[Abbreviated] We search for scaling relations between the fundamental AGN parameters and rest-frame UV/optical variability properties for a sample of $\sim$90 X-ray selected AGNs covering a wide redshift range from the XMM-COSMOS survey, with optical light curves in four bands provided by the Pan-STARRS1 (PS1) Medium Deep Field 04 survey. To estimate the variability amplitude we utilize the normalized excess variance ($\sigma_{\mathrm{rms}}^{2}$) and probe variability on rest-frame timescales of several months and years by calculating $\sigma_{\mathrm{rms}}^{2}$ from different parts of our light curves. In addition, we derive the rest-frame optical PSD for our sources using continuous-time autoregressive moving average (CARMA) models. We observe that the excess variance and the PSD amplitude are strongly anti-correlated with wavelength, bolometric luminosity and Eddington ratio. There is no evidence for a dependency of the variability amplitude on black hole mass and redshift. These results suggest that the accretion rate is the fundamental physical quantity determining the rest-frame UV/optical variability amplitude of quasars on timescales of months and years. The optical PSD of all of our sources is consistent with a broken power law showing a characteristic bend at rest-frame timescales ranging between $\sim$100 and $\sim$300 days. The break timescale exhibits no significant correlation with any of the fundamental AGN parameters. The low frequency slope of the PSD is consistent with a value of $-1$ for most of our objects, whereas the high frequency slope is characterized by a broad distribution of values between $\sim-2$ and $\sim-4$. These findings unveil significant deviations from the simple "damped random walk" model, frequently used in previous optical variability studies. We find a weak tendency for AGNs with higher black hole mass having steeper high frequency PSD slopes.
Energetic feedback from active galactic nuclei (AGN) is an important ingredient for regulating the star-formation history of galaxies in models of galaxy formation, which makes it important to study how AGN feedback actually occurs in practice. In order to catch AGNs in the act of quenching star formation we have used the interstellar NaD absorption lines to look for cold-gas outflows in a sample of 456 nearby galaxies for which we could unambigously ascertain the presence of radio AGN activity, thanks to radio imaging at milli-arcsecond scales. While compact radio emission indicating a radio AGN was found in 103 galaxies (23% of the sample), and 23 objects (5%) exhibited NaD absorption-line kinematics suggestive of cold-gas outflows, not one object showed evidence of a radio AGN and of a cold-gas outflow simultaneously. Radio AGN activity was found predominantly in early-type galaxies, while cold-gas outflows were mainly seen in spiral galaxies with central star-formation or composite star-formation/AGN activity. Optical AGNs also do not seem capable of driving galactic winds in our sample. Our work adds to a picture of the low-redshift Universe where cold-gas outflows in massive galaxies are generally driven by star formation and where radio-AGN activity occurs most often in systems in which the gas reservoir has already been significantly depleted.
Radio relics are patches of diffuse synchrotron radio emission that trace shock waves. Relics are thought to form when intra-cluster medium electrons are accelerated by cluster merger induced shock waves through the diffusive shock acceleration mechanism. In this paper, we present observations spanning 150 MHz to 30 GHz of the `Sausage' and `Toothbrush' relics from the Giant Metrewave and Westerbork telescopes, the Karl G. Jansky Very Large Array, the Effelsberg telescope, the Arcminute Microkelvin Imager and Combined Array for Research in Millimeter-wave Astronomy. We detect both relics at 30 GHz, where the previous highest frequency detection was at 16 GHz. The integrated radio spectra of both sources clearly steepen above 2 GHz, at the >6$\sigma$ significance level, supports the spectral steepening previously found in the `Sausage' and the Abell 2256 relic. Our results challenge the widely adopted simple formation mechanism of radio relics and suggest more complicated models have to be developed that, for example, involve re-acceleration of aged seed electrons.
We present high-resolution Atacama Large Millimeter Array (ALMA) 870um imaging of five z~1.5-4.5 X-ray detected AGN (with luminosities of L(X)>10^42 erg/s). The sub-millimetre emission is extended on scales of FWHM~0.2"-0.5", corresponding to physical sizes of 1-3 kpc (median value of 1.8 kpc). These sizes are comparable to the majority of z=1-5 sub-millimetre galaxies (SMGs) with equivalent ALMA measurements. In combination with spectral energy distribution analyses, we attribute this rest-frame far-infrared (FIR) emission to dust heated by star formation. The implied star-formation rate surface densities are 20-200 Msol/yr/kpc^2, which are consistent with SMGs of comparable FIR luminosities (i.e., L(IR)~ [1-5]x10^12 Lsol). Although limited by a small sample of AGN, which all have high FIR luminosities, our study suggests that the kpc-scale spatial distribution and surface density of star formation in high-redshift star-forming galaxies is the same irrespective of the presence of X-ray detected AGN.
We present constraints on the abundance of carbon-monoxide in the early Universe from the CO Power Spectrum Survey (COPSS). We utilize a data set collected between 2005 and 2008 using the Sunyaev-Zel'dovich Array (SZA), which were previously used to measure arcminute-scale fluctuations of the CMB. This data set features observations of 44 fields, covering an effective area of 1.7 square degrees, over a frequency range of 27 to 35 GHz. Using the technique of intensity mapping, we are able to probe the CO(1-0) transition, with sensitivity to spatial modes between $k=0.5{-}2\ h\,\textrm{Mpc}^{-1}$ over a range in redshift of $z=2.3{-}3.3$, spanning a comoving volume of $3.6\times10^{6}\ h^{-3}\,\textrm{Mpc}^{3}$. We demonstrate our ability to mitigate foregrounds, and present estimates of the impact of continuum sources on our measurement. We constrain the CO power spectrum to $P_{\textrm{CO}}<2.6\times10^{4}\ \mu\textrm{K}^{2} (h^{-1}\,\textrm{Mpc})^{3}$, or $\Delta^{2}_{\textrm{CO}}(k\! = \! 1 \ h\,\textrm{Mpc}^{-1})<1.3 \times10^{3}\ \mu\textrm{K}^{2}$, at $95\%$ confidence. This limit resides near optimistic predictions for the CO power spectrum. Under the assumption that CO emission is proportional to halo mass during bursts of active star formation, this corresponds to a limit on the ratio of $\textrm{CO}(1{-}0)$ luminosity to host halo mass of $A_{\textrm{CO}}<1.2\times10^{-5}\ L_{\odot}\ M_{\odot}^{-1}$. Further assuming a Milky Way-like conversion factor between CO luminosity and molecular gas mass ($\alpha_{\textrm{CO}}=4.3\ M_{\odot}\ (\textrm{K}\ \textrm{km}\ \textrm{s}^{-1}\ \textrm{pc}^{-2})^{-1}$), we constrain the global density of molecular gas to $\rho_{z\sim3}(M_{\textrm{H}_{2}})\leq 2.8 \times10^{8}\ M_{\odot}\ \textrm{Mpc}^{-3}$.
Determining the level of chemical homogeneity in open clusters is of fundamental importance in the study of the evolution of star-forming clouds and that of the Galactic disk. Yet limiting the initial abundance spread in clusters has been hampered by difficulties in obtaining consistent spectroscopic abundances for different stellar types. Without reference to any specific model of stellar photospheres, a model for a homogeneous cluster is that it forms a one-dimensional sequence, with any differences between members due to variations in stellar mass and observational uncertainties. I present a novel method for investigating the abundance spread in open clusters that tests this one-dimensional hypothesis at the level of observed stellar spectra, rather than constraining homogeneity using derived abundances as traditionally done. Using high-resolution APOGEE spectra for 49 giants in M67, NGC 6819, and NGC 2420 I demonstrate that these spectra form one-dimensional sequences for each cluster. With detailed forward modeling of the spectra and Approximate Bayesian Computation, I derive strong limits on the initial abundance spread of 15 elements: <0.01 (0.02) dex for C and Fe, <~0.015 (0.03) dex for N, O, Mg, Si, and Ni, <~0.02 (0.03) dex for Al, Ca, and Mn, and <~0.03 (0.05) dex for Na, S, K, Ti, and V (at 68% and 95% confidence, respectively). The strong limits on C and O imply that no pollution by massive core-collapse supernovae occurred during star formation in open clusters, which, thus, need to form within <~6 Myr. Further development of this and related techniques will bring the power of differential abundances to stars other than solar twins in large spectroscopic surveys and will help unravel the history of star formation and chemical enrichment in the Milky Way through chemical tagging.
A sample of 576 X-ray selected LINERs was constructed by combining data from the 3XMM-DR4 and SDSS-DR7 catalogues. The sample was used to investigate the fraction of galaxies hosting a LINER, finding that the fraction is a strong function of both stellar mass and black hole mass (scaling to the power of 1.6 +/- 0.2 and 0.6 +/- 0.1 respectively) and that it rises close to unity at the highest black hole masses and lowest X-ray luminosities. After obtaining radio flux densities from the FIRST survey, the sample was also used to investigate the Fundamental Plane of black hole activity - a scale-invariant relationship between black hole mass, X-ray luminosity and radio luminosity that is believed to hold across at least nine orders of magnitude of mass. There are key advantages in using only LINERs for the derivation as these are the counterparts of the "low-hard" X-ray binaries for which the relationship is tightest. The Fundamental Plane was found to be log (L_R / erg/s) = (0.65 +/- 0.07) log (L_X / 10^42 erg/s) + (0.69 +/- 0.10) log (M_BH / 10^8 M_solar) + (38.35 +/- 0.10). The scatter around the plane was 0.73 +/- 0.03 dex, too large to suggest that the Fundamental Plane can be used as a tool to estimate black hole mass from the observables of X-ray and radio luminosity. The black hole mass scaling is sensitive to the slope of the mass - velocity dispersion relation and, in order to achieve consistency with X-ray binaries, the analysis favours a steep gradient for this relationship, as found in recent research.
We have mapped cold atomic gas in 21cm line HI self-absorption (HISA) at arcminute resolution over more than 90% of the Milky Way's disk. To probe the formation of H2 clouds, we have compared our HISA distribution with CO J=1-0 line emission. Few HISA features in the outer Galaxy have CO at the same position and velocity, while most inner-Galaxy HISA has overlapping CO. But many apparent inner-Galaxy HISA-CO associations can be explained as chance superpositions, so most inner-Galaxy HISA may also be CO-free. Since standard equilibrium cloud models cannot explain the very cold HI in many HISA features without molecules being present, these clouds may instead have significant CO-dark H2.
Measurement of the local dark matter density plays an important role in both Galactic dynamics and dark matter direct detection experiments. However, the estimated values from previous works are far from agreeing with each other. In this work, we provide a well-defined observed sample with 1427 G \& K type main-sequence stars from the LAMOST spectroscopic survey, taking into account selection effects, volume completeness, and the stellar populations. We apply a vertical Jeans equation method containing a single exponential stellar disk, a razor thin gas disk, and a constant dark matter density distribution to the sample, and obtain a total surface mass density of $\rm {78.7 ^{+3.9}_{-4.7}\ M_{\odot}\ pc^{-2}}$ up to 1 kpc and a local dark matter density of $0.0159^{+0.0047}_{-0.0057}\,\rm M_{\odot}\,\rm pc^{-3}$. We find that the sampling density (i.e. number of stars per unit volume) of the spectroscopic data contributes to about two-thirds of the uncertainty in the estimated values. We discuss the effect of the tilt term in the Jeans equation and find it has little impact on our measurement. Other issues, such as a non-equilibrium component due to perturbations and contamination by the thick disk population, are also discussed.
