatial anisotropies, including one similar to that of the MW, confirming that such a configuration is not unexpected in {\Lambda}CDM. Finally, based on the observed velocity dispersion, size, and stellar mass, we provide new estimates of the maximum circular velocity for the halos of nine MW dwarf spheroidals.
The elegance of inflationary cosmology and cosmological perturbation theory ends with the formation of the first stars and galaxies, the initial sources of light that launched the phenomenologically rich process of cosmic reionization. Here we review the current understanding of early star formation, emphasizing unsolved problems and technical challenges. We begin with the first generation of stars to form after the Big Bang and trace how they influenced subsequent star formation. The onset of chemical enrichment coincided with a sharp increase in the overall physical complexity of star forming systems. Ab-initio computational treatments are just now entering the domain of the predictive and are establishing contact with local observations of the relics of this ancient epoch.
The initial mass function (IMF) is an essential tool with which to study star formation processes. We have initiated the photometric survey of young open clusters in the Galaxy, from which the stellar IMFs are obtained in a homogeneous way. A total of 16 famous young open clusters have preferentially been studied up to now. These clusters have a wide range of surface densities (log sigma = -1 to 3 [stars pc^2] for stars with mass larger than 5M_sun) and cluster masses (M_cl = 165 to 50,000M_sun), and also are distributed in five different spiral arms in the Galaxy. It is possible to test the dependence of star formation processes on the global properties of individual clusters or environmental conditions. We present a preliminary result on the variation of the IMF in this paper.
The presence of elements heavier than helium ("metals") is of fundamental importance for a large number of astrophysical processes occurring in planet, star and galaxy formation; it also affects cosmic structure formation and evolution in several ways. Even a small amount of heavy elements can dramatically alter the chemistry of the gas, opening the path to complex molecules. Metals might enhance the ability of the gas to radiate away its thermal energy, thus favoring the formation of gravitationally bound objects; they can also condensate in a solid phase (dust grains), partly or totally blocking radiation from luminous sources. Finally, they represent useful tracers of energy deposition by stars and probe the physical properties of the environment by absorption or emission lines. Last, but certainly not least, life -- as we know it on Earth -- is tightly related to the presence of at least some of the heavy elements. In this pedagogical review I will concentrate on the connection between early metal enrichment and cosmic reionization. As we will see these two processes are intimately connected and their joint study might turn out to be fundamental in understanding the overall evolution of the Universe during the first billion years after the Big Bang, an epoch corresponding to redshifts z>6.
Star-forming galaxies in the early universe provide us with perhaps the most natural way of explaining the reionization of the universe. Current observational results are sufficiently comprehensive, as to allow us to approximately calculate how the ionizing radiation from galaxies varies as a function of cosmic time. Important uncertainties in modeling reionization by galaxies revolve around the escape fraction and its luminosity and redshift dependence, a possible truncation of the galaxy luminosity function at the faint end, and an evolution in the production efficiency of Lyman-continuum photons with cosmic time. Despite these uncertainties, plausible choices for these parameters naturally predict a cosmic ionizing emissivity at z~6-10 whose evolution and overall normalization is in excellent agreement with that derived from current observational constraints. This strongly suggests that galaxies provide the necessary photons to reionize the universe.
We present continuum and molecular line observations at 230 GHz and 345 GHz from the Sub-millimeter Array (SMA) toward three protostars in the Perseus L1448N region. The data are from the large project "Mass Assembly of Stellar Systems and their Evolution with the SMA" (MASSES). Three dust continuum sources, Source B, Source NW, and Source A, are detected at both frequencies. These sources have corresponding emission peaks in C18O (J=2-1), 13CO (J=2-1), and HCO+ (J=4-3), and have offsets with N2D+ (J=3-2) peaks. High angular resolution data from a complimentary continuum survey with the Karl G. Jansky Very Large Array show that Source B is associated with three 8 mm continuum objects, Source NW with two, and Source A remains single. These results suggest that multiplicity in L1448N exists at different spatial scales from a few thousand AU to < 100 AU. Velocity gradients in each source obtained from two-dimensional fits to the SMA C18O emission are found to be perpendicular to within 20 degrees of the outflow directions as revealed by 12CO (J=2-1). We have observed that Sources B and NW with multiplicity have higher densities than Source A without multiplicity. This suggests that thermal Jeans fragmentation can be relevant in the fragmentation process. However, we have not observed a difference in the ratio between rotational and gravitational energy between sources with and without multiplicity. We also have not observed a trend between non-thermal velocity dispersions and the level of fragmentation. Our study has provided the first direct and comprehensive comparison between multiplicity and core properties in low-mass protostars, although based on small number statistics.
We exploit the first public data release of VIPERS to investigate environmental effects in galaxy evolution between $z\sim0.5$ and $0.9$. The large number of spectroscopic redshifts over an area of about $10\,\mathrm{deg}^2$ provides a galaxy sample with high statistical power. The accurate redshift measurements, with $\sigma_z = 0.00047(1+z_\mathrm{spec})$, allow us to robustly isolate galaxies living in the lowest- and highest-density environments, as defined in terms of spatial 3D density contrast. We estimate the stellar mass function (SMF) of galaxies residing in these two environments, and constrain its high-mass end with unprecedented precision. We find that the galaxy SMF in the densest regions has a different shape than that measured at low densities, with an enhancement of massive galaxies and a hint of a flatter (less negative) slope at $z<0.8$. We normalise each SMF to the comoving volume occupied by the corresponding environment, and relate estimates from different redshift bins. We observe an evolution of the SMF of VIPERS galaxies in high densities, while the low-density one is nearly constant. We compare these results to semi-analytical models and find consistent environmental signatures. We discuss how the halo mass function and fraction of central/satellite galaxies depend on the environments considered, making intrinsic and environmental properties of galaxies physically coupled, and therefore difficult to disentangle. The evolution of our low-density regions is well described by the formalism introduced by Peng et al.~(2010), and is consistent with the idea that galaxies become progressively passive because of internal physical processes. The same formalism could also describe the evolution of the SMF in the high density regions, but only if a significant contribution from dry mergers is considered. [Abridged]
Winds outflowing from Active Galactic Nuclei (AGNs) may carry significant amount of mass and energy out to their host galaxies. In this paper we report the detection of a sub-relativistic outflow observed in the Narrow Line Seyfert 1 Galaxy IRAS17020+4544 as a series of absorption lines corresponding to at least 5 absorption components with an unprecedented wide range of associated column densities and ionization levels and velocities in the range of 23,000-33,000 km/s, detected at X-ray high spectral resolution (E/Delta E ~1000) with the ESA's observatory XMM-Newton. The charge states of the material constituting the wind clearly indicate a range of low to moderate ionization states in the outflowing gas and column densities significantly lower than observed in highly ionized ultra fast outflows. We estimate that at least one of the outflow components may carry sufficient energy to substantially suppress star formation, and heat the gas in the host galaxy. IRAS17020+4544 provides therefore an interesting example of feedback by a moderately luminous AGN hosted in a spiral galaxy, a case barely envisaged in most evolution models, which often predict that feedback processes take place in massive elliptical galaxies hosting luminous quasars in a post merger phase.
