We present deep $g,i$-band DECam stellar photometry of the Hercules Milky Way satellite galaxy, and its surrounding field, out to a radial distance of 5.4 times the tidal radius. We have identified nine extended stellar substructures associated with the dwarf; preferentially distributed along the major axis of the galaxy. Two significant over-densities lie outside the 95\% confidence band for the likely orbital path of the galaxy and appear to be free-floating tidal debris. We estimate the luminosity of the new stellar substructures, and find that approximately the same amount of stellar flux is lying in these extended structures as inside the main body of Hercules. We also analyse the distribution of candidate blue-horizontal-branch stars and find agreement with the alignment of the substructures at a confidence level greater than 98\%. Our analysis provides a quantitative demonstration that Hercules is a strongly tidally disrupted system, with noticeable stellar features at least 1.9 kpc away from the galaxy.
We use a sample of 262 spectroscopically confirmed star-forming galaxies at redshifts $2.08\leq z\leq 2.51$ to compare H$\alpha$, UV, and IR star-formation-rate diagnostics and to investigate the dust properties of the galaxies. At these redshifts, the H$\alpha$ line shifts to the $K_{s}$-band. By comparing $K_{s}$-band photometry to underlying stellar population model fits to other UV, optical, and near-infrared data, we infer the H$\alpha$ flux for each galaxy. We obtain the best agreement between H$\alpha$- and UV-based SFRs if we assume that the ionized gas and stellar continuum are reddened by the same value and that the Calzetti attenuation curve is applied to both. Aided with MIPS 24$\mu$m data, we find that an attenuation curve steeper than the Calzetti curve is needed to reproduce the observed IR/UV ratios of galaxies younger than 100 Myr. Furthermore, using the bolometric star-formation rate inferred from the UV and mid-IR data (SFR$_{IR}$+SFR$_{UV}$), we calculated the conversion between the H$\alpha$ luminosity and SFR to be $(7.5\pm1.3) \times 10^{-42}$ for a Salpeter IMF, which is consistent with the Kennicutt (1998) conversion. The derived conversion factor is independent of any assumption of the dust correction and is robust to stellar population model uncertainties.
We present a first application of the subhalo abundance matching (SHAM) method to describe the redshift-space clustering of galaxies including the non-linear redshift-space distortion, i.e., the Fingers-of-God. We find that the standard SHAM connecting the luminosity of galaxies to the maximum circular velocity of subhalos well reproduces the luminosity dependence of redshift-space clustering of galaxies in the Sloan Digital Sky Survey in a wide range of scales from 0.3 to 40 Mpc/h. The result indicates that the SHAM approach is very promising for establishing a theoretical model of redshift-space galaxy clustering without additional parameters. We also test color abundance matching using two different proxies for colors: subhalo age and local dark matter density following the method by Masaki et al. (2013b). Observed clustering of red galaxies exhibits much stronger Fingers-of-God effect than blue galaxies. We find that the subhalo age model describes the observed color-dependent redshift-space clustering much better than the local dark matter density model. The result infers that the age of subhalos is a key ingredient to determine the color of galaxies.
Based on stellar population models without (SSP) and with (BSP) binary interactions, we investigate the effects of binary interactions on parameter determinations for early-type galaxies (ETGs). We present photometric redshift (photo-z), age and spectral type for photometric data sample by fitting observed magnitudes with the SSP and BSP models. Our results show that binary interactions have no effect on photo-z estimation. Once we neglect binary interactions, the age of ETGs will be underestimated, by contrast, the effects on the age estimations can be negligible for other type of galaxies. For ETG sample, we derive their properties by fitting their spectra with the SSP and BSP models. When comparing these galaxy properties, we find no variation of the overall metallicities for ETGs among the SSP and BSP models. Moreover, the inclusion of binary interactions can affect age estimations. Our results show that the BSP-fitted ages in ~33.3% of ETG sample are around 0.5-1.0 Gyr larger than the SSP-fitted ages; ~44.2\% are only 0.1-0.5 Gyr larger; the rest ~22.5% are approximately equal. By comparisons, we find the difference of the star formation rate between the SSP and BSP models is large at the late evolution stage.
We use recent very extended (out to 48 kpc) HI kinematics alongside with previous H$\alpha$ kinematics of the spiral galaxy NGC 3198 in order to derive its distribution of Dark Matter (DM). First, we use a chi-square method to model the Rotation Curve of this galaxy in terms of different profiles of its DM distribution: the Universal Rotation Curve (URC) mass model (stellar disk $+$ Burkert halo $+$ gaseous disk), the NFW mass model (stellar disk $+$ NFW halo $+$ gaseus disk) and the Baryon$\Lambda$CDM mass model (stellar disk $+$ NFW halo modified by baryonic physics $+$ gaseous disk). Secondly, in order to derive the DM halo density distribution we apply a new method developed by Salucci et al.(2010) which does not require a global and often uncertain mass modelling. We find that, while, according to the standard method, both URC and NFW mass models can account for the RC, the new method instead leads to a density profile which is in sharp disagreement with the dark halo density distribution predicted within the Lambda Cold Dark Matter ($\Lambda$CDM) scenario. We find that the effects of baryonic physics proposed by Di Cintio et al. (2014) modify the original $\Lambda$CDM halo densities in such a way that the resulting profile is more compatible with the DM density of NGC 3198 derived using our new method. However, at large distances, r $\sim$ 25 kpc, also this modified Baryon$\Lambda$CDM halo profile appears in tension with the derived DM halo density.