The stellar distribution derived from an $H$ and $K_{\mathrm S}$-band survey of the central region of our Galaxy is compared with the Fe XXV K$\alpha$ (6.7 keV) line intensity observed with the Suzaku satellite. The survey is for the Galactic coordinates $|l| \lesssim 3^{\circ}.0$ and $|b| \lesssim 1^{\circ}.0$ (equivalent to 0.8 kpc $\times$ 0.3 kpc for $R_0 = 8$ kpc), and the number-density distribution $N(K_{\mathrm S,0}; l, b)$ of stars is derived using the extinction-corrected magnitude $K_{\mathrm S,0}=10.5$. This is deep enough to probe the old red giant population and in turn to estimate the ($l$, $b$) distribution of faint X-ray point sources such as coronally active binaries and cataclysmic variables. In the Galactic plane ($b=0^{\circ}$), $N(10.5; l, b)$ increases to the Galactic center as $|l|^{-0.30 \pm 0.03}$ in the range of $-0^{\circ}.1 \geq l \geq -0^{\circ}.7$, but this increase is significantly slower than the increase ($|l|^{-0.44 \pm 0.02}$ ) of the Fe XXV K$\alpha$ line intensity. If normalized with the ratios in the outer region $1^{\circ}.5 \leq |l| \leq 2^{\circ}.8$, where faint X-ray point sources are argued to dominate the diffuse Galactic X-ray ridge emission, the excess of the Fe XXV K$\alpha$ line intensity over the stellar number density is at least a factor of two at $|l| = 0^{\circ}.1$. This indicates that a significant part of the Galactic center diffuse emission arises from a truly diffuse optically-thin thermal plasma, and not from an unresolved collection of faint X-ray point sources related to the old stellar population.
Small galaxies are thought to be the main contributors to the ionising budget of the Universe before reionisation was complete. There have been a number of numerical studies trying to quantify their ionising efficiency through the escape fraction $f_{esc}$. While there is a clear trend that $f_{esc}$ is higher for smaller haloes, there is a large scatter in the distribution of $f_{esc}$ for a single halo mass. We propose that this is due to the intrinsic burstiness of star formation in low mass galaxies. We performed high resolution radiative hydrodynamics simulations with Ramses-RT to model the evolution of three galaxies and their ionising efficiency. We found that the variability of $f_{esc}$ follows that of the star formation rate. We then discuss the consequences of this variability on the observability of such galaxies by JWST.
We investigate the ionization state of the Extended Emission-Line Regions (EELRs) around two compact steep-spectrum (CSS) radio galaxies, 3C~268.3 and 3C~303.1, in order to identify the contribution of photoionization and shock-ionization. We perform a new spectroscopical (long-slit) analysis with GMOS/Gemini with the slit oriented in the radio-jet direction, where outflows are known to exist. The [Ne V]$\lambda 3426$ emission is the most interesting feature of the spectra and the key to breaking the degeneracy between the models: since this emission-line is more extended than HeII, it challenges the ionization structure proposed by any photoionization model, also its intensity relative to H$\beta$ does not behave as expected with respect to the ionization parameter U in the same scenario. On the contrary, when it is compared to the intensity of [OII]$\lambda3727$/H$\beta$ and all these results are joined, the whole scenario is plausible to be explained as emission coming from the hot, compressed, shocked gas in shock-ionization models. Although the model fitting is strongly sensitive to the chosen line-ratios, it argues for the presence of external and strong ionizing fields, such as the precursor field created by the shock or/and the AGN radiation field. In this paper, we show how AGN photoionization and shock-ionization triggered by jet-cloud interaction work together in these EELRs in order to explain the observed trends and line-ratio behaviours in a kinematically acceptable way.
New low-resolution UV spectra of a sample of reddened OB stars in M31 were obtained with HST/STIS to study the wavelength dependence of interstellar extinction and the nature of the underlying dust grain populations. Extinction curves were constructed for four reddened sightlines in M31 paired with closely matching stellar atmosphere models. The new curves have a much higher S/N than previous studies. Direct measurements of N(H I) were made using the Ly$\alpha$ absorption lines enabling gas-to-dust ratios to be calculated. The sightlines have a range in galactocentric distance of 5 to 14 kpc and represent dust from regions of different metallicities and gas-to-dust ratios. The metallicities sampled range from Solar to 1.5 Solar. The measured curves show similarity to those seen in the Milky Way and the Large Magellanic Cloud. The Maximum Entropy Method was used to investigate the dust composition and size distribution for the sightlines observed in this program finding that the extinction curves can be produced with the available carbon and silicon abundances if the metallicity is super-Solar.
In this paper we present and test chemical models for three-dimensional hydrodynamical simulations of galaxy evolution. The microphysics is modelled by employing the public chemistry package KROME and the chemical networks have been tested to work in a wide range of densities and temperatures. We describe a simple H/He network following the formation of H2, and a more sophisticated network which includes metals. Photochemistry, thermal processes, and different prescriptions for the H2 catalysis on dust are presented and tested within a simple one-zone framework. We explore the effect of changing some of the key parameters such as metallicity, radiation and non-equilibrium versus equilibrium metal cooling approximations on the transition between the different gas phases. We find that employing an accurate treatment of the dust-related processes induces a faster HI-H2 transition. In addition, we show when the equilibrium assumption for metal cooling holds, and how a non-equilibrium approach affects the thermal evolution of the gas and the HII-HI transition. These models can be employed in any hydrodynamical code via an interface to KROME and can be applied to different problems including isolated galaxies, cosmological simulations of galaxy formation and evolution, and supernova explosions in molecular clouds. The metal network can be used for a comparison with observational data of CII 158 {\mu}m emission both for high-redshift as well as for local galaxies.
Supernovae are important probes of the properties of stars at high redshifts because they can be detected at early epochs and their masses can be inferred from their light curves. Direct detection of the first cosmic explosions in the universe will only be possible with JWST, WFIRST and the next generation of extremely large telescopes. But strong gravitational lensing by massive clusters, like those in the Frontier Fields, could reveal supernovae at slightly lower redshifts now by magnifying their flux by factors of 10 or more. We find that Frontier Fields will likely discover dozens of core-collapse supernovae at 5 $ < z <$ 12. Future surveys of cluster lenses similar in scope to Frontier Fields by JWST might find hundreds of these events out to $z \sim$ 15 - 17. Besides revealing the masses of early stars, these ancient supernovae could also constrain cosmic star formation rates in the era of first galaxy formation.
We re-analyse high redshift and high resolution Lyman-{\alpha} forest spectra from Viel et al. [1] seeking to constrain properties of warm dark matter particles. Compared to the previous work we consider a wider range on thermal histories of the intergalactic medium and find that both warm and cold dark matter models can explain the cut-off observed in the flux power spectra of high-resolution observations equally well. This implies, however, very different thermal histories and underlying re-ionisation models. We discuss how to remove this degeneracy.
We study the shapes and intrinsic alignments of disks and elliptical galaxies in the MassiveBlack-II (MBII) and Illustris cosmological hydrodynamic simulations, with volumes of ($100h^{-1}$Mpc)$^{3}$ and ($75h^{-1}$Mpc)$^{3}$ respectively. We find that simulated disk galaxies are more oblate in shape and more misaligned with the shape of their host dark matter subhalo when compared with ellipticals. The disk major axis is found to be oriented towards the location of nearby elliptical galaxies. We also find that the disks are thinner in MBII and misalignments with dark matter halo orientations are smaller in both disks and ellipticals when compared with Illustris. As a result, the intrinsic alignment correlation functions at fixed mass have a higher amplitude in MBII than in Illustris. Despite significant differences in the treatments of hydrodynamics and baryonic physics in the simulations, we find that the correlation functions scale similarly with transverse separation (yet both have a different scale dependence to the correlation functions of the shapes of dark matter subhalos within the same simulation). This is true for both disks and ellipticals. This result makes it likely that we should be able to use information from hydrodynamic simulations to understand intrinsic alignment two-point statistics. Finally, in scales above $\sim 0.1h^{-1}$Mpc, the intrinsic alignment two-point correlation functions for disk galaxies in both simulations are consistent with a null detection, unlike those for ellipticals.
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Multiwavelength data are essential in order to provide a complete picture of galaxy evolution and to inform studies of galaxies' morphological properties across cosmic time. Here we present results of a multiwavelength investigation of the morphologies of "tadpole" galaxies at intermediate redshift (0.314<z<3.175) in the Hubble Ultra Deep Field. These galaxies were previously selected from deep Hubble Space Telescope (HST) F775W data based on their distinct asymmetric knot-plus-tail morphologies (Straughn et al. 2006). Here we use deep Wide Field Camera 3 near-infrared imaging in addition to the HST optical data in order to study the rest-frame UV/optical morphologies of these galaxies across the redshift range 0.3<z<3.2. This study reveals that the majority of these galaxies do retain their general asymmetric morphology in the rest-frame optical over this redshift range, if not the distinct "tadpole" shape. The average stellar mass of tadpole galaxies is lower than field galaxies, with the effect being slightly greater at higher redshift within the errors. Estimated from SED fits, the average age of tadpole galaxies is younger than field galaxies in the lower redshift bin, and the average metallicity is lower (whereas the specific star formation rate for tadpoles is roughly the same as field galaxies across the redshift range probed here). These average effects combined support the conclusion that this subset of galaxies is in an active phase of assembly, either late-stage merging or cold gas accretion causing localized clumpy star-formation.
We derive X-ray mass, luminosity, and temperature profiles for 45 galaxy clusters to explore relationships between halo mass, AGN feedback, and central cooling time. We find that radio--mechanical feedback power (referred to here as "AGN power") in central cluster galaxies correlates with halo mass, but only in halos with central atmospheric cooling times shorter than 1 Gyr. This timescale corresponds approximately to the cooling time (entropy) threshold for the onset of cooling instabilities and star formation in central galaxies (Rafferty et al. 2008). No correlation is found in systems with central cooling times greater than 1 Gyr. The trend with halo mass is consistent with self-similar scaling relations assuming cooling is regulated by feedback. The trend is also consistent with galaxy and central black hole co-evolution along the $M_{BH} - \sigma $ relation. AGN power further correlates with X-ray gas mass and the host galaxy's K-band luminosity. AGN power in clusters with central atmospheric cooling times longer than ~1 Gyr typically lies two orders of magnitude below those with shorter central cooling times. Galaxies centred in clusters with long central cooling times nevertheless experience ongoing and occasionally powerful AGN outbursts. We further investigate the impact of feedback on cluster scaling relations. We find L-T, and M-T relations, excluding regions directly affected by AGN, that are consistent with the cluster population as a whole. While the gas mass rises, the stellar mass remains nearly constant with rising total mass, consistent with earlier studies. This trend is found regardless of central cooling time, implying tight regulation of star formation in central galaxies as their halos grew, and long-term balance between AGN heating and atmospheric cooling. Our scaling relations are presented in forms that can be incorporated easily into galaxy evolution models.