We analyze the spatial distribution of dusty young stellar objects (YSOs) identified in the Spitzer Survey of the Orion Molecular clouds, augmenting these data with Chandra X-ray observations to correct for incompleteness in dense clustered regions. We also devise a scheme to correct for spatially varying incompleteness when X-ray data are not available. The local surface densities of the YSOs range from 1 pc$^{-2}$ to over 10,000 pc$^{-2}$, with protostars tending to be in higher density regions. This range of densities is similar to other surveyed molecular clouds with clusters, but broader than clouds without clusters. By identifying clusters and groups as continuous regions with surface densities $\ge10$ pc$^{-2}$, we find that 59% of the YSOs are in the largest cluster, the Orion Nebular Cluster (ONC), while 13% of the YSOs are found in a distributed population. A lower fraction of protostars in the distributed population is evidence that it is somewhat older than the groups and clusters. An examination of the structural properties of the clusters and groups show that the peak surface densities of the clusters increase approximately linearly with the number of members. Furthermore, all clusters with more than 70 members exhibit asymmetric and/or highly elongated structures. The ONC becomes azimuthally symmetric in the inner 0.1 pc, suggesting that the cluster is only $\sim 2$ Myr in age. We find the star formation efficiency (SFE) of the Orion B cloud is unusually low, and that the SFEs of individual groups and clusters are an order of magnitude higher than those of the clouds. Finally, we discuss the relationship between the young low mass stars in the Orion clouds and the Orion OB 1 association, and we determine upper limits to the fraction of disks that may be affected by UV radiation from OB stars or by dynamical interactions in dense, clustered regions.
We carried out observations toward the giant molecular cloud W 37 with the $J = 1 - 0$ transitions of $^{12}$CO, $^{13}$CO, and C$^{18}$O using the 13.7 m single-dish telescope at the Delingha station of Purple Mountain Observatory. Based on the three CO lines, we calculated the column densities, cloud masses for the molecular clouds with radial velocities at around $+20 \mathrm{km s}^{-1}$. The gas mass of W 37, calculated from $^{13}$CO emission, is $1.7\times10^5 M_\odot$, above the criteria of giant molecular cloud. The dense ridge of W 37 is a dense filament, which is supercritical in linear mass ratio. Dense clumps found by C$^{18}$O emission are aligned along the dense ridge with a regular interval about 2.8 pc, similar to the clump separation caused by large-scale `sausage instability'. We confirm the identification of the giant molecular filament (GMF) G 18.0-16.8 by \cite{2014A&A...568A..73R} and find a new giant filament, G16.5-15.8, located in the west 0.8 degree of G 18.0-16.8. Both GMFs are not gravitationally bound, as indicated by their low linear mass ratio ($\sim80 M_\odot \mathrm{pc}^{-1}$). We compared the gas temperature map with the dust temperature map from \emph{Herschel} images, and find similar structures. The spatial distributions of class I objects and the dense clumps is reminiscent of triggered star formation occurring in the northwestern part of W 37, which is close to NGC 6611.
Recently, the presence of fullerenes in the interstellar medium (ISM) has been confirmed, especially with the first confirmed identification of two strong diffuse interstellar bands (DIBs) with C60+. This justifies reassesing the importance of interstellar fullerenes of various sizes with endohedral or exohedral inclusions and heterofullerenes (EEHFs). The phenomenology of fullerenes is complex. In addition to formation in shock shattering, fully dehydrogenated PAHs in diffuse interstellar (IS) clouds could perhaps efficiently transform into fullerenes including EEHFs. But it is extremely difficult to assess their expected abundance, composition and size distribution, except for C60+. As often suggested, EEHFs share many properties with C60, as regards stability, formation/destruction, chemical processes and many basic spectral features. We address the importance of various EEHFs as possible DIB carriers. Specifically, we discuss IS properties and the contributions of fullerenes of various sizes and charge such as C60+, metallofullerenes, heterofullerenes, fulleranes, fullerene-PAH compounds, H2@C60. We conclude that the landscape of IS fullerenes is probably much richer than heretofore realized. As has been suggested previously, EEHFs, together with pure fullerenes of various sizes, have properties necessary to be suitably carriers of DIBs (carbonaceous nature, resilience in the ISM, various heteroatoms and ionization states, relatively easy formation, few isomers, right wavelength range, energy internal conversion, Jahn-Teller fine structure) as supported by the C60+ DIBs. But, the lack of information about optical spectra other than C60 and IS abundances still precludes defined assessment of the importance of fullerenes as DIB carriers. Their compounds could significantly contribute to DIBs, but it still seems difficult that they are the only important DIB carriers.
Integral field spectroscopy studies based on CALIFA data have recently revealed the presence of ongoing low-level star formation (SF) in the periphery of ~10% of local early-type galaxies (ETGs), witnessing a still ongoing inside-out galaxy growth process. A distinctive property of the nebular component in these ETGs, classified i+, is a two-radial-zone structure, with the inner zone displaying LINER emission with a H\alpha equivalent width EW~1{\AA}, and the outer one (3{\AA}<EW<~20{\AA}) showing HII-region characteristics. Using CALIFA IFS data, we empirically demonstrate that the confinement of nebular emission to the galaxy periphery leads to a strong aperture (or, redshift) bias in spectroscopic single-fiber studies of type i+ ETGs: At low redshift (<~0.45), SDSS spectroscopy is restricted to the inner (SF-devoid LINER) zone, thereby leading to their erroneous classification as "retired" galaxies (systems lacking SF and whose faint emission is powered by pAGB stars). Only at higher z's the SDSS aperture can encompass the outer SF zone, permitting their unbiased classification as "composite SF/LINER". We also demonstrate that the principal effect of a decreasing aperture on the classification of i+ ETGs via standard BPT emission-line ratios consists in a monotonic up-right shift precisely along the upper-right wing of the "seagull" distribution. Motivated by these insights, we also investigate theoretically these biases in aperture-limited studies of inside-out growing galaxies as a function of z. To this end, we devise a simple model, which involves an outwardly propagating SF process, that reproduces the radial extent and two-zone EW distribution of i+ ETGs. By simulating on this model the spectroscopic SDSS aperture, we find that SDSS studies at z<~1 are progressively restricted to the inner LINER-zone, and miss an increasingly large portion of the H\alpha-emitting periphery.
Magnetic fields are considered as a vital ingredient of contemporary star formation, and may have been important during the formation of the first stars in the presence of an efficient amplification mechanism. Initial seed fields are provided via plasma fluctuations, and are subsequently amplified by the small-scale dynamo, leading to a strong tangled magnetic field. Here we explore how the magnetic field provided by the small-scale dynamo is further amplified via the $\alpha-\Omega$ dynamo in a protostellar disk and assess its implications. For this purpose, we consider two characteristic cases, a typical Pop.~III star with $10$~M$_\odot$ and an accretion rate of $10^{-3}$~M$_\odot$~yr$^{-1}$, and a supermassive star with $10^5$~M$_\odot$ and an accretion rate of $10^{-1}$~M$_\odot$~yr$^{-1}$. For the $10$~M$_\odot$ Pop.~III star, we find that coherent magnetic fields can be produced on scales of at least $100$~AU, which are sufficient to drive a jet with a luminosity of $100$~L$_\odot$ and a mass outflow rate of $10^{-3.7}$~M$_\odot$~yr$^{-1}$. For the supermassive star, the dynamical timescales in its environment are even shorter, implying smaller orbital timescales and an efficient magnetization out to at least $1000$~AU. The jet luminosity corresponds to $\sim10^{6.0}$~L$_\odot$, and a mass outflow rate of $10^{-2.1}$~M$_\odot$~yr$^{-1}$. We expect that the feedback from the supermassive star can have a relevant impact on its host galaxy.