Initially cold and spherically symmetric self-gravitating systems may give rise to a virial equilibrium state which is far from spherically symmetric, and typically triaxial. We focus here on how the degree of symmetry breaking in the final state depends on the initial density profile. We note that the most asymmetric structures result when, during the collapse phase, there is a strong injection of energy preferentially into the particles which are localized initially in the outer shells. These particles are still collapsing when the others, initially located in the inner part, are already re-expanding; the motion of particles in a time varying potential allow them to gain kinetic energy --- in some cases enough to be ejected from the system. We show that this mechanism of energy gain amplifies the initial small deviations from perfect spherical symmetry due to finite $N$ fluctuations. This amplification is more efficient when the initial density profile depends on radius, because particles have a greater spread of fall times compared to a uniform density profile, for which very close to symmetric final states are obtained}. These effects lead to a distinctive correlation of the orientation of the final structure with the distribution of ejected mass, and also with the initial (very small) angular fluctuations.
Photoionization modelling allows to follow the transport, the emergence, and the absorption of photons taking into account all important processes in nebular plasmas. Such modelling needs the spatial distribution of density, chemical abundances and temperature, that can be provided by chemo-dynamical simulations (ChDS) of dwarf galaxies. We perform multicomponent photoionization modelling (MPhM) of the ionized gas using 2-D ChDSs of dwarf galaxies. We calculate emissivity maps for important nebular emission lines. Their intensities are used to derive the chemical abundance of oxygen by the so-called Te- and R23-methods. Some disagreements are found between oxygen abundances calculated with these methods and the ones coming from the ChDSs. We investigate the fraction of ionizing radiation emitted in the star-forming region which is able to leak out the galaxy. The time- and direction-averaged escape fraction in our simulation is 0.35-0.4. Finally, we have calculated the total Halpha lumi- nosity of our model galaxy using Kennicutt's calibration to derive the star-formation rate. This value has been compared to the 'true' rate in the ChDSs. The Halpha-based star-formation rate agrees with the true one only at the beginning of the simulation. Minor deviations arise later on and are due in part to the production of high-energy photons in the warm-hot gas, in part to the leakage of energetic photons out of the galaxy. The effect of artificially introduced thin dense shells (with thicknesses smaller than the ChDSs spatial resolution) is investigated, as well.
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We present a study of the dynamical properties of 125 compact stellar systems (CSSs) in the nearby giant elliptical galaxy NGC5128, using high-resolution spectra (R~26,000) obtained with VLT/FLAMES. Our results provide evidence for a new type of star cluster, based on the CSS dynamical mass scaling relations. All radial velocity (v_r) and line-of-sight velocity dispersion (sigma_los) measurements are performed with the penalized pixel fitting (ppxf) technique, which provided sigma_ppxf estimates for 115 targets. The sigma_ppxf estimates are corrected to the 2D projected half-light radii, sigma_{1/2}, as well as the cluster cores, sigma_0, accounting for observational/aperture effects and are combined with structural parameters, from high spatial resolution imaging, in order to derive total dynamical masses (M_dyn) for 112 members of NGC5128's star cluster system. In total, 89 CSSs have dynamical masses measured for the first time along with the corresponding dynamical mass-to-light ratios (Upsilon_V^dyn). We find two distinct sequences in the Upsilon_V^dyn - M_dyn plane, which are well approximated by power laws of the forms Upsilon_V^dyn ~ M_dyn^0.33+\-0.04 and Upsilon_V^dyn - M_dyn^0.79+\-0.04. The shallower sequence corresponds to the very bright tail of the globular cluster luminosity function (GCLF), while the steeper relation appears to be populated by a distinct group of objects which require significant dark gravitating components such as central massive black holes and/or exotically concentrated dark matter distributions. This result would suggest that the formation and evolution of these CSSs are markedly different from the "classical" globular clusters in NGC5128 and the Local Group, despite the fact that these clusters have luminosities similar to the GCLF turn-over magnitude.~We include a thorough discussion of myriad factors potentially influencing our measurements.