We present a study of photometric redshift accuracy in the 3D-HST photometric catalogs, using 3D-HST grism redshifts to quantify and dissect trends in redshift accuracy for galaxies brighter than $H_{F140W}<24$ with an unprecedented and representative high-redshift galaxy sample. We find an average scatter of $0.0197\pm0.0003(1+z)$ in the Skelton et al. (2014) photometric redshifts. Photometric redshift accuracy decreases with magnitude and redshift, but does not vary monotonically with color or stellar mass. The 1-$\sigma$ scatter lies between $0.01-0.03$(1+z) for galaxies of all masses and colors below $z<2.5$ (for $H_{F140W}{<}24$), with the exception of a population of very red ($U-V > 2$), dusty star-forming galaxies for which the scatter increases to $\sim0.1(1+z)$. Although the overall photometric redshift accuracy for quiescent galaxies is better than for star-forming galaxies, scatter depends more strongly on magnitude and redshift than on galaxy type. We verify these trends using the redshift distributions of close pairs and extend the analysis to fainter objects, where photometric redshift errors further increase to $\sim0.046(1+z)$ at $H_{F160W}=26$. We demonstrate that photometric redshift accuracy is strongly filter-dependent and quantify the contribution of multiple filter combinations. We evaluate the widths of redshift probability distribution functions and find that error estimates are underestimated by a factor of $\sim1.1-1.6$, but that uniformly broadening the distribution does not adequately account for fitting outliers. Finally, we suggest possible applications of these data in planning for current and future surveys and simulate photometric redshift performance in the LSST, DES, and combined DES and VHS surveys.
We search for environmental dependence of the HI mass function in the ALFALFA 70% catalogue. The catalogue is split into quartiles of environment density based on the projected neighbour density of neighbours found in both SDSS and 2MRS volume limited reference catalogues. We find the Schechter function 'knee' mass to be dependent on environment, with the value of $\log ({M_{*}/\mathrm{M_{\odot}}})$ shifting from $9.81 \pm 0.02$ to $10.00 \pm 0.03$ between the lowest and highest density quartiles. However, this dependence was only observed when defining environment based on the SDSS reference catalogue, not 2MRS. We interpret these results as meaning that the local environment is the dominant cause of the shift in $M_{*}$, and that the larger scales that 2MRS probes (compared to SDSS) are almost irrelevant. In addition, we also use a fixed aperture method to probe environment, and find tentative evidence that HI-deficiency depresses the value of $M_{*}$ in the highest density regions. We find no significant dependence of the low-mass slope on environment in any test, using either method. Tensions between these results and those from the literature, are discussed and alternative explanations are explored.
In this paper, we study the age and spatial distributions of Cepheids in the Small Magellanic Cloud (SMC) as a function of their ages using the data from the OGLE III photometric catalogue. A period-age (PA) relation derived for the Classical Cepheids in the Large Magellanic Cloud (LMC) has been used to find the ages of Cepheids. The age distribution of the SMC Classical Cepheids is found to have a peak at log(Age) = 8.40+/-0.10 which suggests that a major star formation event might have occurred in the SMC at about 250+/-50 Myrs ago. It is believed that this star forming burst had been triggered by close interactions of the SMC with the LMC and/or the Milky Way (MW). A comparison of the observed spatial distributions of the Cepheids and open star clusters has also been carried out to study the star formation scenario in the SMC.
We present photometric analysis of the two W UMa type binaries identified in the field of distant open star cluster NGC6866. Although these systems, namely ID487 and ID494, were reported in the Joshi et al. (2012), but a detailed study of these stars has not been carried out earlier. The orbital periods of these stars are found to be 0.415110+/-0.000001 day and 0.366709+/-0.000004 day, respectively. Based on the photometric and infrared colours, we find their respective spectral types as K0 and K3. The photometric light variations of both the stars show O'Connell effect which could be explained by employing a dark spot on the secondary components. The V and I bands light curves are analyzed using the Wilson-Devinney (WD) code and relations given by Gazeas (2009) which yield radii and mass of the primary and secondary components of the star ID487 as R1 = 1.24+/-0.01 Rsun, R2 = 1.11+/-0.02 Rsun, and M1 = 1.24+/-0.02 Msun, M2 = 0.96+/-0.05 Msun, and for the star ID494 as R1 = 1.22+/-0.02 Rsun, R2 = 0.81+/-0.01 Rsun, and M1 = 1.20+/-0.06 Msun, M2 = 0.47+/-0.01 Msun.
Aims: We resolve the length-scales for filament formation and fragmentation
(res. <=0.1pc), in particular the Jeans length and cylinder fragmentation
scale.
Methods: We observed the prototypical high-mass star-forming filament
IRDC18223 with the Plateau de Bure Interferometer (PdBI) in the 3.2mm continuum
and N2H+(1-0) line emission in a ten field mosaic at a spatial resolution of
~4'' (~14000AU).
Results: The dust continuum emission resolves the filament into a chain of at
least 12 relatively regularly spaced cores. The mean separation between cores
is ~0.40(+-0.18)pc. While this is approximately consistent with the
fragmentation of an infinite, isothermal, gravitationally bound gas cylinder, a
high mass-to-length ratio of M/l~1000M_sun/pc requires additional turbulent
and/or magnetic support against radial collapse of the filament. The N2H+(1-0)
data reveal a velocity gradient perpendicular to the main filament. Although
rotation of the filament cannot be excluded, the data are also consistent with
the main filament being comprised of several velocity-coherent sub-filaments.
Furthermore, this velocity gradient perpendicular to the filament resembles
recent results toward Serpens south that are interpreted as signatures of
filament formation within magnetized and turbulent sheet-like structures.
Lower-density gas tracers ([CI] and C18O) reveal a similar red/blueshifted
velocity structure on scales around 60'' east and west of the IRDC18223
filament. This may tentatively be interpreted as a signature of the large-scale
cloud and the smaller-scale filament being kinematically coupled. We do not
identify a velocity gradient along the axis of the filament. This may either be
due to no significant gas flows along the filamentary axis, but it may partly
also be caused by a low inclination angle of the filament with respect to the
plane of the sky that could minimize such signature.
Keck OSIRIS/LGSAO observations of the ultraluminous galaxy IRAS 23365+3604 resolve a non-axisymmetric, circumnuclear structure of semi-major axis 0.42" (520 pc) in Paschen-alpha emission. The line-of-sight velocity of the ionized gas increases from the northeast towards the southwest; this gradient is perpendicular to the photometric major axis of the infrared emission. Two pairs of bends in the zero velocity line are detected. The inner bend provides evidence for gas inflow onto the circumnuclear structure. We interpret the gas kinematics on kiloparsec scales in relation to the molecular gas disk and multiphase outflow discovered previously. In particular, the fast component of the outflow (detected previously with lower spatial resolution) is not detected, adding support to the conjecture that the fast wind originates well-beyond the nucleus. These data directly show the dynamics of gas inflow and outflow in the central kiloparsec of a late-stage, gas-rich merger and demonstrate the potential of integral field spectroscopy to improve our understanding of the role of gas flows during the growth phase of bulges and supermassive black holes.
We searched for z > 7 Lyman-break galaxies (LBGs) in the optical-to-mid-infrared Hubble Frontier Field and associated parallel field observations of the strong-lensing cluster MACS J0416-2403. We discovered 22 candidates, of which six lie at z > 9 and one lies at z > 10. Based on the Hubble and Spitzer photometry, all have secure photometric redshifts and a negligible probability of being at lower redshifts, according to their peak probability ratios, R. This substantial increase in the number of known high-redshift galaxies allows a solid determination of the luminosity function at z > 8. The number of high-z candidates in the parallel field is considerably higher than that in the Abell 2744 parallel field. Our candidates have median stellar masses of log(M_*) ~ 8.40^{+0.55}_{-0.31}~Msun, SFRs of ~ 1.6^{+0.5}_{-0.4} Msun yr^-1, and SFR-weighted ages of < 310^{+70}_{-140} Myr. Finally, we are able to put strong constraints on the z = 7,8,9 and 10 luminosity functions. One of the objects in the cluster field is a z ~ 10 candidate, with a magnification of mu ~ 20 +- 13. This object is likely the faintest z ~ 10 object known to date, allowing a first look into the extreme faint-end (L ~ 0.04L*) of the z ~ 10 luminosity function.
Most molecular clouds are filamentary or elongated. Among those forming low-mass stars, their long axes tend to be either parallel or perpendicular to the large-scale (10-100 pc) magnetic field (B-field) in the surrounding inter cloud medium. This arises because, along the dynamically dominant B-fields, the competition between self-gravity and turbulent pressure will shape the cloud to be elongated either perpendicular or parallel to the fields. Recent study also suggested that, on the scales of 0.1-0.01 pc, fields are dynamically important within cloud cores forming massive stars. But whether the core field morphologies are inherited from the inter cloud medium or governed by cloud turbulence is under vigorous debate, so is the role played by B-fields in cloud fragmentation at 10 - 0.1 pc scales. Here we report B-field maps covering 100-0.01 pc scales inferred from polarimetric observations of a massive-star forming region, NGC 6334. First, the main filament also lies perpendicular to the ambient field. NGC 6334 hosts young star-forming sites where fields are not severely affected by stellar feedback, and their directions do not change significantly over the entire scale range. This means that the fields are dynamically important. At various scales, we find that the hourglass-shaped field lines are pinched where the gas column density peaks and the field strength is proportional to the 0.4-power of the density. We conclude that B-fields play a crucial role in the fragmentation of NGC 6334.
Intermediate surface brightness (ISB) galaxies are less numerous than their counterparts at high and low surface brightness (HSB and LSB). Investigating ISB characteristics from a sample from the S4G survey, complete down to M_B=-16, we find that they have intermediate stellar, gas and baryonic masses and on average as much gas as stars. They lie on the (baryonic) Tully-Fisher relation between HSBs and LSBs, although they present a higher scatter than the latter. Their stellar to baryonic mass ratios have intermediate values unlike their condensed baryonic fractions. By comparing their environments, as classified by the eigenvalues of the velocity shear tensor of local constrained simulations, ISBs have a 5-10% probability higher (smaller) to be in sheets (filaments) with respect to HSBs and LSBs. Additionally, for galaxies in filaments (with close neighbors), the mass and mu_0 are correlated at 2.5 (2) sigma more than for those in sheets. ISBs live in regions where the divergence of the velocity field is smaller than where HSBs and LSBs live, a result at more than 50% significance. ISBs may exist as an unstable transition state between LSBs and HSBs, the low flow activity environment maximally encouraging their formation. Interaction events altering the central baryon fraction could happen at a lower rate in these less dense environment, whilst in the higher density environments the LSBs are primarily satellite galaxies, whose accretion is sufficiently constrained that it fails to promote them to HSBs.
We present DEIMOS multi-object spectroscopy (MOS) of 22 star-forming dwarf galaxies located in four gas-rich groups, including six newly-discovered dwarfs. Two of the galaxies are strong tidal dwarf galaxy (TDG) candidates based on our luminosity-metallicity relation definition. We model the rotation curves of these galaxies. Our sample shows low mass-to-light ratios (M/L=0.73$\pm0.39M_\odot/L_\odot$) as expected for young, star-forming dwarfs. One of the galaxies in our sample has an apparently strongly-falling rotation curve, reaching zero rotational velocity outside the turnover radius of $r_{turn}=1.2r_e$. This may be 1) a polar ring galaxy, with a tilted bar within a face-on disk; 2) a kinematic warp. These scenarios are indistinguishable with our current data due to limitations of slit alignment inherent to MOS-mode observations. We consider whether TDGs can be detected based on their tidal radius, beyond which tidal stripping removes kinematic tracers such as H$\alpha$ emission. When the tidal radius is less than about twice the turnover radius, the expected falling rotation curve cannot be reliably measured. This is problematic for as much as half of our sample, and indeed more generally, galaxies in groups like these. Further to this, the H$\alpha$ light that remains must be sufficiently bright to be detected; this is only the case for three (14%) galaxies in our sample. We conclude that the falling rotation curves expected of tidal dwarf galaxies are intrinsically difficult to detect.