We present [C/Fe] and [N/Fe] abundance ratios and CH({\lambda}4300) and S({\lambda}3883) index measurements for 94 red giant branch (RGB) stars in the Sculptor dwarf spheroidal galaxy from VLT/VIMOS MOS observations at a resolving power R= 1150 at 4020 {\AA}. This is the first time that [N/Fe] abundances are derived for a large number of stars in a dwarf spheroidal. We found a trend for the [C/Fe] abundance to decrease with increasing luminosity on the RGB across the whole metallicity range, a phenomenon observed in both field and globular cluster giants, which can be interpreted in the framework of evolutionary mixing of partially processed CNO material. Both our measurements of [C/Fe] and [N/Fe] are in good agreement with the theoretical predictions for stars at similar luminosity and metallicity. We detected a dispersion in the carbon abundance at a given [Fe/H], which cannot be ascribed to measurement uncertainties alone. We interpret this observational evidence as the result of the contribution of different nucleosynthesis sources over time to a not well-mixed interstellar medium. We report the discovery of two new carbon-enhanced, metal-poor stars. These are likely the result of pollution from material enriched by asymptotic giant branch stars, as indicated by our estimates of [Ba/Fe]> +1. We also attempted a search for dissolved globular clusters in the field of the galaxy by looking for the distinctive C-N pattern of second population globular clusters stars in a previously detected, very metal-poor, chemodynamical substructure. We do not detect chemical anomalies among this group of stars. However, small number statistics and limited spatial coverage do not allow us to exclude the hypotheses that this substructure forms part of a tidally shredded globular cluster.
Flux magnification is an interesting complement to shear-based lensing measurements, especially at high redshift where sources are harder to resolve. One measures either changes in the source density (magnification bias) or in the shape of the flux distribution (e.g. magnitude-shift). The interpretation of these measurements relies on theoretical estimates of how the observables change under magnification. Here we present simulations to create multi-band photometric mock catalogues of Lyman-break galaxies in a CFHTLenS-like survey that include several observational effects that can change these relations, making simple theoretical estimates unusable. In particular, we show how the magnification bias can be affected by photometric noise, colour selection, and dust extinction. We find that a simple measurement of the slope of the number-counts is not sufficient for the precise interpretation of virtually all observations of magnification bias. We also explore how sensitive the shift in the mean magnitude of a source sample in different photometric bands is to magnification including the same observational effects. Again we find significant deviations from simple analytical estimates. We also discover a wavelength-dependence of the magnitude-shift effect when applied to a colour-selected noisy source sample. Such an effect can mimic the reddening by dust in the lens. It has to be disentangled from the dust extinction before the magnitude-shift/colour-excess can be used to measure the distribution of either dark matter or extragalactic dust. Using simulations like the ones presented here these observational effects can be studied and eventually removed from observations making precise measurements of flux magnification possible.
Owing to the paucity of sub-arcsecond (sub)mm observations required to probe the innermost regions of newly forming protostars, several fundamental questions are still being debated, such as the existence and coevality of close multiple systems. We study the physical and chemical properties of the jets and protostellar sources in the NGC1333-IRAS4A proto-binary system using continuum emission and molecular tracers of shocked gas. We observed NGC1333-IRAS4A in the SiO(6-5), SO(6_5-5_4), and CO(2-1) lines and the continuum emission at 1.3, 1.4, and 3 mm using the IRAM Plateau de Bure Interferometer in the framework of the CALYPSO large program. We clearly disentangle for the first time the outflow emission from the two sources A1 and A2. The two protostellar jets have very different properties: the A1 jet is faster, has a short dynamical timescale (<10^3 yr), and is associated with H2 shocked emission, whereas the A2 jet, which dominates the large-scale emission, is associated with diffuse emission, bends, and emits at slower velocities. The observed bending of the A2 jet is consistent with the change of propagation direction observed at large scale and suggests jet precession on very short timescales (~200-600 yr). In addition, a chemically rich spectrum with emission from several COMs (e.g. HCOOH, CH3OCHO, CH3OCH3) is only detected towards A2. Finally, very high-velocity shocked emission (~50 km s^-1) is observed along the A1 jet. An LTE analysis shows that SiO, SO, and H2CO abundances in the gas phase are enhanced up to (3-4)x10^{-7}, (1.4-1.7)x10^{-6}, and (3-7.9)x10^{-7}, respectively. The intrinsic different properties of the jets and driving sources in NGC1333-IRAS4A suggest different evolutionary stages for the two protostars, with A1 being younger than A2, in a very early stage of star formation previous to the hot-corino phase.
We present dust column densities and dust temperatures for $\sim3000$ young high-mass molecular clumps from the Millimetre Astronomy Legacy Team 90 GHz (MALT90) survey, derived from adjusting single temperature dust emission models to the far-infrared intensity maps measured between 160 and 870 \micron\ from the Herschel/Hi-Gal and APEX/ATLASGAL surveys. We discuss the methodology employed in analyzing the data, calculating physical parameters, and estimating their uncertainties. The population average dust temperature of the clumps are: $16.8\pm0.2$ K for the clumps that do not exhibit mid-infrared signatures of star formation (Quiescent clumps), $18.6\pm0.2$ K for the clumps that display mid-infrared signatures of ongoing star formation but have not yet developed an HII region (Protostellar clumps), and $23.7\pm0.2$ and $28.1\pm0.3$ K for clumps associated with HII and photo-dissociation regions, respectively. These four groups exhibit large overlaps in their temperature distributions, with dispersions ranging between 4 and 6 K. The median of the peak column densities of the Protostellar clump population is $0.20\pm0.02$ gr cm$^{-2}$, which is about 50% higher compared to the median of the peak column densities associated with clumps in the other evolutionary stages. We compare the dust temperatures and column densities measured toward the center of the clumps with the mean values of each clump. We find that in the Quiescent clumps the dust temperature increases toward the outer regions and that they are associated with the shallowest column density profiles. In contrast, molecular clumps in the Protostellar or HII region phase have dust temperature gradients more consistent with internal heating and are associated with steeper column density profiles compared with the Quiescent clumps.