We present a study of extended galaxy halo gas through HI and OVI absorption over two decades in projected distance at $z\approx0.2$. The study is based on a sample of $95$ galaxies from a highly complete ($ > 80\%$) survey of faint galaxies ($L > 0.1L_*$) with archival quasar absorption spectra and $53$ galaxies from the literature. A clear anti-correlation is found between HI (OVI) column density and virial radius normalized projected distance, $d/R_{\rm h}$. Strong HI (OVI) absorption systems with column densities greater than $10^{14.0}$ ($10^{13.5}$) cm$^{-2}$ are found for $48$ of $54$ ($36$ of $42$) galaxies at $d < \,R_{\rm h}$ indicating a mean covering fraction of $\langle\kappa_{\rm HI}\rangle=0.89$ ($\langle\kappa_{\rm OVI}\rangle=0.86$). OVI absorbers are found at $d\approx R_{\rm h}$, beyond the extent observed for lower ionization species. At $d/R_{\rm h}=1-3$ strong HI (OVI) absorption systems are found for only $7$ of $43$ ($5$ of $34$) galaxies ($\langle\kappa_{\rm HI}\rangle=0.16$ and $\langle\kappa_{\rm OVI}\rangle=0.15$). Beyond $d=3\,R_{\rm h}$, the HI and OVI covering fractions decrease to levels consistent with coincidental systems. The high completeness of the galaxy survey enables an investigation of environmental dependence of extended gas properties. Galaxies with nearby neighbors exhibit a modest increase in OVI covering fraction at $d>R_{\rm h}$ compared to isolated galaxies ($\kappa_{\rm OVI}\approx0.13$ versus $0.04$) but no excess HI absorption. These findings suggest that environmental effects play a role in distributing heavy elements beyond the enriched gaseous halos of individual galaxies. Finally, we find that differential HI and OVI absorption between early- and late-type galaxies continues from $d < R_{\rm h}$ to $d\approx3\,R_{\rm h}$.
Radio-loud Active Galactic Nuclei at z~2-4 are typically located in dense environments and their host galaxies are among the most massive systems at those redshifts, providing key insights for galaxy evolution. Finding radio-loud quasars at the highest accessible redshifts (z~6) is important to study their properties and environments at even earlier cosmic time. They would also serve as background sources for radio surveys intended to study the intergalactic medium beyond the epoch of reionization in HI 21 cm absorption. Currently, only five radio-loud ($R=f_{\nu,5{\rm GHz}}/f_{\nu,4400\AA}>10$) quasars are known at z~6. In this paper we search for 5.5 < z < 7.2 quasars by cross-matching the optical Pan-STARRS1 and radio FIRST surveys. The radio information allows identification of quasars missed by typical color-based selections. While we find no good 6.4 < z <7.2 quasar candidates at the sensitivities of these surveys, we discover two new radio-loud quasars at z~6. Furthermore, we identify two additional z~6 radio-loud quasars which were not previously known to be radio-loud, nearly doubling the current z~6 sample. We show the importance of having infrared photometry for z>5.5 quasars to robustly classify them as radio-quiet or radio-loud. Based on this, we reclassify the quasar J0203+0012 (z=5.72), previously considered radio-loud, to be radio-quiet. Using the available data in the literature, we constrain the radio-loud fraction of quasars at z~6, using the Kaplan--Meier estimator, to be $8.1^{+5.0}_{-3.2}\%$. This result is consistent with there being no evolution of the radio-loud fraction with redshift, in contrast to what has been suggested by some studies at lower redshifts.
Gamma-ray bursts (GRBs) offer a route to characterizing star-forming galaxies and quantifying high-$z$ star-formation that is distinct from the approach of traditional galaxy surveys: GRB selection is independent of dust and probes even the faintest galaxies that can evade detection in flux-limited surveys. However, the exact relation between GRB rate and Star Formation Rate (SFR) throughout all redshifts is controversial. The TOUGH survey includes observations of all GRB hosts (69) in an optically unbiased sample and we utilize these to constrain the evolution of the UV GRB-host-galaxy Luminosity Function (LF) between $z=0$ and $z=4.5$, and compare this with LFs derived from both Lyman-break galaxy (LBG) surveys and simulation modeling. At all redshifts we find the GRB hosts to be most consistent with a Luminosity Function derived from SFR weighted models incorporating GRB production via both metallicity-dependent and independent channels with a relatively high level of bias towards low metallicity hosts. In the range $1<z<3$ an SFR weighted LBG derived (i.e. non-metallicity biased) LF is also a reasonable fit to the data. Between $z\sim3$ and $z\sim6$, we observe an apparent lack of UV bright hosts in comparison with Lyman-break galaxies, though the significance of this shortfall is limited by nine hosts of unknown redshift.
The radial profiles of gas, stars, and far ultraviolet radiation in 20 dwarf Irregular galaxies are converted to stability parameters and scale heights for a test of the importance of two-dimensional (2D) instabilities in promoting star formation. A detailed model of this instability involving gaseous and stellar fluids with self-consistent thicknesses and energy dissipation on a perturbation crossing time give the unstable growth rates. We find that all locations are effectively stable to 2D perturbations, mostly because the disks are thick. We then consider the average volume densities in the midplanes, evaluated from the observed HI surface densities and calculated scale heights. The radial profiles of the star formation rates are equal to about 1% of the HI surface densities divided by the free fall times at the average midplane densities. This 1% resembles the efficiency per unit free fall time commonly found in other cases. There is a further variation of this efficiency with radius in all of our galaxies, following the exponential disk with a scale length equal to about twice the stellar mass scale length. This additional variation is modeled by the molecular fraction in a diffuse medium using radiative transfer solutions for galaxies with the observed dimensions and properties of our sample. We conclude that star formation is activated by a combination of three-dimensional gaseous gravitational processes and molecule formation. Implications for outer disk structure and formation are discussed.