We present the first results from the Las Cumbres Observatory Global Telescope (LCOGT) Network's Active Galactic Nuclei Key Project, a large program devoted to using the robotic resources of LCOGT to perform time domain studies of active galaxies. We monitored the Seyfert 1 galaxy Arp~151 (Mrk~40) for $\sim$200 days with robotic imagers and with the FLOYDS robotic spectrograph at Faulkes Telescope North. Arp~151 was highly variable during this campaign, with $V$-band light curve variations of $\sim$0.3 mag and H$\beta$ flux changing by a factor of $\sim$3. We measure robust time lags between the $V$-band continuum and the H$\alpha$, H$\beta$ and H$\gamma$ emission lines, with $\tau_\mathrm{cen} = 13.89^{+1.39}_{-1.41}$, 7.52$^{+1.43}_{-1.06}$ and 7.40$^{+1.50}_{-1.32}$ days, respectively. The lag for the \ion{He}{2} $\lambda4686$ emission line is unresolved. We measure a velocity-resolved lag for the H$\beta$ line, which is clearly asymmetric with higher lags on the blue wing of the line which decline to the red, possibly indicative of radial inflow, and is similar in morphology to past observations of the H$\beta$ transfer function shape. Assuming a virialization factor of $f$=5.5, we estimate a black hole mass of $M_\mathrm{BH}=6.2^{+1.4}_{-1.2}\times$10$^{6}$~$M_{\odot}$, also consistent with past measurements for this object. These results represent the first step to demonstrate the powerful robotic capabilities of LCOGT for long-term, AGN time domain campaigns that human intensive programs cannot easily accomplish. Arp 151 is now one of just a few AGN where the virial product is known to remain constant against substantial changes in H$\beta$ lag and luminosity.
The WISE Catalog of Galactic HII Regions contains $\sim2000$ HII region candidates lacking ionized gas spectroscopic observations. All candidates have the characteristic HII region mid-infrared morphology of WISE $12\,\,\mu\,m$ emission surrounding $22\,\mu\,m$ emission, and additionally have detected radio continuum emission. We here report Green Bank Telescope (GBT) hydrogen radio recombination line (RRL) and radio continuum detections at X-band (9GHz; 3cm) of 302 WISE HII region candidates (out of 324 targets observed) in the zone $225^{\circ} > l > -20^{\circ}$, $|b| \le 6^{\circ}$. Here we extend the sky coverage of our HII region Discovery Survey (HRDS), which now contains nearly 800 HII regions distributed across the entire northern sky. We provide LSR velocities for the 302 detections and kinematic distances for 131 of these. Of the 302 new detections, five have ($l, b, v$) coordinates consistent with the Outer Scutum-Centaurus Arm (OSC), the most distant molecular spiral arm of the Milky Way. Due to the Galactic warp, these nebulae are found at Galactic latitudes $>1^{\circ}$ in the first Galactic quadrant, and therefore were missed in previous surveys of the Galactic plane. One additional region has a longitude and velocity consistent with the OSC but lies at a negative Galactic latitude (G039.183$-$01.422; $-$54.9 kms). With Heliocentric distances >22 kpc and Galactocentric distances >16 kpc, the OSC HII regions are the most distant known in the Galaxy. We detect an additional three HII regions near $l \simeq 150^{\circ}$ whose LSR velocities place them at Galactocentric radii >19 kpc. If their distances are correct, these nebulae may represent the limit to Galactic massive star formation.
In order to find out capable molecular source of astronomically well observed infrared (IR) spectrum, asymmetric molecular configuration polycyclic aromatic hydrocarbon (PAH) was analyzed by the density functional theory (DFT) analysis. Starting molecules were benzene C6H6, naphthalene C10H8 and 1H-phenalene C13H9. In interstellar space, those molecules will be attacked by high energy photon and proton, which may bring cationic molecules as like C6H6n+ (n=0~3 in calculation), C10H8n+, and C13H9n+, also CH lacked molecules C5H5n+, C9H7n+, and C12H8n+. IR spectra of those molecules were analyzed based on DFT based Gaussian program. Results suggested that symmetrical configuration molecules as like benzene, naphthalene , 1H-phenalene and those cation ( +, 2+, and 3+) show little resemblance with observed IR. Contrast to such symmetrical molecules, several cases among cationic and asymmetric configuration molecules show fairly good IR tendency. One typical example was C12H83+, of which calculated harmonic IR wavelength were 3.2, 6.3, 7.5, 7.8, 8.7, 11.3, and 12.8 micro meter, which correspond well to astronomically observed wavelength of 3.3, 6.2, 7.6, 7.8, 8.6, 11.2, and 12.7 micro meter. It was amazing agreement. Also, some cases like C5H5+, C9H7+, C9H72+, C9H73+ and C12H82+ show fairly good coincidence. Such results suggest that asymmetric and cationic PAH may be capable source of interstellar dust.
We present the detection of molecular gas from galaxies located in nearby voids using the CO line emission as a tracer. The observations were done using the 45m Nobeyama Radio Telescope. Void galaxies lie in the most under dense parts of our universe and a significant fraction of them are gas rich, late type spiral galaxies. Although isolated, they have ongoing star formation but appear to be slowly evolving compared to galaxies in denser environments. Not much is known about their star formation properties or cold gas content. In this study we searched for molecular gas in five void galaxies. The galaxies were selected based on their relatively high IRAS fluxes or Ha line luminosities, both of which signify ongoing star formation. All five galaxies appear to be isolated and two lie within the Bootes void. We detected CO line emission from four of the five galaxies in our sample and the molecular gas masses lie between 10^8 to 10^9 Msolar. We did follow-up Ha imaging observations of three detected galaxies using the Himalayan Chandra Telescope and determined their star formation rates (SFRs). The SFR varies from 0.2 to 1 Msolar/yr, which is similar to that observed in local galaxies. Our study indicates that although void galaxies reside in under dense regions, their disks contain molecular gas and have star formation rates similar to galaxies in denser environments.
We present the first stellar density profile of the Milky Way bulge reaching
latitude $b=0^\circ$. It is derived by counting red clump stars within the
colour\--magnitude diagram constructed with the new PSF-fitting photometry from
VISTA Variables in the V\'\i a L\'actea (VVV) survey data. The new stellar
density map covers the area between $|l|\leq 10^\circ$ and $|b|\leq 4.5^\circ$
with unprecedented accuracy, allowing to establish a direct link between the
stellar kinematics from the Giraffe Inner Bulge Spectroscopic Survey (GIBS) and
the stellar mass density distribution. In particular, the location of the
central velocity dispersion peak from GIBS matches a high overdensity in the
VVV star count map. By scaling the total luminosity function (LF) obtained from
all VVV fields to the LF from Zoccali et al.(2003), we obtain the first fully
empirical estimate of the mass in stars and remnants of the Galactic bulge.
The Milky Way bulge stellar mass within ($|b|<9.5^\circ$, $|l|<10^\circ$) is
$2.0\pm0.3\times 10^{10}M_{\odot}$.
We aim at analyzing the (sub-)millimeter emission in a nearby blazar, PKS 0521-365 , to study the synchrotron and thermal emission in the different components detected at low frequency. We analyze the archive public data of the ALMA Cycle 0 where PKS 0521-365 is used as a calibrator. A total of 13 projects with 23 dataset is analyzed in band 3, 6 and 7 and combined. The whole set of data is combined and wavelet filtered to obtain a deep image reaching a dynamic range of 47000. The individual emission flux is measured at different date over a period of 11 months in various components. Finally we analyze the Spectral Energy Distribution (SED) in each different component, including the radio jet and counter jet. The point sources detected in the field follow a similar distribution to previous studies. The blazar flux shows large variation especially in band 3. Different components are observed: core, radio jet and newly detected counter jet, Hot Spot (HS) and a disky structure roughly perpendicular to the jet. The HS emission is formed by a point source surrounded by an extended emission. The viewing angle of the jet is about 30 with a Doppler factor of 1.6$. The HS is at a distance of 19 kpc from the center. The SED analysis shows a strong variation of the core spectral index, especially in band 3. The two components in the radio jet have roughly a flat spectral index in band 6 and 7. Using these ALMA data the different weak and extended components are detected. The analysis of both jets constrains the geometrical distance of the HS to the center. The SED presents a different shape in time and frequency for each component. Finally a new structure is detected roughly perpendicular to the radio jet and a thermal emission origin is currently favoured. Further observations at higher spatial resolution are needed to confirm that hypothesis.
We present a Bayesian approach to the redshift classification of emission-line galaxies when only a single emission line is detected spectroscopically. We consider the case of surveys for high-redshift ${\rm Ly{\alpha}}$-emitting galaxies (LAEs), which have traditionally been classified via an inferred rest-frame equivalent width $(W_{\rm Ly\alpha})$ greater than $20 {\rm \,\AA}$. Our Bayesian method relies on known prior probabilities in measured emission-line luminosity functions and equivalent width distributions for the galaxy populations in question, and it returns the probability that an object is an LAE given the characteristics observed. This approach will be directly relevant for the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX), which seeks to classify $\sim$$10^6$ emission-line galaxies into LAEs and low-redshift [O II] emitters. For a simulated HETDEX catalog with realistic measurement noise, our Bayesian method recovers $86\%$ of LAEs missed by the traditional $W_{\rm Ly\alpha} > 20 {\rm \,\AA}$ cutoff over $2 < z < 3$, outperforming the equivalent width (EW) cut in both contamination and incompleteness. Our method can trade off between contamination and incompleteness by adjusting the stringency of the probability requirement for classifying an observed object as an LAE in order to maximize the recovery of cosmological information. In our simulations of HETDEX, the Bayesian method reduces the uncertainty in cosmological distance measurements by $14\%$ with respect to the EW cut, equivalent to obtaining $29\%$ more data. This method enables us to use classification probabilities, rather than just object labels, in large-scale structure analyses, and can be applied to narrowband emission-line surveys as well as upcoming large spectroscopic surveys including Euclid and WFIRST.
In this Letter, we constrain the dust-to-gas ratio in the intergalactic medium (IGM) at high redshifts. We employ models for dust in the local Universe to contrain the dust-to-gas ratio during the epoch of reionization at redshifts z ~ 6-10. The observed level of reddening of high redshift galaxies implies that the IGM was enriched to an intergalactic dust-to-gas ratio of less than 3% of the Milky Way value by a redshift of z=10.
Though widely accepted, it is not proven that supermassive compact objects (SMCOs) residing in galactic centers are black holes. In particular, the Milky Way's SMCO can be a giant nontopological soliton, Q-ball, made of a scalar field: this fits perfectly all observational data. Similar but tiny Q-balls produced in the early Universe may constitute, partly or fully, the dark matter. This picture explains in a natural way, why our SMCO has very low accretion rate and why the observed angular size of the corresponding radio source is much smaller than expected. Interactions between dark-matter Q-balls may explain how SMCOs were seeded in galaxies and resolve well-known problems of standard (non-interacting) dark matter.