The Galactic Center (GC) has been long known to host gamma-ray emission detected to >10 TeV. HESS data now points to two plausible origins: the supermassive black hole (perhaps with >PeV cosmic rays and neutrinos) or high-energy electrons from the putative X-ray pulsar wind nebula G359.95-0.04. We show that if the magnetic field experienced by PWN electrons is near the several mG ambient field strength suggested by radio observations of the nearby GC magnetar SGR J1745-29, synchrotron losses constrain the TeV gamma-ray output to be far below the data. Accounting for the peculiar geometry of GC infrared emission, we also find that the requisite TeV flux could be reached if the PWN is ~1 pc from Sgr A* and the magnetic field is two orders of magnitude weaker, a scenario that we discuss in relation to recent data and theoretical developments. Otherwise, Sgr A* is left, which would then be a PeV link to other AGN.
Neutral diffuse intergalactic gas that existed during the Epoch of Reionization (EoR) suppresses Lyman Alpha (Lya) flux emitted by background galaxies. In this chapter I summarise the increasing observational support for the claim that Lya photons emitted by galaxies at z>6 are suppressed by intervening HI gas. I describe key physical processes that affect Lya transfer during the EoR. I argue that in spite of the uncertainties associated with this complex multiscale problem, the data on Lya emitting galaxies at z=0-6 strongly suggests that the observed reduction in Lya flux from galaxies at z>6 is due to additional intervening HI gas. The main question is what fraction of this additional HI gas is in the diffuse neutral IGM. I summarise how future surveys on existing and upcoming instruments are expected to reduce existing observational uncertainties enormously. With these improved data we will likely be able to nail down reionization with Lya emitting galaxies.
Links to: arXiv, form interface, find, astro-ph, recent, 1511, contact, help (Access key information)
Low-mass dwarf galaxies are very sensitive test-beds for theories of cosmic structure formation since their weak gravitational fields allow the effects of the relevant physical processes to clearly stand out. Up to now, no unified account exists of the sometimes seemingly conflicting properties of the faintest isolated dwarfs in and around the Local Group, such as Leo T and the recently discovered Leo P and Pisces A systems. Using new numerical simulations, we show that this serious challenge to our understanding of galaxy formation can be effectively resolved by taking into account the regulating influence of the ultraviolet radiation of the first population of stars on a dwarf's star formation rate while otherwise staying within the standard cosmological paradigm for structure formation. These simulations produce faint, gas-dominated, star-forming dwarf galaxies that lie on the baryonic Tully-Fisher relation and that successfully reproduce a broad range of chemical, kinematical, and structural observables of real late-type dwarf galaxies. Furthermore, we stress the importance of obtaining properties of simulated galaxies in a manner as close as possible to the typically employed observational techniques.
We present a comprehensive chemical abundance analysis of five red giants and two horizontal branch (HB) stars towards the southern Galactic bulge, at (l,b)$\sim$(0$^{\rm o}$,-11$^{\rm o}$). Based on high-resolution spectroscopy obtained with the Magellan/MIKE spectrograph, we derived up to 23 chemical element abundances and identify a mixed bag of stars, representing various populations in the central regions of the Galaxy. Although cosmological simulations predict that the inner Galaxy was host to the first stars in the Universe, we see no chemical evidence of the ensuing massive supernova explosions: all of our targets exhibit halo-like, solar [Sc/Fe] ratios, which is in contrast to the low values predicted from Population III nucleosynthesis. One of the targets is a CEMP-s star at [Fe/H]=-2.52 dex, and another one is a moderately metal-poor ([Fe/H]=-1.53 dex) CH star with strong enrichment in s-process elements (e.g., [Ba/Fe]=1.35). These individuals provide the first contenders of these classes of stars towards the bulge. Four of the carbon-normal stars exhibit abundance patterns reminiscent of halo star across a metallicity range spanning -2.0 to -2.6 dex, i.e., enhanced $\alpha$-elements and solar Fe-peak and n-capture elements, and the remaining one is a regular metal-rich bulge giant. The position, distance, and radial velocity of one of the metal-poor HB stars coincides with the old trailing arm of the disrupted Sagittarius dwarf galaxy. While their uncertain proper motions prohibit a clear kinematic separation, the stars' abundances and distances suggest that these metal-poor candidates, albeit located towards the bulge, are not of the bulge, but rather inner halo stars on orbits that make them pass through the central regions. Thus, we caution similar claims of detections of metal-poor stars as true habitants of the bulge. (Abridged)
We present a statistical analysis of the environments of 11 supernovae (SNe) which occurred in 6 nearby galaxies (z $\lesssim$ 0.016). All galaxies were observed with MUSE, the high spatial resolution integral field spectrograph mounted to the 8m VLT UT4. These data enable us to map the full spatial extent of host galaxies up to $\sim$3 effective radii. In this way, not only can one characterise the specific host environment of each SN, one can compare their properties with stellar populations within the full range of other environments within the host. We present a method that consists of selecting all HII regions found within host galaxies from 2D extinction-corrected H$\alpha$ emission maps. These regions are then characterised in terms of their H$\alpha$ equivalent widths, star formation rates, and oxygen abundances. Identifying HII regions spatially coincident with SN explosion sites, we are thus able to determine where within the distributions of host galaxy e.g. metallicities and ages each SN is found, thus providing new constraints on SN progenitor properties. This initial pilot study using MUSE opens the way for a revolution in SN environment studies where we are now able to study multiple environment SN progenitor dependencies using a single instrument and single pointing.
We report on the serendipitous discovery of a star-forming galaxy at redshift z=0.116 with morphological features that indicate an ongoing merger. This object exhibits two clearly separated components with significantly different colors, plus a possible tidal stream. Follow-up spectroscopy of the bluer component revealed a low star-forming activity of 0.09 M$_{\odot}$/year and a high metallicity of 12+log(O/H)=8.6. Based on comparison with mass-star-formation-rate and mass-metallicity relations, and on fitting of spectral energy distributions, we obtain a stellar mass of 3x10$^9$ M$_{\odot}$, which renders this object comparable to the Large Magellanic Cloud (LMC). Thus our finding provides a further piece of evidence of a major merger already acting on small, dwarf galaxy-like scales.
Dynamical mass calculations have suggested that the Milky Way globular cluster NGC 6535 belongs to a population of clusters with high mass-to-light ratios, possibly due to a bottom-heavy stellar initial mass function. We use published Hubble Space Telescope data to measure the present day stellar mass function of this cluster within its half-light radius and instead find that it is bottom-light, exacerbating the discrepancy between the dynamical measurement and its known stellar content. The cluster's proximity to the Milky Way bulge and its relatively strong velocity anisotropy are both reasons to be suspicious of the dynamical mass measurement, but we find that neither straightforwardly explains the sense and magnitude of the discrepancy. Although there are alternative potential explanations for the high mass-to-light ratio, such as the presence of large numbers of stellar remnants or dark matter, we find this cluster to be sufficiently perplexing that we now exclude it from a discussion of possible variations in the initial mass function. Because this was the sole known old, Milky Way cluster in the population of high dynamical mass-to-light ratio clusters, some possible explanations for the difference in cluster properties are again open for consideration.