We report preliminary results of large-scale distribution toward the Magellanic supernova remnant N132D using Mopra and Chandra archival datasets. We identified a cavity-like CO structure along the X-ray shell toward the southern half of it. The total mass of associating molecular gas is $\sim10^4 M_\odot$, which is smaller than the previous study by an order of magnitude. Further observations using ALMA, ASTE, and Mopra will reveal the detailed spatial structures and its physical conditions.
The scenario of galaxy formation is believed to follow a structure that builds up from the bottom, with large galaxies being formed by several merging episodes of smaller ones. In this scenario a number of galaxies can be expected to be seen in the merging phase, with their external regions already mixed, while their nuclei, with stronger self-gravitation, are still recognizable as such. During a photometric monitoring of AGNs in the field of a long-exposure INTEGRAL pointing, we serendipitously found an elliptical galaxy in the center of the X-ray cluster (EXO 0422-086) with two nuclei. We performed surface photometry on our images and those of the SDSS archive and obtained slit spectra of both nuclei. Aperture photometry of the two stellar-like nuclei showed very similar colors in the SDSS image and in our Johnson BVRI images, which is typical of an elliptical galaxy nucleus. The spectra of the nuclei showed the typical absorption lines of an elliptical galaxy without appreciable emission lines. The redshifts derived from each nucleus were equal and fully consistent with the literature value (0.0397). We can therefore exclude the possibility that one of the nuclei is a foreground star or a background AGN and consider this elliptical galaxy as a bona fide example of a galaxy merger.
NGC 4258 is the galaxy with the most accurate (maser-based) determination for the mass of the supermassive black hole (SMBH) in its nucleus. In this work we present a two-dimensional mapping of the stellar kinematics in the inner 3.0 x 3.0 arcsec = 100 x 100 pc of NGC 4258 using adaptative-optics observations obtained with the Near-Infrared Integral Field Spectrograph of the GEMINI North telescope at a 0.11 arcsec (4 pc) angular resolution. The observations resolve the radius of influence of the SMBH, revealing an abrupt increase in the stellar velocity dispersion within 10 pc from the nucleus, consistent with the presence of a SMBH there. Assuming that the galaxy nucleus is in a steady state and that the velocity dispersion ellipsoid is aligned with a cylindrical coordinate system, we constructed a Jeans anisotropic dynamical model to fit the observed kinematics distribution. Our dynamical model assumes that the galaxy has axial symmetry and is constructed using the multi-gaussian expansion method to parametrize the observed surface brightness distribution. The Jeans dynamical model has three free parameters: the mass of the central SMBH, the mass-luminosity ratio of the galaxy and the anisotropy of the velocity distribution. We test two types of models: one with constant velocity anisotropy, and another with variable anisotropy. The model that best reproduces the observed kinematics was obtained considering that the galaxy has radially varying anisotropy, being the best-fitting parameters with 3$\sigma$ significance $M_\bullet=4.8^{+0.8}_{-0.9}\times 10^7\,{\rm M_\odot}$ and $\Gamma_k = 4.1^{+0.4}_{-0.5}$. This value for the mass of the SMBH is just 25 per cent larger than that of the maser determination and 50 per cent larger that a previous stellar dynamical determination obtained via Schwarzschild models.
Galactic bulges are complex systems. Once thought to be small-scale versions of elliptical galaxies, advances in astronomical instrumentation (spectroscopy in particular) has revealed a wealth of photometric and kinematic substructure in otherwise simple-looking components. This review provides an overview of how our perspective on galactic bulges has changed over the years. While it is mainly focused on aspects related to the dynamical state of their stars, there will be natural connections to other properties (e.g. morphology, stellar populations) discussed in other reviews in this volume.
Aims. The main goal of this study is to detect the stellar overdensity associated with the Perseus arm in the anticenter direction. Methods. We used the physical parameters derived from Str\"omgren photometric data to compute the surface density distribution as a function of galactocentric distance for different samples of intermediate young stars. The radial distribution of the interstellar absorption has also been derived. Results. We detected the Perseus arm stellar overdensity at 1.6+-0.2 kpc from the Sun with a significance of 4-5{\sigma} and a surface density amplitude of around 10%, slightly depending on the sample used. Values for the radial scale length of the Galactic disk have been simultaneously fitted obtaining values in the range [2.9,3.5] kpc for the population of the B4-A1 stars. Moreover, the interstellar visual absorption distribution is congruent with a dust layer in front of the Perseus arm. Conclusions. This is the first time that the presence of the Perseus arm stellar overdensity has been detected through individual star counts, and its location matches a variation in the dust distribution. The offset between the dust lane and the overdensity indicates that the Perseus arm is placed inside the co-rotation radius of the Milky Way spiral pattern.