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The ability to accurately derive black hole (BH) masses at progressively higher redshifts and over a wide range of continuum luminosities has become indispensable in the era of large-area extragalactic spectroscopic surveys. In this paper we present an extension of existing comparisons between rest-frame UV and optical virial BH mass estimators to intermediate redshifts and luminosities comparable to the local H$\beta$ reverberation mapped active galactic nuclei (AGN). We focus on the MgII, CIV, and CIII] broad emission lines and compare them to both H$\alpha$ and H$\beta$. We use newly acquired near-infrared spectra from the FMOS instrument on the Subaru telescope for 89 broad-lined AGN at redshifts between 0.3 and 3.5, complemented by data from the AGES survey. We employ two different prescriptions for measuring the emission line widths and compare the results. We confirm that MgII shows a tight correlation with H$\alpha$ and H$\beta$, with a scatter of ~0.25 dex. The CIV and CIII] estimators, while showing larger scatter, are viable virial mass estimators after accounting for a trend with the UV-to-optical luminosity ratio. We find an intrinsic scatter of ~0.37 dex between Balmer and carbon virial estimators by combining our dataset with previous high redshift measurements. This updated comparison spans a total of 3 decades in BH mass. We calculate a virial factor for CIV/CIII] logf(CIV/CIII])=0.87 with an estimated systematic uncertainty of ~0.4 dex and find excellent agreement between the local reverberation mapped AGN sample and our high-z sample.
Compact groups of galaxies provide a unique environment to study the evolution of galaxies amid frequent gravitational encounters. These nearby groups have conditions similar to those in the earlier universe when galaxies were assembled and give us the opportunity to witness hierarchical formation in progress. To understand how the compact group environment affects galaxy evolution, we examine the gas and dust in these groups. We present new single-dish GBT neutral hydrogen (HI) observations of 30 compact groups and define a new way to quantify the group HI content as the HI-to-stellar mass ratio of the group as a whole. We compare the HI content with mid-IR indicators of star formation and optical [g-r] color to search for correlations between group gas content and star formation activity of individual group members. Quiescent galaxies tend to live in HI-poor groups, and galaxies with active star formation are more commonly found in HI-rich groups. Intriguingly, we also find "rogue" galaxies whose star formation does not correlate with group HI content. In particular, we identify three galaxies (NGC 2968 in RSCG 34, KUG 1131+202A in RSCG 42, and NGC 4613 in RSCG 64) whose mid-IR activity is discrepant with the HI. We speculate that this mismatch between mid-IR activity and HI content is a consequence of strong interactions in this environment that can strip HI from galaxies and abruptly affect star-formation. Ultimately, characterizing how and on what timescales the gas is processed in compact groups will help us understand the interstellar medium in complex, dense environments similar to the earlier Universe.
We obtain a new determination of the metallicity distribution function (MDF) of stars within $\sim5$-$10$ kpc of the Sun, based on recently improved co-adds of $ugriz$ photometry for Stripe 82 from the Sloan Digital Sky Survey. Our new estimate uses the methodology developed previously by An et al. to study in situ halo stars, but is based on a factor of two larger sample than available before, with much-improved photometric errors and zero-points. The newly obtained MDF can be divided into multiple populations of halo stars, with peak metallicities at [Fe/H] $\approx -1.4$ and $-1.9$, which we associate with the inner-halo and outer-halo populations of the Milky Way, respectively. We find that the kinematics of these stars (based on proper-motion measurements at high Galactic latitude) supports the proposed dichotomy of the halo, as stars with retrograde motions in the rest frame of the Galaxy are generally more metal-poor than stars with prograde motions, consistent with previous claims. In addition, we generate mock catalogs of stars from a simulated Milk Way halo system, and demonstrate for the first time that the chemically- and kinematically-distinct properties of the inner- and outer-halo populations are qualitatively in agreement with our observations. The decomposition of the observed MDF and our comparison with the mock catalog results suggest that the outer-halo population contributes on the order of $\sim35\%$-$55\%$ of halo stars in the local volume.
We extend the catalogue of two-dimensional, PSF-corrected de Vacouleurs, Sersic, de Vacouleurs+Exponential, and Sersic+Exponential fits of ~7x10^5 galaxies presented in Meert, Vikram & Bernardi (2015) to include the g- and i-bands. Fits are analysed using the physically motivated flagging system presented in the original text, making adjustments for the differing signal-to-noise when necessary. We compare the fits in each of the g-, r-, and i-bands. Fixed aperture magnitudes and colours are also provided for all galaxies. The catalogues are available in electronic format.
We present high-resolution Magellan/MIKE spectra of the four brightest confirmed red giant stars in the ultra-faint dwarf galaxy Bootes II (Boo II). These stars all inhabit the metal-poor tail of the Boo II metallicity distribution function. The chemical abundance pattern of all detectable elements in these stars is consistent with that of the Galactic halo. However, all four stars have undetectable amounts of neutron-capture elements Sr and Ba, with upper limits comparable to the lowest ever detected in the halo or in other dwarf galaxies. One star exhibits significant radial velocity variations over time, suggesting it to be in a binary system. Its variable velocity has likely increased past determinations of the Boo II velocity dispersion. Our four stars span a limited metallicity range, but their enhanced {\alpha}-abundances and low neutron-capture abundances are consistent with the interpretation that Boo II has been enriched by very few generations of stars. The chemical abundance pattern in Boo II confirms the emerging trend that the faintest dwarf galaxies have neutron-capture abundances distinct from the halo, suggesting the dominant source of neutron-capture elements in halo stars may be different than in ultra-faint dwarfs.
In this study we use multi-epoch near-infrared observations from the VISTA survey of the Magellanic Cloud system (VMC) to measure the proper motion of different stellar populations in a tile of 1.5 deg sq. in size in the direction of the Galactic globular cluster 47 Tuc. We obtain the proper motion of the cluster itself, of the Small Magellanic Cloud (SMC), and of the field Milky Way stars. Stars of the three main stellar components are selected from their spatial distribution and their distribution in colour-magnitude diagrams. Their average coordinate displacement is computed from the difference between multiple Ks-band observations for stars as faint as Ks=19 mag. Proper motions are derived from the slope of the best-fitting line among 10 VMC epochs over a time baseline of ~1 yr. Background galaxies are used to calibrate the absolute astrometric reference frame. The resulting absolute proper motion of 47 Tuc is (mu_alpha cos(delta), mu_delta)=(+7.26+/-0.03, -1.25+/-0.03) mas/yr. This measurement refers to about 35000 sources distributed between 10 and 60 arcmin from the cluster centre. For the SMC we obtain (mu_alpha cos(delta), mu_delta)=(+1.16+/-0.07, -0.81+/-0.07) mas/yr from about 5250 red clump and red giant branch stars. The absolute proper motion of the Milky Way population in the line-of-sight (l =305.9, b =-44.9) of this VISTA tile is (mu_alpha cos(delta), mu_delta)=(+10.22+/-0.14, -1.27+/-0.12) mas/yr and results from about 4000 sources. Systematic uncertainties associated to the astrometric reference system are 0.18 mas/yr. Thanks to the proper motion we detect 47 Tuc stars beyond its tidal radius.
[Abridged] In high density environment, the gas content of galaxies is stripped, leading to a rapid quenching of their star formation activity. This dramatic environmental effect is generally not taken into account in the SFHs usually assumed to perform spectral energy distribution (SED) fitting of these galaxies, yielding to a poor fit of their stellar emission and, consequently, a biased estimate of the SFR. We aim at reproducing the SFH of galaxies that underwent a rapid star formation quenching using a truncated delayed SFH that we implemented in the SED fitting code CIGALE. We show that the ratio between the instantaneous SFR and the SFR just before the quenching ($r_{SFR}$) is well constrained as long as rest frame UV data are available. This SED modelling is applied to the Herschel Reference Survey (HRS) containing isolated galaxies and sources falling in the dense environment of the Virgo cluster. The latter are HI-deficient due to ram pressure stripping. We show that the truncated delayed SFH successfully reproduces their SED while typical SFH assumptions fail. A good correlation is found between $r_{SFR}$ and HI-def, the parameter quantifying the gas deficiency of cluster galaxies, meaning that SED fitting results can be used to provide a tentative estimate of the gas deficiency of galaxies for which HI observations are not available. The HRS galaxies are placed on the SFR-$M_*$ diagram showing that the HI-deficient sources lie in the quiescent region confirming previous studies. Using the $r_{SFR}$ parameter, we derive the SFR of these sources before quenching and show that they were previously on the main sequence relation. We show that the $r_{SFR}$ parameter is also well recovered for deeply obscured high redshift sources, as well as in absence of IR data. SED fitting is thus a powerful tool to identify galaxies that underwent a rapid star formation quenching.
We extend a machine learning (ML) framework presented previously to model galaxy formation and evolution in a hierarchical universe using N-body + hydrodynamical simulations. In this work, we show that ML is a promising technique to study galaxy formation in the backdrop of a hydrodynamical simulation. We use the Illustris Simulation to train and test various sophisticated machine learning algorithms. By using only essential dark matter halo physical properties and no merger history, our model predicts the gas mass, stellar mass, black hole mass, star formation rate, $g-r$ color, and stellar metallicity fairly robustly. Our results provide a unique and powerful phenomenological framework to explore the galaxy-halo connection that is built upon a solid hydrodynamical simulation. The promising reproduction of the listed galaxy properties demonstrably place ML as a promising and a significantly more computationally efficient tool to study small-scale structure formation. We find that ML mimics a full-blown hydrodynamical simulation surprisingly well in a computation time of mere minutes. The population of galaxies simulated by ML, while not numerically identical to Illustris, is statistically and physically robust and follows the same fundamental observational constraints. Machine learning offers an intriguing and promising technique to create quick mock galaxy catalogs in the future.
We present results from 21 cm radio synthesis imaging of 28 spiral galaxies from the DiskMass Survey obtained with the VLA, WSRT, and GMRT facilities. We detail the observations and data reduction procedures and present a brief analysis of the radio data. We construct 21 cm continuum images, global HI emission-line profiles, column-density maps, velocity fields, and position-velocity diagrams. From these we determine star formation rates (SFRs), HI line widths, total HI masses, rotation curves, and azimuthally-averaged radial HI column-density profiles. All galaxies have an HI disk that extends beyond the readily observable stellar disk, with an average ratio and scatter of R_{HI}/R_{25}=1.35+/-0.22, and a majority of the galaxies appear to have a warped HI disk. A tight correlation exists between total HI mass and HI diameter, with the largest disks having a slightly lower average column density. Galaxies with relatively large HI disks tend to exhibit an enhanced stellar velocity dispersion at larger radii, suggesting the influence of the gas disk on the stellar dynamics in the outer regions of disk galaxies. We find a striking similarity among the radial HI surface density profiles, where the average, normalized radial profile of the late-type spirals is described surprisingly well with a Gaussian profile. These results can be used to estimate HI surface density profiles in galaxies that only have a total HI flux measurement. We compare our 21 cm radio continuum luminosities with 60 micron luminosities from IRAS observations for a subsample of 15 galaxies and find that these follow a tight radio-infrared relation, with a hint of a deviation from this relation at low luminosities. We also find a strong correlation between the average SFR surface density and the K-band surface brightness of the stellar disk.