We use a set of high-resolution N-body simulations of the Galactic disk to study its interactions with the population of satellites predicted cosmologically. One simulation illustrates that multiple passages of massive satellites with different velocities through the disk generate a wobble, having the appearance of rings in face-on projections of the stellar disk. They also produce flares in the disk outer parts and gradually heat the disk through bending waves. A different numerical experiment shows that an individual satellite as massive as the Sagittarius dwarf galaxy passing through the disk will drive coupled horizontal and vertical oscillations of stars in underdense regions, with no significant associated heating. This experiment shows that vertical excursions of stars in these low-density regions can exceed 1 kpc in the Solar neighborhood, resembling the coherent vertical oscillations recently detected locally. They can also induce non-zero vertical streaming motions as large as 10-20 km s$^{-1}$, consistent with recent observations in the Galactic disk. This phenomenon appears as a local ring, with no associated disk heating.
We perform dry merger simulations to investigate the role of dry mergers in the size growth of early-type galaxies in high density environments. We replace the virialized dark matter haloes obtained by a large cosmological $N$-body simulation with $N$-body galaxy models consisting of two components, a stellar bulge and a dark matter halo, which have higher mass resolution than the cosmological simulation. We then re-simulate nine cluster forming regions, whose masses range from 1e+14 Msun to 5e+14 Msun. Masses and sizes of stellar bulges are also assumed to satisfy the stellar mass--size relation of high-z compact massive early-type galaxies. We find that dry major mergers considerably contribute to the mass and size growth of central massive galaxies. One or two dry major mergers double the average stellar mass and quadruple the average size between $z=2$ and $z=0$. These growths favorably agree with observations. Moreover, the density distributions of our simulated central massive galaxies grow from the inside-out, which is consistent with recent observations. The mass--size evolution is approximated as R propto M_{*}^{alpha}, with alpha ~ 2.24. Most of our simulated galaxies are efficiently grown by dry mergers, and their stellar mass--size relations match the ones observed in the local Universe. Our results show that the central galaxies in the cluster haloes are potential descendants of high-z (z ~ 2-3) compact massive early-type galaxies. This conclusion is consistent with previous numerical studies which investigate the formation and evolution of compact massive early-type galaxies.
The optical light curve of the quasar PG 1302-102 at $z = 0.278$ shows a strong, smooth 5.2 yr periodic signal, detectable over a period of $\sim 20$ yr. Although the interpretation of this phenomenon is still uncertain, the most plausible mechanisms involve a binary system of two supermassive black holes with a subparsec separation. At this close separation, the nuclear black holes in PG 1302-102 will likely merge within $\sim 10^{5}$ yr due to gravitational wave emission alone. Here we report the rest-frame near-infrared time lags for PG 1302-102. Compiling data from {\it WISE} and {\it Akari}, we confirm that the periodic behavior reported in the optical light curve from Graham et al. (2015) is reproduced at infrared wavelengths, with best-fit observed-frame 3.4 and $4.6 \mu$m time lags of $(2219 \pm 153, 2408 \pm 148)$ days for a near face-on orientation of the torus, or $(4103\pm 153, 4292 \pm 148)$ days for an inclined system with relativistic Doppler boosting in effect. The periodicity in the infrared light curves and the light-travel time of the accretion disk photons to reach the dust glowing regions support that a source within the accretion disk is responsible for the optical variability of PG 1302-102, echoed at the further out dusty regions. The implied distance of this dusty, assumed toroidal region is $\sim$ 1.5 pc for a near face-on geometry, or $\sim$1.1 pc for the relativistic Doppler boosted case.
We present a study of stellar populations in a sample of spectroscopically-confirmed Lyman-break galaxies (LBGs) and Ly$\alpha$ emitters (LAEs) at $5.7<z<7$. These galaxies have deep optical and infrared images from Subaru, $HST$, and $Spitzer$/IRAC. We focus on a subset of 27 galaxies with IRAC detections, and characterize their stellar populations utilizing galaxy synthesis models based on the multi-band data and secure redshifts. By incorporating nebular emission estimated from the observed Ly$\alpha$ flux, we are able to break the strong degeneracy of model spectra between young galaxies with prominent nebular emission and older galaxies with strong Balmer breaks. The results show that our galaxies cover a wide range of ages from several to a few hundred million years (Myr), and a wide range of stellar masses from $\sim10^8$ to $\sim10^{11}$ $M_{\odot}$. These galaxies can be roughly divided into an `old' subsample and a `young' subsample. The `old' subsample consists of galaxies older than 100 Myr, with stellar masses higher than $10^9$ $M_{\odot}$. The galaxies in the `young' subsample are younger than $\sim$30 Myr, with masses ranging between $\sim10^8$ and $\sim3\times10^9$ $M_{\odot}$. Both subsamples display a correlation between stellar mass and star-formation rate (SFR), but with very different normalizations. The average specific SFR (sSFR) of the `old' subsample is 3--4 Gyr$^{-1}$, consistent with previous studies of `normal' star-forming galaxies at $z\ge6$. The average sSFR of the `young' subsample is an order of magnitude higher, likely due to starburst activity. Our results also indicate little or no dust extinction in the majority of the galaxies, as already suggested by their steep rest-frame UV slopes. Finally, LAEs and LBGs with strong Ly$\alpha$ emission are indistinguishable in terms of age, stellar mass, and SFR.
We present the first attempt at a reverberation mapping (RM) experiment that combines broad and intermediate-band photometry, targeting a sample of 13 quasars at $0.2<z<0.9$. The quasars were selected to have strong H$\alpha$ or H$\beta$ emission lines that are located in one of three intermediate bands (with FWHM around 200 \AA) centered at 8045, 8505, and 9171 \AA. The imaging observations were carried out in the intermediate bands and the broad $i$ and $z$ bands using the prime-focus imager 90Prime on the 2.3m Bok telescope. Because of the large ($\sim$1 deg$^2$) field-of-view (FoV) of 90Prime, we were able to include the 13 quasars within only five telescope pointings or fields. The five fields were repeatedly observed over 20-30 epochs that were unevenly distributed over a duration of 5-6 months. The combination of the broad- and intermediate-band photometry allows us to derive accurate light curves for both optical continuum (from the accretion disk) and line (from the broad-line region, or BLR) emission. We detect time lags between the continuum and line emission in 6 quasars. These quasars are at a relatively low redshift range $0.2<z<0.4$. The measured lags are consistent with the current BLR size-luminosity relation for H$\beta$ at $z<0.3$. We demonstrate that by using a small telescope with a large FoV, intermediate-band photometric RM can be efficiently executed for a large sample of quasars at $z>0.2$.
We consider turbulent synchrotron emitting media that also exhibits Faraday rotation and provide a statistical description of synchrotron polarization fluctuations. In particular, we consider these fluctuations as a function of the spatial separation of the direction of measurements and as a function of wavelength for the same line-of-sight. On the basis of our general analytical approach, we introduce several measures that can be used to obtain the spectral slopes and correlation scales of both the underlying magnetic turbulence responsible for emission and the spectrum of the Faraday rotation fluctuations. We show the synergetic nature of these measures and discuss how the study can be performed using sparsely sampled interferometric data. We also discuss how additional characteristics of turbulence can be obtained, including the turbulence anisotropy, the three dimensional direction of the mean magnetic field. We consider both cases when the synchrotron emission and Faraday rotation regions coincide and when they are spatially separated. Appealing to our earlier study in Lazarian \& Pogosyan (2012) we explain that our new results are applicable to a wide range of spectral indexes of relativistic electrons responsible for synchrotron emission. We expect wide application of our techniques both with existing synchrotron data sets as well as with big forthcoming data sets from LOFAR and SKA.