We present an analysis of the optical nuclear spectra from the active galactic nuclei (AGN) in a sample of giant low surface brightness (GLSB) galaxies. GLSB galaxies are extreme late type spirals that are large, isolated and poorly evolved compared to regular spiral galaxies. Earlier studies have indicated that their nuclei have relatively low mass black holes. Using data from the Sloan Digital Sky Survey (SDSS), we selected a sample of 30 GLSB galaxies that showed broad H$\alpha$ emission lines in their AGN spectra. In some galaxies such as UGC 6284, the broad component of H$\alpha$ is more related to outflows rather than the black hole. One galaxy (UGC 6614) showed two broad components in H$\alpha$, one associated with the black hole and the other associated with an outflow event. We derived the nuclear black hole (BH) masses of 29 galaxies from their broad H$\alpha$ parameters. We find that the nuclear BH masses lie in the range $10^{5}-10^{7} M_{\odot}$. 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. The effects of galaxy orientation in the measurement of $\sigma_e$ and the increase of $\sigma_e$ due to the effects of bar are probable reasons for the observed offset for some galaxies, but in many galaxies the offset is real. A possible explanation for the $M_{BH}-\sigma_{e}$ offset could be lack of mergers and accretion events in the history of these galaxies which leads to a lack of BH-bulge co-evolution. \keywords{galaxies: active, galaxies: bulges, galaxies: nuclei}
A large number of high-dispersion spectra of classical Cepheids were obtained in the region of the CaII H+K spectral lines. The analysis of these spectra allowed us to detect the presence of a strong Balmer line, H$\epsilon$, for several Cepheids, interpreted as the signature of a blue companion: the presence of a sufficiently bright blue companion to the Cepheid results in a discernible strengthening of the CaII H + Hepsilon line relative to the CaII K line. We investigated 103 Cepheids, including those with known hot companions (B5-B6 main-sequence stars) in order to test the method. We could confirm the presence of a companion to WW Car and FN Vel (the existence of the former was only suspected before) and we found that these companions are blue hot stars. The method remains efficient when the orbital velocity changes in a binary system cannot be revealed and other methods of binarity detection are not efficient.
The existence of sub-stellar cold H2 globules in planetary nebulae and the
mere existence of comets suggest that the physics of cold interstellar gas
might be much richer than usually envisioned.
We study the case of a cold gaseous medium in ISM conditions which is subject
to a gas-liquid/solid phase transition.
First the equilibrium of such fluids is studied using the virial theorem and
linear stability analysis. Then the non-linear dynamics is studied by using
simulations in order to characterize the expected formation of solid bodies
analogous to comets. The simulations are run with a state of the art molecular
dynamics code (LAMMPS). The long-range gravitational forces can be taken into
account together with short-range molecular forces with finite limited
computational resources by using super-molecules, provided the right scaling is
followed.
The concept of super-molecule is tested with simulations, allowing to
correctly satisfy the ideal gas Jeans instability criterion for one-phase
fluids. The simulations show that fluids presenting a phase transition are
gravitationally unstable as well, independent of the strength of the
gravitational potential, producing two distinct kinds of sub-stellar bodies,
those dominated by gravity ("planetoids") and those dominated by molecular
attractive force ("comets").
Observations, formal analysis and computer simulations suggest the
possibility of the formation of sub-stellar H2 clumps in cold molecular clouds
due to the combination of phase transition and gravity. Fluids in a phase
transition are gravitationally unstable, independent of the strength of the
gravitational potential. Small H2 clumps may form even at relatively high
temperatures, up to 400 - 600K according to virial analysis. The combination of
phase transition and gravity may be relevant for a wider range of astrophysical
situations, such as proto-planetary disks.
We present the design and performance of a non-imaging concentrator for use in broad-band polarimetry at millimeter through submillimeter wavelengths. A rectangular geometry preserves the input polarization state as the concentrator couples f/2 incident optics to a 2 pi sr detector. Measurements of the co-polar and cross-polar beams in both the few-mode and highly over-moded limits agree with a simple model based on mode truncation. The measured co-polar beam pattern is nearly independent of frequency in both linear polarizations. The cross-polar beam pattern is dominated by a uniform term corresponding to polarization efficiency 94%. After correcting for efficiency, the remaining cross-polar response is -18 dB.
In this article we review the astrophysical application of gravitational microlensing. After introducing the history of gravitational lensing, we present the key equations and concept of microlensing. The most frequent microlensing events are single-lens events and historically it has been used for searching dark matter in the form of compact astrophysical halo objects in the Galactic halo. We discuss about the degeneracy problem in the parameters of lens and perturbation effects that can partially break the degeneracy between the lens parameters. The rest of paper is about the astrophysical applications of microlensing. One of the important applications is in the stellar physics by probing the surface of source stars in the high magnification microlensing events. The astrometric and polarimetric observations will be complimentary for probing the atmosphere and stellar spots on the surface of source stars. Finally we discuss about the future projects as space based telescopes for parallax and astrometry observations of microlensing events. With this project, we would expect to produce a complete stellar and remnant mass function and study the structure of Galaxy in term of distribution of stars along our line of sight towards the centre of galaxy.