Quasar feedback models often predict an expanding hot gas bubble which drives a galaxy-scale outflow. In many circumstances the hot gas is predicted to radiate inefficiently, making the hot bubble hard to observe directly. We present an indirect method to detect the presence of a hot bubble using hydrostatic photoionization models of the cold (10^4 K) line-emitting gas. These models assume that the cold gas is in pressure equilibrium with either the hot gas pressure or with the radiation pressure, whichever is larger. We compare our models with observations of the broad line region (BLR), the inner face of the dusty torus, the narrow line region (NLR), and the extended NLR, and thus constrain the hot gas pressure over a dynamical range of 10^5 in radius, from 0.1 pc to 10 kpc. We find that the emission line ratios observed in the average quasar spectrum are consistent with radiation-pressure-dominated models on all scales. On scales <40 pc a dynamically significant hot gas pressure is ruled out for an average quasar spectrum, while on larger scales the hot gas pressure cannot exceed six times the local radiation pressure. In individual quasars, ~25% of the objects exhibit narrow line ratios that are inconsistent with radiation-pressure-dominated models by a factor of ~2, though in these objects the hot gas pressure is also unlikely to exceed the radiation pressure by an order of magnitude or more. The upper limits we derive on the hot gas pressure imply that the instantaneous gas pressure force acting on galaxy-scale outflows falls short of the time-averaged force needed to explain the large momentum fluxes \dot{p} >> L_AGN/c inferred for galaxy-scale outflows in luminous quasars. This apparent discrepancy can be reconciled if the optical quasars observed today previously experienced a buried, fully-obscured phase, (abridged)
We utilize zoom-in cosmological simulations to study the nature of violent disc instability (VDI) in clumpy galaxies at high redshift, $z=1$--$5$. Our simulated galaxies are not in the ideal state assumed in Toomre instability, of linear fluctuations in an isolated, uniform, rotating disk. There, instability is characterised by a $Q$ parameter below unity, and lower when the disk is thick. Instead, the high-redshift discs are highly perturbed. Over long periods they consist of non-linear perturbations, compact massive clumps and extended structures, with new clumps forming in inter-clump regions. This is while the galaxy is subject to frequent external perturbances. We compute the local, two-component $Q$ parameter for gas and stars, smoothed on a $\sim1~{\rm kpc}$ scale to capture clumps of $10^{8-9}~{\rm M}_\odot$. The $Q<1$ regions are confined to collapsed clumps due to the high surface density there, while the inter-clump regions show $Q$ significantly higher than unity. Tracing the clumps back to their relatively smooth Lagrangian patches, we find that $Q$ prior to clump formation typically ranges from unity to a few. This is unlike the expectations from standard Toomre instability. We discuss possible mechanisms for high-$Q$ clump formation, e.g. rapid turbulence decay leading to small clumps that grow by mergers, non-axisymmetric instability, or clump formation induced by non-linear perturbations in the disk. Alternatively, the high-$Q$ non-linear VDI may be stimulated by the external perturbations, e.g. mergers and counter-rotating streams. The high $Q$ may represent excessive compressive modes of turbulence, possibly induced by tidal interactions.
We describe the luminosity function, based on Sersic fits to the light profiles, of CMASS galaxies at z ~ 0.55. Compared to previous estimates, our Sersic-based reductions imply more luminous, massive galaxies, consistent with the effects of Sersic- rather than Petrosian or de Vaucouleur-based photometry on the Sloan Digital Sky Survey (SDSS) main galaxy sample at z ~ 0.1. This implies a significant revision of the high mass end of the correlation between stellar and halo mass. Inferences about the evolution of the luminosity and stellar mass functions depend strongly on the assumed, and uncertain, k+e corrections. In turn, these depend on the assumed age of the population. Applying k+e corrections taken from fitting the models of Maraston et al. (2009) to the colors of both SDSS and CMASS galaxies, the evolution of the luminosity and stellar mass functions appears impressively passive, provided that the fits are required to return old ages. However, when matched in comoving number- or luminosity-density, the SDSS galaxies are less strongly clustered compared to their counterparts in CMASS. This rules out the passive evolution scenario, and, indeed, any minor merger scenarios which preserve the rank ordering in stellar mass of the population. Potential incompletenesses in the CMASS sample would further enhance this mismatch. Our analysis highlights the virtue of combining clustering measurements with number counts.
We present an analysis of the optical nuclear spectra from the active galactic nuclei (AGN) in a sample of low surface brightness (LSB) galaxies. Using data from the Sloan Digital Sky Survey (SDSS), we derived the virial black hole (BH) masses of 24 galaxies from their broad H$\alpha$ parameters. We find that our estimates of nuclear BH masses lie in the range $10^{5}-10^{7}~M_{\odot}$, with a median mass of 5.62 x 10$^{6}~M_{\odot}$. The bulge stellar velocity dispersion $\sigma_{e}$ was determined from the underlying stellar spectra. We compared our results with the existing BH mass - velocity dispersion ($M_{BH}-\sigma_{e}$) correlations and found that the majority of our sample lie in the low BH mass regime and below the $M_{BH}-\sigma_{e}$ correlation. We analysed the effects of any systematic bias in the M$_{BH}$ estimates, the effects of galaxy orientation in the measurement of $\sigma_e$ and the increase of $\sigma_e$ due to the presence of bars and found that these effects are insufficient to explain the observed offset in M$_{BH}$ - $\sigma_e$ correlation. Thus the LSB galaxies tend to have low mass BHs which probably are not in co-evolution with the host galaxy bulges. A detailed study of the nature of the bulges and the role of dark matter in the growth of the BHs is needed to further understand the BH-bulge co-evolution in these poorly evolved and dark matter dominated systems.
We study the sensitivity of the methods available for abundance determinations in H II regions to potential observational problems. We compare the dispersions they introduce around the oxygen and nitrogen abundance gradients when applied to 5 different sets of spectra of H II regions in the galaxy M81. Our sample contains 116 H II regions with galactocentric distances of 3 to 33 kpc, including 48 regions observed by us with the OSIRIS long-slit spectrograph at the 10.4-m GTC telescope. The direct method can be applied to 31 regions, where we can get estimates of the electron temperature. The different methods imply oxygen abundance gradients with slopes of -0.010 to -0.002 dex kpc-1, and dispersions in the range 0.06-0.25 dex. The direct method produces the shallowest slope and the largest dispersion, illustrating the difficulty of obtaining good estimates of the electron temperature. Three of the strong-line methods, C, ONS, and N2, are remarkably robust, with dispersions of ~ 0.06 dex, and slopes in the range -0.008 to -0.006 dex kpc-1. The robustness of each method can be directly related to its sensitivity to the line intensity ratios that are more difficult to measure properly. Since the results of the N2 method depend strongly on the N/O abundance ratio and on the ionization parameter, we recommend the use of the C and ONS methods when no temperature estimates are available or when they have poor quality, although the behaviour of these methods when confronted with regions that have different properties and different values of N/O should be explored.
We present a new technique for the statistical evaluation of the Tully-Fisher
relation (TFR) using spectral line stacking. This technique has the potential
to extend TFR observations to lower masses and higher redshifts than possible
through a galaxy-by-galaxy analysis. It further avoids the need for individual
galaxy inclination measurements.
To quantify the properties of stacked HI emission lines, we consider a
simplistic model of galactic disks with analytically expressible line profiles.
Using this model, we compare the widths of stacked profiles with those of
individual galaxies. We then follow the same procedure using more realistic
mock galaxies drawn from the S3-SAX model (a derivative of the Millennium
simulation). Remarkably, when stacking the apparent HI lines of galaxies with
similar absolute magnitude and random inclinations, the width of the stack is
very similar to the width of the deprojected (= corrected for inclination) and
dedispersed (= after removal of velocity dispersion) input lines. Therefore,
the ratio between the widths of the stack and the deprojected/dedispersed input
lines is approximately constant - about 0.93 - with very little dependence on
the gas dispersion, galaxy mass, galaxy morphology, and shape of the rotation
curve.
Finally, we apply our technique to construct a stacked TFR using HIPASS data
which already has a well defined TFR based on individual detections. We obtain
a B-band TFR with a slope of $-8.5\pm0.4$ and a K-band relation with a slope of
$-11.7\pm0.6$ for the HIPASS data set which is consistent with the existing
results.
The origin of cosmic rays is one of the long-standing mysteries in physics and astrophysics. Simple arguments suggest that a scenario of supernova remnants (SNRs) in the Milky Way as the dominant sources for the cosmic ray population below the knee could work: in a generic calculation, it can be shown that these objects can provide the energy budget necessary to explain the observed flux of cosmic rays. However, this argument is based on the assumption that all sources behave in the same way, i.e.\ they all have the same energy budget, spectral behavior and maximum energy. In this paper, we investigate if a realistic population of SNRs is capable of producing the cosmic ray flux as it is observed below the knee. We use 21 SNRs that are well-studied from radio wavelengths up to gamma-ray energies. It could be shown previously (Mandelartz & Becker Tjus 2015) that the high-energy bump in the energy spectrum of these 21 sources can be dominated by hadronic emission. Here, gamma-rays are produced via $\pi^{0}-$decays from cosmic ray interactions in molecular clouds near the supernova remnant, which serves as the cosmic ray accelerator. The cosmic ray spectra show a large variety in their energy budget, spectral behavior and maximum energy. These sources are assumed to be representative for the total class of SNRs, where we assume that about 100 - 200 cosmic ray emitting SNRs should be present today. Finally, we use these source spectra to simulate the cosmic ray transport from individual SNRs in the Galaxy with the GALPROP code for cosmic ray propagation. We find that the cosmic ray budget can be matched well for a diffusion coefficient that is close to $D\propto E^{0.3}$. A stronger dependence on the energy, e.g. $E^{0.5}$, would lead to a spectrum at Earth that is too steep when compared to what is detected and the energy budget cannot be matched, in particular toward high energies.
A recent study of soft X-ray absorption in native and hydrogenated coronene cations, C$_{24}$H$_{12+m}^+$ $m=0-7$, led to the conclusion that additional hydrogen atoms protect (interstellar) Polycyclic Aromatic Hydrocarbon (PAH) molecules from fragmentation [Reitsma et al., Phys. Rev. Lett. 113, 053002 (2014)]. The present experiment with collisions between fast (30-200 eV) He atoms and pyrene (C$_{16}$H$_{10+m}^+$, $m=0$, 6, and 16) and simulations without reference to the excitation method suggests the opposite. We find that the absolute carbon-backbone fragmentation cross section does not decrease but increases with the degree of hydrogenation for pyrene molecules.
A detailed model of the tidal disruption events (TDE) has been constructed using stellar dynamical and gas dynamical inputs that include black hole mass $M_{\bullet}$, specific orbital energy $E$ and angular momentum $J$, star mass $M_{\star}$ and radius $R_{\star}$ and pericenter of the star orbit $r_{p}(E,\hspace{1mm}J,\hspace{1mm}M_{\bullet})$. We have solved the steady state Fokker- Planck equation using the standard loss cone theory for the galactic density profile $\rho (r) \propto r^{-\gamma}$ and stellar mass function $\xi(m) $ where $m=M_{\star}/M_{\odot}$ and obtained the feeding rate of stars to the black hole integrated over the phase space as $\dot{N}_{t} \propto M_{\bullet}^\beta$ where $\beta= -0.3\pm 0.01$ for $M_{\bullet}>10^7 M_{\odot}$ and $\sim 6.8 \hspace{1mm} \times 10^{-5}$ Yr$^{-1}$ for $\gamma=0.7$. Using this we model the in fall rate of the disrupted debris, $\dot{M}(E,\hspace{1mm}J,\hspace{1mm}m,\hspace{1mm}t)$ and discuss conditions for the disk formation and find that the accretion disk is formed almost always for the fiduciary range of the physical parameters. We also find the conditions under which the disk formed from the tidal debris of a given star has a super Eddington accretion phase. We have simulated the light curve profiles in relevant optical g band and soft X-rays for both super and sub Eddington accretion disks as function of $\dot{M}(E,\hspace{1mm}J,\hspace{1mm}t)$. Using this, standard cosmological parameters and mission instrument details, we predict the detectable TDE rates for various forthcoming surveys finally as a function of $\gamma$.