Using a sample of ~410 000 galaxies to depth I_AB = 24 over 8.26 deg^2 in the Bootes field (~10 times larger than z~1 luminosity function studies in the prior literature), we have accurately measured the evolving B-band luminosity function of red galaxies at z<1.2 and blue galaxies at z<1.0. In addition to the large sample size, we utilise photometry that accounts for the varying angular sizes of galaxies, photometric redshifts verified with spectroscopy, and absolute magnitudes that should have very small random and systematic errors. Our results are consistent with the migration of galaxies from the blue cloud to the red sequence as they cease to form stars, and with downsizing in which more massive and luminous blue galaxies cease star formation earlier than fainter less massive ones. Comparing the observed fading of red galaxies with that to be expected from passive evolution alone, we find that the stellar mass contained within the red galaxy population has increased by a factor of ~3.6 from z~1.1 to z~0.1. The bright end of the red galaxy luminosity function fades with decreasing redshift, the rate of fading increasing from ~0.2 mag per unit redshift at z = 1.0 to ~0.8 at z = 0.2. The overall decrease in luminosity implies that the stellar mass in individual highly luminous red galaxies increased by a factor of ~2.2 from z = 1.1 to z = 0.1.
Recent studies suggest that faint active galactic nuclei may be responsible for the reionization of the universe. Confirmation of this scenario requires spectroscopic identification of faint quasars ($M_{1450}>-24$ mag) at $z \gtrsim6$, but only a very small number of such quasars have been spectroscopically identified so far. Here, we report the discovery of a faint quasar IMS J220417.92+011144.8 at z~6 in a 12.5 deg$^{2}$ region of the SA22 field of the Infrared Medium-deep Survey (IMS). The spectrum of the quasar shows a sharp break at $\sim8443~\rm{\AA}$, with emission lines redshifted to $z=5.944 \pm 0.002$ and rest-frame ultraviolet continuum magnitude $M_{1450}=-23.59\pm0.10$ AB mag. The discovery of IMS J220417.92+011144.8 is consistent with the expected number of quasars at z~6 estimated from quasar luminosity functions based on previous observations of spectroscopically identified low-luminosity quasars . This suggests that the number of $M_{1450}\sim-23$ mag quasars at z~6 may not be high enough to fully account for the reionization of the universe. In addition, our study demonstrates that faint quasars in the early universe can be identified effectively with a moderately wide and deep near-infrared survey such as the IMS.
In this work, we investigate the molecular gas and star formation properties in the barred spiral galaxy NGC 6946 using multiple molecular lines and star formation tracers. High-resolution image (100 pc) of $^{13}$CO (1-0) is created by single dish NRO45 and interferometer CARMA for the inner 2 kpc disk, which includes the central region (nuclear ring and bar) and the offset ridges of the primary bar. Single dish HCN (1-0) observations were also made to constrain the amount of dense gas. Physical properties of molecular gas are inferred by (1) the Large Velocity Gradient (LVG) calculations using our observations and archival $^{12}$CO (1-0), $^{12}$CO(2-1) data, (2) dense gas fraction suggested by HCN to $^{12}$CO (1-0) luminosity ratio, and (3) infrared color. The results show that the molecular gas in the central region is warmer and denser than that of the offset ridges. Dense gas fraction of the central region is similar with that of LIRGs/ULIRGs, while the offset ridges are close to the global average of normal galaxies. The coolest and least dense region is found in a spiral-like structure, which was misunderstood to be part of the southern primary bar in previous low-resolution observations. Star formation efficiency (SFE) changes by ~ 5 times in the inner disk. The variation of SFE agrees with the prediction in terms of star formation regulated by galactic bar. We find a consistency between star-forming region and the temperature inferred by the infrared color, suggesting that the distribution of sub-kpc scale temperature is driven by star formation.
Newly born and young radio sources are in a delicate phase of their life. Their jets are fighting their way through the surrounding gaseous medium, strongly experiencing this interaction while, at the same time, impacting and affecting the interstellar medium (ISM). Here we present the results from two studies of HI (in absorption) and molecular gas illustrating what can be learned from these phases of the gas. We first describe a statistical study with the WSRT. The study shows that the young radio sources not only have an higher detection rate of HI, but also systematically broader and more asymmetric HI profiles, most of them blueshifted. This supports the idea that we are looking at young radio jets making their way through the surrounding ISM, which also appears to be, on average, richer in gas than in evolved radio sources. Signatures of the impact of the jet are seen in the kinematics of the gas. However, even among the young sources, we identify a population that remains undetected in HI even after stacking their profiles. Orientation effects can only partly explain the result. These objects either are genuinely gas-poor or have different conditions of the medium, e.g. higher spin temperature. We further present the ALMA study of molecular gas in IC5063 to trace in detail the jet impacting the ISM. The kinematics of the cold, molecular gas co-spatial with the radio plasma shows this process in action. The ALMA data reveal a fast outflow of molecular gas extending along the entire radio jet (~1 kpc), with the highest outflow velocities at the location of the brighter hot-spot. We propose a scenario where the radio jet is expanding into a clumpy medium, interacting directly with the clouds and inflating a cocoon that drives a lateral outflow into the ISM.
We present results of long-term multi-wavelength polarization observations of the powerful blazar 3C~279 after its $\gamma$-ray flare on 2013~December 20. We followed up this flare with single-dish polarization observations using two 21-m telescopes of the Korean VLBI Network. Observations carried out weekly from 2013~December~25 to 2015~January~11, at 22~GHz, 43~GHz, 86~GHz simultaneously, as part of the Monitoring Of GAmma-ray Bright AGN (MOGABA) program. We measured 3C~279 total flux densities of 22--34~Jy at 22~GHz, 15--28~Jy (43~GHz), and 10--21~Jy (86~GHz), showing mild variability of $\leq 50\,\%$ over the period of our observations. The spectral index between 22~GHz and 86~GHz ranged from $-0.13$ to $-0.36$. Linear polarization angles were 27$^{\circ}$--38$^{\circ}$, 30$^{\circ}$--42$^{\circ}$, and 33$^{\circ}$--50$^{\circ}$ at 22~GHz, 43~GHz, and 86~GHz, respectively. The degree of linear polarization was in the range of 6--12\,\%, and slightly decreased with time at all frequencies. We investigated Faraday rotation and depolarization of the polarized emission at 22--86~GHz, and found Faraday rotation measures (RM) of $-300$ to $-1200$~rad~m$^{-2}$ between 22~GHz and 43~GHz, and $-800$ to $-5100$~rad~m$^{-2}$ between 43~GHz and 86~GHz. The RM values follow a power law with a mean power law index $a$ of $2.2$, implying that the polarized emission at these frequencies travels through a Faraday screen in or near the jet. We conclude that the regions emitting polarized radio emission may be different from the region responsible for the 2013 December $\gamma$-ray flare and are maintained by the dominant magnetic field perpendicular to the direction of the radio jet at milliarcsecond scales.