We examine the stellar mass assembly in galaxy cluster cores using data from the Cluster Lensing and Supernova survey with Hubble (CLASH). We measure the growth of brightest cluster galaxy (BCG) stellar mass, the fraction of the total cluster light which is in the intracluster light (ICL) and the numbers of mergers that occur in the BCG over the redshift range of the sample, 0.18<z<0.90. We find that BCGs grow in stellar mass by a factor of 1.4 on average from accretion of their companions, and this growth is reduced to a factor of 1.2 assuming 50% of the accreted stellar mass becomes ICL, in line with the predictions of simulations. We find that the ICL shows significant growth over this same redshift range, growing by a factor of of 4--5 in its contribution to the total cluster light. This result is in line with our previous findings for ICL at higher redshifts, however our measured growth is somewhat steeper than is predicted by simulations of ICL assembly. We find high mass companions and hence major merging (mergers with objects of masses $\geq$1/2 of the BCG) to be very rare for our sample. We conclude that minor mergers (mergers with objects with masses $<$ 1/2 of the BCG) are the dominant process for stellar mass assembly at low redshifts, with the majority of the stellar mass from interactions ending up contributing to the ICL rather than building up the BCG. From a rough estimate of the stellar mass growth of the ICL we also conclude that the majority of the ICL stars must come from galaxies which fall from outside of the core of the cluster, as is predicted by simulations. It appears that the growth of the ICL is the major evolution event in galaxy cluster cores during the second half of the lifetime of the Universe.
We present the results obtained from linear stability analysis and 2.5-dimensional magnetohydrodynamic (MHD) simulations of the magnetorotational instability (MRI), including the effects of cosmic rays (CRs). We took into account of the CR diffusion along the magnetic field but neglect the cross-field-line diffusion. Two models are considered in this paper: shearing box model and differentially rotating cylinder model. We studied how MRI is affected by the initial CR pressure (i.e., energy) distribution. In the shearing box model, the initial state is uniform distribution. Linear analysis shows that the growth rate of MRI does not depend on the value of CR diffusion coefficient. In the differentially rotating cylinder model, the initial state is a constant angular momentum polytropic disk threaded by weak uniform vertical magnetic field. Linear analysis shows that the growth rate of MRI becomes larger if the CR diffusion coefficient is larger. Both results are confirmed by MHD simulations. The MHD simulation results show that the outward movement of matter by the growth of MRI is not impeded by the CR pressure gradient, and the centrifugal force which acts to the concentrated matter becomes larger. Consequently, the growth rate of MRI is increased. On the other hand, if the initial CR pressure is uniform, then the growth rate of the MRI barely depends on the value of the CR diffusion coefficient.
Recent results by Bensby and collaborators on the ages of microlensed stars in the Galactic bulge have challenged the picture of an exclusively old stellar population. However, these age estimates have not been independently confirmed. In this paper we verify these results by means of a grid-based method and quantify the systematic biases that might be induced by some assumptions adopted to compute stellar models. We explore the impact of increasing the initial helium abundance, neglecting the element microscopic diffusion, and changing the mixing-length calibration in theoretical stellar track computations. We adopt the SCEPtER pipeline with a novel stellar model grid for metallicities [Fe/H] from -2.00 to 0.55 dex, and masses in the range [0.60; 1.60] Msun from the ZAMS to the helium flash at the red giant branch tip. We show for the considered evolutionary phases that our technique provides unbiased age estimates. Our age results are in good agreement with Bensby and collaborators findings and show 16 stars younger than 5 Gyr and 28 younger than 9 Gyr over a sample of 58. The effect of a helium enhancement as large as Delta Y/Delta Z = 5 is quite modest, resulting in a mean age increase of metal rich stars of 0.6 Gyr. Even simultaneously adopting a high helium content and the upper values of age estimates, there is evidence of 4 stars younger than 5 Gyr and 15 younger than 9 Gyr. For stars younger than 5 Gyr, the use of stellar models computed by neglecting microscopic diffusion or by assuming a super-solar mixing-length value leads to a mean increase in the age estimates of about 0.4 Gyr and 0.5 Gyr respectively. Even considering the upper values for the age estimates, there are four stars estimated younger than 5 Gyr is in both cases. Thus, the assessment of a sizeable fraction of young stars among the microlensed sample in the Galactic bulge appears robust.
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Bars have a complex three-dimensional shape. In particular their inner part is vertically much thicker than the parts further out. Viewed edge-on, the thick part of the bar is what is commonly known as a boxy-, peanut- or X- bulge and viewed face-on it is referred to as a barlens. These components are due to disc and bar instabilities and are composed of disc material. I review here their formation, evolution and dynamics, using simulations, orbital structure theory and comparisons to observations.