We study the dynamical response of extended systems, hosts, to smaller systems, satellites, orbiting around the hosts using extremely high-resolution N-body simulations with up to one billion particles. This situation corresponds to minor mergers which are ubiquitous in the scenario of hierarchical structure formation in the universe. According to Chandrasekhar (1943), satellites create density wakes along the orbit and the wakes cause a deceleration force on satellites, i.e. dynamical friction. This study proposes an analytical model to predict the dynamical response of hosts in the density distribution and finds not only traditional wakes but also mirror images of over- and underdensities centered on the host. Controlled N-body simulations with high resolutions verify the predictions of the analytical model directly. We apply our analytical model to the expected dynamical response of nearby interacting galaxy pairs, the Milky Way - Large Magellanic Cloud system and the M31 - M33 system.
Active galactic nuclei (AGN) are complex phenomena. At the heart of an AGN is a relativistic accretion disk around a spinning supermassive black hole (SMBH) with an X-ray emitting corona and, sometimes, a relativistic jet. On larger scales, the outer accretion disk and molecular torus act as the reservoirs of gas for the continuing AGN activity. And on all scales from the black hole outwards, powerful winds are seen that probably affect the evolution of the host galaxy as well as regulate the feeding of the AGN itself. In this review article, we discuss how X-ray spectroscopy can be used to study each of these components. We highlight how recent measurements of the high-energy cutoff in the X-ray continuum by NuSTAR are pushing us to conclude that X-ray coronae are radiatively-compact and have electron temperatures regulated by electron-positron pair production. We show that the predominance of rapidly-rotating objects in current surveys of SMBH spin is entirely unsurprising once one accounts for the observational selection bias resulting from the spin-dependence of the radiative efficiency. We review recent progress in our understanding of fast (v~0.1-0.3c), highly-ionized (mainly visible in FeXXV and FeXXVI lines), high-column density winds that may dominate quasar-mode galactic feedback. Finally, we end with a brief look forward to the promise of Astro-H and future X-ray spectropolarimeters.
Synthetic RGBB magnitudes are generated with the most recent theoretical stellar evolution models computed with the Dartmouth Stellar Evolution Program (DSEP) code. They are compared to the observational work of Nataf et al., who present RGBB magnitudes for 72 globular clusters. A DSEP model using a chemical composition with enhanced $\alpha$ capture [$\alpha$/Fe] $ =+0.4$ and an age of 13 Gyr shows agreement with observations over metallicities ranging from [Fe/H] = $0$ to [Fe/H] $\approx-1.5$, with discrepancy emerging at lower metallicities.
Astronomers commonly quote the properties of celestial objects in units of parameters for the Sun, Jupiter, or the Earth. The resolution presented here was proposed by the IAU Inter-Division Working Group on Nominal Units for Stellar and Planetary Astronomy and passed by the XXIXth IAU General Assembly in Honolulu. IAU 2015 Resolution B3 adopts a set of nominal solar, terrestrial, and jovian conversion constants for stellar and (exo)planetary astronomy which are defined to be exact SI values. While the nominal constants are based on current best estimates (CBEs; which have uncertainties, are not secularly constant, and are updated regularly using new observations), they should be interpreted as standard values and not as CBEs. IAU 2015 Resolution B3 adopts five solar conversion constants (nominal solar radius, nominal total solar irradiance, nominal solar luminosity, nominal solar effective temperature, and nominal solar mass parameter) and six planetary conversion constants (nominal terrestrial equatorial radius, nominal terrestrial polar radius, nominal jovian equatorial radius, nominal jovian polar radius, nominal terrestrial mass parameter, and nominal jovian mass parameter).
We suggest that dark matter in the universe has quantum entanglement among dark matter particles if the dark matter is a Bose-Einstein condensation of ultra-light scalar particles. In this theory any two regions of a galaxy are quantum entangled due to the quantum nature of the condensate. We calculate the entanglement entropy of a typical galactic halo, which turns out to be at least $O(ln(M/m))$ where $M$ is the mass of the halo and $m$ is the dark matter particle mass.
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Quasi-stellar object (QSO) spectral templates are important both to QSO physics and for investigations that use QSOs as probes of intervening gas and dust. However, combinations of various QSO samples obtained at different times and with different instruments so as to expand a composite and to cover a wider rest frame wavelength region may create systematic effects, and the contribution from QSO hosts may contaminate the composite. We have constructed a composite spectrum from luminous blue QSOs at 1 < z < 2.1 selected from the Sloan Digital Sky Survey (SDSS). The observations with X-shooter simultaneously cover ultraviolet (UV) to near- infrared (NIR) light, which ensures that the composite spectrum covers the full rest-frame range from Ly$\beta$ to 11350 $\AA$ without any significant host contamination. Assuming a power-law continuum for the composite we find a spectral slope of $\alpha_\lambda$ = 1.70+/-0.01, which is steeper than previously found in the literature. We attribute the differences to our broader spectral wavelength coverage, which allows us to effectively avoid fitting any regions that are affected either by strong QSO emissions lines (e.g., Balmer lines and complex [Fe II] blends) or by intrinsic host galaxy emission. Finally, we demonstrate the application of the QSO composite spectrum for evaluating the reddening in other QSOs.
We investigate the degree to which the inclusion of baryonic physics can overcome two long-standing problems of the standard cosmological model on galaxy scales: (i) the problem of satellite planes around Local Group galaxies, and (ii) the "too big to fail" problem. By comparing dissipational and dissipationless simulations, we find no indication that the addition of baryonic physics results in more flattened satellite distributions around Milky-Way-like systems. Recent claims to the contrary are shown to derive in part from a non-standard metric for the degree of flattening, which ignores the satellites' radial positions. If the full 3D positions of the satellite galaxies are considered, none of the simulations we analyse reproduce the observed flattening nor the observed degree of kinematic coherence of the Milky Way satellite system. Our results are consistent with the expectation that baryonic physics should have little or no influence on the structure of satellite systems on scales of hundreds of kiloparsecs. Claims that the "too big to fail" problem can be resolved by the addition of baryonic physics are also shown to be problematic.
We investigate correlations between different physical properties of star-forming galaxies in the "Evolution and Assembly of GaLaxies and their Environments" (EAGLE) cosmological hydrodynamical simulation suite over the redshift range $0\le z\le 4.5$. A principal component analysis reveals that neutral gas fraction ($f_{\rm gas, neutral}$), stellar mass ($M_{\rm stellar}$) and star formation rate (SFR) account for most of the variance seen in the population, with galaxies tracing a two-dimensional, nearly flat, surface in the three-dimensional space of $f_{\rm gas, neutral}-M_{\rm stellar}-\rm SFR$ with little scatter. The location of this plane varies little with redshift, whereas galaxies themselves move along the plane as their $f_{\rm gas, neutral}$ and SFR drop with redshift. The positions of galaxies along the plane are highly correlated with gas metallicity. The metallicity can therefore be robustly predicted from $f_{\rm gas, neutral}$, or from the $M_{\rm stellar}$ and SFR. We argue that the appearance of this "fundamental plane of star formation" is a consequence of self-regulation, with the plane's curvature set by the dependence of the SFR on gas density and metallicity. We analyse a large compilation of observations spanning the redshift range $0\lesssim \rm z\lesssim 2.5$, and find that such a plane is also present in the data. The properties of the observed fundamental plane of star formation are in good agreement with EAGLE's predictions.
We propose a method to substantially increase the flexibility and power of template fitting-based photometric redshifts by transforming a large numbers of galaxy spectral templates into a corrresponding collection of "fuzzy archetypes" using a suitable set of perturbative priors designed to account for empirical variation in dust attenuation and emission line strengths. To bypass widely seperated degeneracies in parameter space (e.g., the redshift-reddening degeneracy), we train Self-Organizing Maps (SOMs) on a large "model catalogs" generated from appropriate Monte Carlo sampling of our fuzzy archetypes to cluster the predicted observables in a topologically smooth fashion. Subsequent sampling over the SOM then allows full reconstruction of the relevant probability distribution functions (PDFs) using the associated set of inverse mappings from the SOM to the underlying model parameters. This combined approach enables the multi-modal exploration of known variation among galaxy spectral energy distributions (SEDs) using large numbers of archetypes with minimal modeling assumptions. We demonstrate the power of this approach to recover full redshift PDFs using discrete Markov Chain Monte Carlo (MCMC) sampling methods combined with SOMs constructed from model catalogs based on LSST $ugrizY$ and Euclid $YJH$ mock photometry.
The Magellanic Clouds provide the only laboratory to study the effect of metallicity and galaxy mass on molecular gas and star formation at high (~20 pc) resolution. We use the dust emission from HERITAGE Herschel data to map the molecular gas in the Magellanic Clouds, avoiding the known biases of CO emission as a tracer of H2. Using our dust-based molecular gas estimates, we find molecular gas depletion times of ~0.4 Gyr in the LMC and ~0.6 SMC at 1 kpc scales. These depletion times fall within the range found for normal disk galaxies, but are shorter than the average value, which could be due to recent bursts in star formation. We find no evidence for a strong intrinsic dependence of the molecular gas depletion time on metallicity. We study the relationship between gas and star formation rate across a range in size scales from 20 pc to ~1 kpc, including how the scatter in molecular gas depletion time changes with size scale, and discuss the physical mechanisms driving the relationships. We compare the metallicity-dependent star formation models of Ostriker, McKee, and Leroy (2010) and Krumholz (2013) to our observations and find that they both predict the trend in the data, suggesting that the inclusion of a diffuse neutral medium is important at lower metallicity, but do not capture the full extent of the scatter in the relationship between gas and star formation.
We present the optical luminosity functions (LFs) of galaxies for the CLASH-VLT cluster MACS J1206.2-0847 at z=0.439, based on HST and SUBARU data, including ~600 spectroscopically confirmed member galaxies. The LFs on the wide SUBARU FoV are well described by a single Schechter function down to M~M*+3, whereas this fit is poor for HST data, due to a faint-end upturn visible down M~M*+7, suggesting a bimodal behaviour. We also investigate the effect of local environment by deriving the LFs in four different regions, according to the distance from the centre, finding an increase in the faint-end slope going from the core to the outer rings. Our results confirm and extend our previous findings on the analysis of mass functions, which showed that the galaxies with stellar mass below 10^10.5, M_sun have been significantly affected by tidal interaction effects, thus contributing to the intra cluster light.