We present neutral hydrogen (HI) and warm molecular hydrogen (H2) observations of the young (10^2 years) radio galaxy PKS B1718-649. We study the morphology and the kinematics of both gas components, focusing, in particular, on their properties in relation to the triggering of the radio activity. The regular kinematics of the large scale HI disk, seen in emission, suggests that an interaction event occurred too long ago to be responsible for the recent triggering of the radio activity. In absorption, we detect two absorption lines along the narrow line of sight of the compact (r<2 pc) radio source. The lines trace two clouds with opposite radial motions. These may represent a population of clouds in the very inner regions of the galaxy, which may be involved in triggering the radio activity. The warm molecular hydrogen (H2 1-0 S(1) ro-vibrational line) in the innermost kilo-parsec of the galaxy appears to be distributed in a circum-nuclear disk following the regular kinematics of the HI and of the stellar component. An exception to this behaviour arises only in the very centre, where a highly dispersed component is detected. These particular HI and H2 features suggest that a strong interplay between the radio source and the surrounding ISM is on-going. The physical properties of the cold gas in the proximity of the radio source may regulate the accretion recently triggered in this AGN.
We present preliminary results of the quasar survey in Large Sky Area Multi- Object Fiber Spectroscopic Telescope (LAMOST) first data release (DR1), which includes pilot survey and the first year regular survey. There are 3921 quasars identified with reliability, among which 1180 are new quasars discovered in the survey. These quasars are at low to median redshifts, with highest z of 4.83. We compile emission line measurements around the H{\alpha}, H{\beta}, Mg II, and C IV regions for the new quasars. The continuum luminosities are inferred from SDSS photo- metric data with model fitting as the spectra in DR1 are non-flux-calibrated. We also compile the virial black hole mass estimates, and flags indicating the selec- tion methods, broad absorption line quasars. The catalog and spectra for these quasars are available online. 28% of the 3921 quasars are selected with optical- infrared colours independently, indicating that the method is quite promising in completeness of quasar survey. LAMOST DR1 and the on-going quasar survey will provide valuable data in the studies of quasars.
<Context.> Nuclear star clusters (NSCs) at the dynamical center of galaxies appear to have a complex star formation history. This suggests repeated star formation even in the influence of the strong tidal field from supermassive black holes. <Aim.> In our previous study, we have detected 31 so far unknown early-type star candidates throughout the Galactic NSC (at 0.5 - 3 pc from Sgr A*; Nishiyama and Schoedel 2013). The aim of this study is a confirmation of the spectral type for the candidates. <Method.> We have carried out NIR spectroscopic observations of the candidates using Subaru/IRCS/AO188/LGS. K-band spectra for 20 out of the 31 candidates were obtained. By determining an equivalent width, EW(CO), of the 12CO absorption feature at 2.294 um, we have derived an effective temperature and a bolometric magnitude for each candidate, and then constructed an HR diagram. <Results.> No young (~ Myr), massive stars are included in the 20 candidates we observed; however, 13 candidates are most likely intermediate-age giants (50 - 500 Myr). Two other sources have ages of ~1 Gyr, and the remaining five sources are old (> 1 Gyr), late-type giants. <Conclusions.> Although none of the early-type star candidates from our previous narrow-band imaging observations can be confirmed as a young star, we find that the photometric technique is sensitive to distinguish old, late-type giants from young and intermediate-age populations. The intermediate-age stars could be so far unknown members of a population formed in a starburst ~100 Myr ago. Finding no young (~ a few Myr) stars at R = 0.5 - 3 pc favors the in-situ formation scenario for the presence of the young stars at R < 0.5 pc. Furthermore, the different spatial distributions of the young and the intermediate-age stars imply that the Galactic NSC is an aggregate of stars born in different places and under different physical conditions.
With the Westerbork Synthesis Radio Telescope, we performed HI observations of a sample of known X-ray emitting Gigahertz-peaked-spectrum galaxies with compact-symmetric-object morphology (GPS/CSOs) that lacked an HI absorption detection. We combined radio and X-ray data of the full sample of X-ray emitting GPS/CSOs and found a significant, positive correlation between the column densities of the total and neutral hydrogen ($N_{\rm H}$ and $N_{\rm HI}$, respectively). Using a Bayesian approach, we simultaneously quantified the parameters of the $N_{\rm H} - N_{\rm HI}$ relation and the intrinsic spread of the data set. For a specific subset of our sample, we found $N_{\rm H} \propto N_{\rm HI}^b$, with $b=0.93^{+0.49}_{-0.33}$, and $\sigma_{int} (N_{\rm H})= 1.27^{+1.30}_{-0.40}$. The $N_{\rm H} - N_{\rm HI}$ correlation suggests a connection between the physical properties of the radio and X-ray absorbing gas.
Context. The compositional properties of hydrogenated amorphous carbons are known to evolve in response to the local conditions. Aims. To present a model for low-temperature, amorphous hydrocarbon solids, based on the microphysical properties of random and defected networks of carbon and hydrogen atoms, that can be used to study and predict the evolution of their properties in the interstellar medium. Methods. We adopt an adaptable and prescriptive approach to model these materials, which is based on a random covalent network (RCN) model, extended here to a full compositional derivation (the eRCN model), and a defective graphite (DG) model for the hydrogen poorer materials where the eRCN model is no longer valid. Results. We provide simple expressions that enable the determination of the structural, infrared and spectral properties of amorphous hydrocarbon grains as a function of the hydrogen atomic fraction, XH. Structural annealing, resulting from hydrogen atom loss, results in a transition from H-rich, aliphatic-rich to H-poor, aromatic-rich materials. Conclusions. The model predicts changes in the optical properties of hydrogenated amorphous carbon dust in response to the likely UV photon-driven and/or thermal annealing processes resulting, principally, from the radiation field in the environment. We show how this dust component will evolve, compositionally and structurally in the interstellar medium in response to the local conditions.
Context. The properties of hydrogenated amorphous carbon (a-C:H) dust are known to evolve in response to the local conditions. Aims. We present an adaptable model for the determination of the optical properties of low-temperature, interstellar a-C:H grains that is based on the fundamental physics of their composition. Methods. The imaginary part of the refractive index, k, for a-C:H materials, from 50 eV to cm wavelengths, is derived and the real part, n, of the refractive index is then calculated using the Kramers-Kronig relations. Results. The formulated optEC(s) model allows a determination of the complex dielectric function, epsilon, and refractive index, m(n, k), for a-C:H materials as a continuous function the band gap, Eg , which is shown to lie in the range = -0.1 to 2.7 eV. We provide expressions that enable a determination of their optical constants and tabulate m(n, k, Eg ) for 14 different values of Eg . We explore the evolution of the likely extinction and emission behaviours of a-C:H grains and estimate the relevant transformation time-scales. Conclusions. With the optEC(s) model we are able to predict how the optical properties of an a-C:H dust component in the interstellar medium will evolve in response to, principally, the local interstellar radiation field. The evolution of a-C:H materials appears to be consistent with many dust extinction, absorption, scattering and emission properties, and also with H2 molecule, daughter PAH and hydrocarbon molecule formation resulting from its photo-driven decomposition.