We investigate the abundance of galactic molecular hydrogen (H$_2$) in the "Evolution and Assembly of GaLaxies and their Environments" (EAGLE) cosmological hydrodynamic simulations. We assign H$_2$ masses to gas particles in the simulations in post-processing using two different prescriptions that depend on the local dust-to-gas ratio and the interstellar radiation field. Both result in H$_2$ galaxy mass functions that agree well with observations in the local and high-redshift Universe. The simulations reproduce the observed scaling relations between the mass of H$_2$ and the stellar mass, star formation rate and stellar surface density. Towards high edshifts, galaxies in the simulations display larger H$_2$ mass fractions, and correspondingly lower H$_2$ depletion timescales, also in good agreement with observations. The comoving mass density of H$_2$ in units of the critical density, $\Omega_{\rm H_2}$, peaks at $z\approx 1.2-1.5$, later than the predicted peak of the cosmic star formation rate activity, at $z\approx 2$. This difference stems from the decrease in gas metallicity and increase in interstellar radiation field with redshift, both of which hamper H$_2$ formation. We find that the cosmic H$_2$ budget is dominated by galaxies with $M_{\rm H_2}>10^9\,\rm M_{\odot}$, star formation rates $>10\,\rm M_{\odot}\,\rm yr^{-1}$ and stellar masses $M_{\rm stellar}>10^{10}\,\rm M_{\odot}$, which are readily observable in the optical and near-IR. The match between the H$_2$ properties of galaxies that emerge in the simulations and observations is remarkable, particularly since it involves no adjustable parameters.
We present the catalogue of blended galaxy spectra from the Galaxy And Mass Assembly (GAMA) survey. These are cases where light from two galaxies are significantly detected in a single GAMA fibre. Galaxy pairs identified from their blended spectrum fall into two principal classes: they are either strong lenses, a passive galaxy lensing an emission-line galaxy; or occulting galaxies, serendipitous overlaps of two galaxies, of any type. Blended spectra can thus be used to reliably identify strong lenses for follow-up observations (high resolution imaging) and occulting pairs, especially those that are a late-type partly obscuring an early-type galaxy which are of interest for the study of dust content of spiral and irregular galaxies. The GAMA survey setup and its autoz automated redshift determination were used to identify candidate blended galaxy spectra from the cross-correlation peaks. We identify 280 blended spectra with a minimum velocity separation of 600 km/s, of which 104 are lens pair candidates, 71 emission-line-passive pairs, 78 are pairs of emission-line galaxies and and 27 are pairs of galaxies with passive spectra. We have visually inspected the candidates in the Sloan Digital Sky Survey (SDSS) and Kilo Degree Survey (KiDS) images. Many blended objects are ellipticals with blue fuzz (Ef in our classification). These latter "Ef" classifications are candidates for possible strong lenses, massive ellipticals with an emission-line galaxy in one or more lensed images. The GAMA lens and occulting galaxy candidate samples are similar in size to those identified in the entire SDSS. This blended spectrum sample stands as a testament of the power of this highly complete, second-largest spectroscopic survey in existence and offers the possibility to expand e.g., strong gravitational lens surveys.
We show the effect of galaxy formation on the dark matter (DM) distribution across a wide range of halo masses. We focus on how baryon physics changes the dark matter halo shape, the so called "pseudo phase-space density distribution" and the velocity distribution within the virial radius, Rvir and in the solar neighborhood. This study is based on the NIHAO galaxy formation simulations, a large suite of cosmological zoom-in simulations. The galaxies reproduce key properties of observed galaxies, and hence offer unique insight into how baryons change the dark matter morphology and kinematics. When compared to dark matter only simulations, the NIHAO haloes have similar shapes at Rvir, but are substantially rounder inside ~0.1 Rvir. In DM-only simulations the inner halo has a minor-to-major axis ratio of c/a~0.5. In hydro simulations c/a increases with halo mass and integrated star formation efficiency, reaching ~0.8 at the Milky Way mass, reconciling a long-standing conflict between observations and DM only simulations. The radial profile of the phase-space Q parameter is best fit with a single power law in DM-only simulations, but shows a substantial flattening within ~0.1 Rvir, with hydro. Finally, the global velocity distribution of DM is similar in both DM-only and hydro simulations, but in the solar neighborhood, hydro galaxies deviate substantially from Maxwellian. Instead, dark matter particles show a more symmetric distribution, roughly Gaussian, around the mean, which has implications for direct DM detection experiments. Our results show that the comparison of theoretical predictions with observational data can no longer rely on pure collisionless simulations, but must include the effects of visible matter.
The growth of galaxies through adiabatic accretion of dark matter is one of the main drivers of galaxy evolution. By isolating it from other processes like mergers, we analyse how it affects the evolution of star clusters. Our study comprises a fast and approximate exploration of the orbital and intrinsic cluster parameter space, and more detailed monitoring of their evolution, through N-body simulations for a handful of cases. We find that the properties of present-day star clusters and their tidal tails differ very little, whether the clusters are embedded in a growing galactic halo for 12 Gyr, or in a static one.