Active Galactic Nuclei (AGN) with bright radio jets offer the opportunity to study the structure of and physical conditions in relativistic outflows. For such studies, multi-frequency polarimetric very long baseline interferometric (VLBI) observations are important as they directly probe particle densities, magnetic field geometries, and several other parameters. We present results from first-epoch data obtained by the Korean VLBI Network (KVN) within the frame of the Plasma Physics of Active Galactic Nuclei (PAGaN) project. We observed seven radio-bright nearby AGN at frequencies of 22, 43, 86, and 129 GHz in dual polarization mode. Our observations constrain apparent brightness temperatures of jet components and radio cores in our sample to $>10^{8.01}$ K and $>10^{9.86}$ K, respectively. Degrees of linear polarization $m_{L}$ are relatively low overall: less than 10%. This indicates suppression of polarization by strong turbulence in the jets. We found an exceptionally high degree of polarization in a jet component of BL Lac at 43 GHz, with $m_{L} \sim$ 40%. Assuming a transverse shock front propagating downstream along the jet, the shock front being almost parallel to the line of sight can explain the high degree of polarization.
We report first results from KVN and VERA Array (KaVA) VLBI observations obtained in the frame of our Plasma-physics of Active Galactic Nuclei (PAGaN) project. We observed eight selected AGN at 22 and 43 GHz in single polarization (LCP) between March 2014 and April 2015. Each source was observed for 6 to 8 hours per observing run to maximize the $uv$ coverage. We obtained a total of 15 deep high-resolution images permitting the identification of individual circular Gaussian jet components and three spectral index maps of BL Lac, 3C 111 and 3C 345 from simultaneous dual-frequency observations. The spectral index maps show trends in agreement with general expectations -- flat core and steep jets -- while the actual value of the spectral index for jets shows indications for a dependence on AGN type. We analyzed the kinematics of jet components of BL Lac and 3C 111, detecting superluminal proper motions with maximum apparent speeds of about $5c$. This constrains the lower limits of the intrinsic component velocities to $\sim0.98c$ and the upper limits of the angle between jet and line of sight to $\sim$20$\deg$. In agreement with global jet expansion, jet components show systematically larger diameters $d$ at larger core distances $r$, following the global relation $d\approx0.2r$, albeit within substantial scatter.
Active galactic nuclei (AGN) are known for irregular variability on all time scales, down to intra-day variability with relative variations of a few percent within minutes to hours. On such short timescales, unexplored territory, such as the possible existence of a shortest characteristic time scale of activity and the shape of the high frequency end of AGN power spectra, still exists. We present the results of AGN single-dish fast photometry performed with the Korean VLBI Network (KVN). Observations were done in a "anti-correlated" mode using two antennas, with always at least one antenna pointing at the target. This results in an effective time resolution of less than three minutes. We used all four KVN frequencies, 22, 43, 86, and 129 GHz, in order to trace spectral variability, if any. We were able to derive high-quality light curves for 3C 111, 3C 454.3, and BL Lacertae at 22 and 43 GHz, and for 3C 279 at 86 GHz, between May 2012 and April 2013. We performed a detailed statistical analysis in order to assess the levels of variability and the corresponding upper limits. We found upper limits on flux variability ranging from $\sim$1.6% to $\sim$7.6%. The upper limits on the derived brightness temperatures exceed the inverse Compton limit by three to six orders of magnitude. From our results, plus comparison with data obtained by the University of Michigan Radio Astronomy Observatory, we conclude that we have not detected source-intrinsic variability which would have to occur at sub-per cent levels.
We present preliminary results of the spectral analysis on the radial distributions of the star formation history in both, a galaxy merger and a spiral isolated galaxy observed with MaNGA. We find that the central part of the isolated galaxy is composed by older stellar population ($\sim$2 Gyr) than in the outskirts ($\sim$7 Gyr). Also, the time-scale is gradually larger from 1 Gyr in the inner part to 3 Gyr in the outer regions of the galaxy. In the case of the merger, the stellar population in the central region is older than in the tails, presenting a longer time-scale in comparison to central part in the isolated galaxy. Our results are in agreement with a scenario where spiral galaxies are built from inside-out. In the case of the merger, we find evidence that interactions enhance star formation in the central part of the galaxy.
We explore the effect of galactic environment on properties of molecular clouds. Using clouds formed in a large-scale galactic disc simulation, we measure the observable properties from synthetic column density maps. We confirm that a significant fraction of unbound clouds forms naturally in a galactic disc environment and that a mixed population of bound and unbound clouds can match observed scaling relations and distributions for extragalactic molecular clouds. By dividing the clouds into inner and outer disc populations, we compare their distributions of properties and test whether there are statistically significant differences between them. We find that clouds in the outer disc have lower masses, sizes, and velocity dispersions as compared to those in the inner disc for reasonable choices of the inner/outer boundary. We attribute the differences to the strong impact of galactic shear on the disc stability at large galactocentric radii. In particular, our Toomre analysis of the disc shows a narrowing envelope of unstable masses as a function of radius, resulting in the formation of smaller, lower mass fragments in the outer disc. We also show that the star formation rate is affected by the environment of the parent cloud, and is particularly influenced by the underlying surface density profile of the gas throughout the disc. Our work highlights the strengths of using galaxy-scale simulations to understand the formation and evolution of cloud properties - and the star formation within them - in the context of their environment.
The time delay experienced by a light ray as it passes through a changing gravitational potential by a non-zero mass distribution along the line of sight is usually referred to as Shapiro delay. Shapiro delay has been extensively measured in the Solar system and in binary pulsars, enabling stringent tests of general relativity as well as measurement of neutron star masses . However, Shapiro delay is ubiquitous and experienced by all astrophysical messengers on their way from the source to the Earth. We calculate the "one-way" static Shapiro delay for the first discovered millisecond pulsar PSR~B1937+21, by including the contributions from both the dark matter and baryonic matter between this pulsar and the Earth. We find a value of approximately 5 days (of which 4.74 days is from the dark matter and 0.22 days from the baryonic matter). We also calculate the modulation of Shapiro delay from the motion of a single dark matter halo, and also evaluate the cumulative effects of the motion of matter distribution on the change in pulsar's period and its derivative. The time-dependent effects are too small to be detected with the current timing noise observed for this pulsar. Finally, we would like to emphasize that although the one-way Shapiro delay is mostly of academic interest for electromagnetic astronomy, its ubiquity should not be forgotten in the era of multi-messenger astronomy.
High velocity stars are stars moving at velocities so high to require an acceleration mechanism involving binary systems or the presence of a massive central black hole. In the frame of a galaxy hosting a supermassive black hole binary (of total mass $10^8$ M$_\odot$), we investigated a mechanism for the production of high velocity stars due to the close interaction between a massive and orbitally decayed globular cluster and the super massive black hole binary. Some stars of the cluster acquire high velocities by conversion of gravitational energy into kinetic energy deriving from their interaction with the black hole binary. After the interaction, few stars reach a velocity sufficient to overcome the galactic gravitational well, while some of them are just stripped from the globular cluster and start orbiting around the galactic centre.
The Herschel Space Observatory was the fourth cornerstone mission in the European Space Agency (ESA) science programme with excellent broad band imaging capabilities in the sub-mm and far-infrared part of the spectrum. Although the spacecraft finished its observations in 2013, it left a large legacy dataset that is far from having been fully scrutinised and still has a large potential for new scientific discoveries. This is specifically true for the photometric observations of the PACS and SPIRE instruments. Some source catalogues have already been produced by individual observing programs, but there are many observations that risk to remain unexplored. To maximise the science return of the SPIRE and PACS data sets, we are in the process of building the Herschel Point Source Catalogue (HPSC) from all primary and parallel mode observations. Our homogeneous source extraction enables a systematic and unbiased comparison of sensitivity across the different Herschel fields that single programs will generally not be able to provide. The catalogue will be made available online through archives like the Herschel Science Archive (HSA), the Infrared Science Archive (IRSA), and the Strasbourg Astronomical Data Center (CDS).
We performed mapping observations of the Class I protostellar binary system L1551 NE in the C$^{18}$O ($J$=3-2), $^{13}$CO ($J$=3-2), CS ($J$=7-6), and SO ($J_N$=7$_8$-6$_7$) lines with Atacama Submillimeter Telescope Experiment (ASTE). The ASTE C$^{18}$O data are combined with our previous SMA C$^{18}$O data, which show a $r \sim$300-AU scale Keplerian disk around the protostellar binary system. The C$^{18}$O maps show a $\sim$20000-AU scale protostellar envelope surrounding the central Keplerian circumbinary disk. The envelope exhibits a northeast (blue) - southwest (red) velocity gradient along the minor axis, which can be interpreted as a dispersing gas motion with an outward velocity of 0.3 km s$^{-1}$, while no rotational motion in the envelope is seen. In addition to the envelope, two $\lesssim$4000 AU scale, high-velocity ($\gtrsim$1.3 km s$^{-1}$) redshifted $^{13}$CO and CS emission components are found to $\sim$40$^{\prime\prime}$ southwest and $\sim$20$^{\prime\prime}$ west of the protostellar binary. These redshifted components are most likely outflow components driven from the neighboring protostellar source L1551 IRS 5, and are colliding with the envelope in L1551 NE. The net momentum, kinetic and internal energies of the L1551 IRS 5 outflow components are comparable to those of the L1551 NE envelope, and the interactions between the outflows and the envelope are likely to cause the dissipation of the envelope and thus suppression of the further growth of the mass and mass ratio of the central protostellar binary in L1551 NE.
[abridged] We present a strong-lensing analysis of MACSJ0717.5+3745, based on the full depth of the Hubble Frontier Field (HFF) observations, which brings the number of multiply imaged systems to 61, ten of which are spectroscopically confirmed. The total number of images comprised in these systems rises to 165. Our analysis uses a parametric mass reconstruction technique, as implemented in the Lenstool software, to constrain a mass distribution composed of four large-scale mass components + galaxy-scale perturbers. We find a superposition of cored isothermal mass components to provide a good fit to the observational constraints, resulting in a very shallow mass distribution for the smooth (large-scale) component. Given the implications of such a flat mass profile, we investigate whether a model composed of "peaky" non-cored mass components can also reproduce the observational constraints. We find that such a non-cored mass model reproduces the observational constraints equally well. Although the total mass distributions of both models are consistent, as well as the integrated two dimensional mass profiles, we find that the smooth and the galaxy-scale components are very different. We conclude that, even in the HFF era, the generic degeneracy between smooth and galaxy-scale components is not broken, in particular in such a complex galaxy cluster. Consequently, insights into the mass distribution of MACS J0717 remain limited, underlining the need for additional probes beyond strong lensing. Our findings also have implications for estimates of the lensing magnification: we show that the amplification difference between the two models is larger than the error associated with either model. This uncertainty decreases the area of the image plane where we can reliably study the high-redshift Universe by 50 to 70%.
We report on the study of interstellar extinction across the Tarantula nebula (30 Doradus), in the Large Magellanic Cloud, using observations from the Hubble Tarantula Treasury Project in the 0.3 - 1.6 micron range. The considerable and patchy extinction inside the nebula causes about 3500 red clump stars to be scattered along the reddening vector in the colour-magnitude diagrams, thereby allowing an accurate determination of the reddening slope in all bands. The measured slope of the reddening vector is remarkably steeper in all bands than in the the Galactic diffuse interstellar medium. At optical wavelengths, the larger ratio of total-to-selective extinction, namely Rv = 4.5 +/- 0.2, implies the presence of a grey component in the extinction law, due to a larger fraction of large grains. The extra large grains are most likely ices from supernova ejecta and will significantly alter the extinction properties of the region until they sublimate in 50 - 100 Myr. We discuss the implications of this extinction law for the Tarantula nebula and in general for regions of massive star formation in galaxies. Our results suggest that fluxes of strongly star forming regions are likely to be underestimated by a factor of about 2 in the optical.
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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.
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