Context. The properties of hydrogenated amorphous carbon (a-C:H) dust evolve in response to the local radiation field in the interstellar medium and the evolution of these properties is particularly dependent upon the particle size. Aims. A model for finite-sized, low-temperature amorphous hydrocarbon particles, based on the microphysical properties of random and defected networks of carbon and hydrogen atoms, with surfaces passivated by hydrogen atoms, has been developed. Methods. The eRCN/DG and the optEC(s) models have been combined, adapted and extended into a new optEC(s)(a) model that is used to calculate the optical properties of hydrocarbon grain materials down into the sub-nanometre size regime, where the particles contain only a few tens of carbon atoms. Results. The optEC(s)(a) model predicts a continuity in properties from large to small (sub-nm) carbonaceous grains. Tabulated data of the size-dependent optical constants (from EUV to cm wavelengths) for a-C:H (nano-)particles as a function of the bulk material band gap [Eg(bulk)], or equivalently the hydrogen content, are provided. The effective band gap [Eg(eff.)] is found to be significantly larger than Eg(bulk) for hydrogen-poor a-C(:H) nano-particles and their predicted long-wavelength ({\lambda} > 30{\mu}m) optical properties differ from those derived for interstellar polycyclic aromatic hydrocarbons (PAHs). Conclusions. The optEC(s)(a) model is used to investigate the size-dependent structural and spectral evolution of a-C(:H) materials under ISM conditions, including: the IR-FUV extinction, the 217 nm bump and the infrared emission bands. The model makes several predictions that can be tested against observations.
We present the first model that couples high-resolution simulations of the
formation of Local Group galaxies with calculations of the galactic habitable
zone (GHZ), a region of space which has sufficient metallicity to form
terrestrial planets without being subject to hazardous radiation. These
simulations allow us to make substantial progress in mapping out the asymmetric
three-dimensional GHZ and its time evolution for the Milky Way (MW) and
Triangulum (M33) galaxies, as opposed to works that generally assume an
azimuthally symmetric GHZ.
Applying typical habitability metrics to MW and M33, we find that while a
large number of habitable planets exist as close as a few kiloparsecs from the
galactic centre, the probability of individual planetary systems being
habitable rises as one approaches the edge of the stellar disc. Tidal streams
and satellite galaxies also appear to be fertile grounds for habitable planet
formation.
In short, we find that both galaxies arrive at similar GHZs by different
evolutionary paths, as measured by the first and third quartiles of surviving
biospheres. For the Milky Way, this interquartile range begins as a narrow band
at large radii, expanding to encompass much of the galaxy at intermediate times
before settling at a range of 2-13kpc. In the case of M33, the opposite
behaviour occurs - the initial and final interquartile ranges are quite
similar, showing gradual evolution. This suggests that galaxy assembly history
strongly influences the time evolution of the GHZ, which will affect the
relative time lag between biospheres in different galactic locations. We end by
noting the caveats involved in such studies and demonstrate that high
resolution cosmological simulations will play a vital role in understanding
habitability on galactic scales, provided that these simulations accurately
resolve chemical evolution.
We report a new detection of neutral deuterium in the sub Damped Lyman Alpha system with low metallicity [O/H]\,=\,$-2.042 \pm 0.005$ at $z_{\rm abs}=2.437$ towards QSO~J\,1444$+$2919. The hydrogen column density in this system is log$N$(H\,{\sc i})~$=19.983\pm0.010$ and the measured value of deuterium abundance is log(D/H)~$=-4.706\pm0.007_{\rm stat}\pm0.067_{\rm syst}$. This system meets the set of strict selection criteria stated recently by Cooke et al. and, therefore, widens the {\it Precision Sample} of D/H. However, possible underestimation of systematic errors can bring bias into the mean D/H value (especially if use a weighted mean). Hence, it might be reasonable to relax these selection criteria and, thus, increase the number of acceptable absorption systems with measured D/H values. In addition, an unweighted mean value might be more appropriate to describe the primordial deuterium abundance. The unweighted mean value of the whole D/H data sample available to date (15 measurements) gives a conservative value of the primordial deuterium abundance (D/H)$_{\rm p}=(2.55\pm 0.19)\times10^{-5}$ which is in good agreement with the prediction of analysis of the cosmic microwave background radiation for the standard Big Bang nucleosynthesis. By means of the derived (D/H)$_{\rm p}$ value the baryon density of the Universe $\Omega_{\rm b}h^2=0.0222\pm0.0013$ and the baryon-to-photon ratio $\eta_{10} = 6.09\pm 0.36$ have been deduced. These values have confident intervals which are less stringent than that obtained for the {\it Precision Sample} and, thus, leave a broader window for new physics. The latter is particularly important in the light of the lithium problem.
Mapping Nearby Galaxies at Apache Point Observatory (MaNGA), one of three core programs in the Sloan Digital Sky Survey-IV (SDSS-IV), is an integral-field spectroscopic (IFS) survey of roughly 10,000 nearby galaxies. It employs dithered observations using 17 hexagonal bundles of 2 arcsec fibers to obtain resolved spectroscopy over a wide wavelength range of 3,600-10,300A. To map the internal variations within each galaxy, we need to perform accurate {\it spectral surface photometry}, which is to calibrate the specific intensity at every spatial location sampled by each individual aperture element of the integral field unit. The calibration must correct only for the flux loss due to atmospheric throughput and the instrument response, but not for losses due to the finite geometry of the fiber aperture. This requires the use of standard star measurements to strictly separate these two flux loss factors (throughput versus geometry), a difficult challenge with standard single-fiber spectroscopy techniques due to various practical limitations. Therefore, we developed a technique for spectral surface photometry using multiple small fiber-bundles targeting standard stars simultaneously with galaxy observations. We discuss the principles of our approach and how they compare to previous efforts, and we demonstrate the precision and accuracy achieved. MaNGA's relative calibration between the wavelengths of H$\alpha$ and H$\beta$ has a root-mean-square (RMS) of 1.7%, while that between [NII] $\lambda$6583A and [OII] $\lambda$3727A has an RMS of 4.7%. Using extinction-corrected star formation rates and gas-phase metallicities as an illustration, this level of precision guarantees that flux calibration errors will be sub-dominant when estimating these quantities. The absolute calibration is better than 5% for more than 89% of MaNGA's wavelength range.
The steep spectrum of neutrinos measured by IceCube extending from >1 PeV down to ~10 TeV has an energy flux now encroaching on the Fermi isotropic GeV background. We examine several implications starting from source energetics requirements for neutrino production. We show how the environment of extragalactic nuclei can extinguish ~10-100 TeV gamma rays and convert their energy to X-rays for plausible conditions of infrared luminosity and magnetic field, so that the Fermi background is not overwhelmed by cascades. We address a variety of scenarios, such as for acceleration by supermassive black holes and hadronic scenarios, and observations that may help elucidate the neutrinos' shadowy origins.
Links to: arXiv, form interface, find, astro-ph, recent, 1511, contact, help (Access key information)