We introduce project NIHAO (Numerical Investigation of a Hundred Astrophysical Objects), a set of 100 cosmological zoom-in hydrodynamical simulations performed using the GASOLINE code, with an improved implementation of the SPH algorithm. The haloes in our study range from dwarf to Milky Way masses, and represent an unbiased sampling of merger histories, concentrations and spin parameters. The particle masses and force softenings are chosen to resolve the mass profile to below 1% of the virial radius at all masses, ensuring that galaxy half-light radii are well resolved. Using the same treatment of star formation and stellar feedback for every object, the simulated galaxies reproduce the observed inefficiency of galaxy formation across cosmic time as expressed through the stellar mass vs halo mass relation, and the star formation rate vs stellar mass relation. We thus conclude that stellar feedback is the chief piece of physics required to limit the efficiency of star formation in galaxies less massive than the MilkyWay.
We report the discovery of significant localized structures in the projected two-dimensional (2D) spatial distributions of the Globular Cluster (GC) systems of the ten brightest galaxies in the Virgo Cluster. We use catalogs of GCs extracted from the HST ACS Virgo Cluster Survey (ACSVCS) imaging data, complemented, when available, by additional archival ACS data. These structures have projected sizes ranging from $\sim\!5$ arcsec to few arc-minutes ($\sim\!1$ to $\sim\!25$ kpc). Their morphologies range from localized, circular, to coherent, complex shapes resembling arcs and streams. The largest structures are preferentially aligned with the major axis of the host galaxy. A few relatively smaller structures follow the minor axis. Differences in the shape and significance of the GC structures can be noticed by investigating the spatial distribution of GCs grouped by color and luminosity. The largest coherent GC structures are located in low-density regions within the Virgo cluster. This trend is more evident in the red GC population, believed to form in mergers involving late-type galaxies. We suggest that GC over-densities may be driven by either accretion of satellite galaxies, major dissipationless mergers or wet dissipation mergers. We discuss caveats to these scenarios, and estimate the masses of the potential progenitors galaxies. These masses range in the interval $10^{8.5}\!-\!10^{9.5}$ solar masses, larger than those of the Local Group dwarf galaxies.
We present 2.5-5.0 $\mu$m spectra of 83 nearby ($0.002\,<\,z\,<\,0.48$) and bright ($K<14$mag) type-1 active galactic nuclei (AGNs) taken with the Infrared Camera (IRC) on board $\it{AKARI}$. The 2.5-5.0 $\mu$m spectral region contains emission lines such as Br$\beta$ (2.63 $\mu$m), Br$\alpha$ (4.05 $\mu$m), and polycyclic aromatic hydrocarbons (PAH; 3.3 $\mu$m), which can be used for studying the black hole (BH) masses and star formation activities in the host galaxies of AGNs. The spectral region also suffers less dust extinction than in the ultra violet (UV) or optical wavelengths, which may provide an unobscured view of dusty AGNs. Our sample is selected from bright quasar surveys of Palomar-Green (PG) and SNUQSO, and AGNs with reverberation-mapped BH masses from Peterson et al. (2004). Using 11 AGNs with reliable detection of Brackett lines, we derive the Brackett-line-based BH mass estimators. We also find that the observed Brackett line ratios can be explained with the commonly adopted physical conditions of the broad line region (BLR). Moreover, we fit the hot and warm dust components of the dust torus by adding photometric data of SDSS, 2MASS, $\it{WISE}$, and $\it{ISO}$ to the $\it{AKARI}$ spectra, finding hot and warm dust temperatures of $\sim1100\,\rm{K}$ and $\sim220\,\rm{K}$, respectively, rather than the commonly cited hot dust temperature of 1500 K.
We report measurements of the Diffuse Galactic Light (DGL) spectrum in the near-infrared, spanning the wavelength range 0.95-1.65 {\mu}m by the Cosmic Infrared Background ExpeRiment (CIBER). Using the low-resolution spectrometer (LRS) calibrated for absolute spectro-photometry, we acquired long-slit spectral images of the total diffuse sky brightness towards four high-latitude fields spread over four sounding rocket flights. To separate the DGL spectrum from the total sky brightness, we correlated the spectral images with a 100 {\mu}m intensity map, which traces the dust column density in optically thin regions. The measured DGL spectrum shows no resolved features and is consistent with other DGL measurements in the optical and at near-infrared wavelengths longer than 1.8 {\mu}m. Our result implies that the continuum is consistently reproduced by models of scattered starlight in the Rayleigh scattering regime with a few large grains.
An isolated, initially cold and ellipsoidal cloud of self-gravitating particles represents a relatively simple system to study the effects of the deviations from spherical symmetry in the mechanism of violent relaxation. Initial deviations from spherical symmetry are shown to play a dynamical role that is equivalent to that of density fluctuations in the case of an initially spherical cloud. Indeed, these deviations control the amount of particles energy change and thus determine the properties of the final energy distribution, particularly the appearance of two species of particles: bound and free. Ejection of mass and energy from the system together with the formation of a density profile decaying as $\rho(r) \sim r^{-4}$ and a Keplerian radial velocity dispersion profile, are the prominent features similar to those observed after the violent relaxation of spherical clouds. In addition, we find that ejected particles are characterized by highly non-spherical shapes, whose features can be traced in the initial deviations from spherical symmetry that are amplified during the dynamical evolution: particles can indeed form anisotropic configurations, like bars and/or disks, even though the initial cloud was very close to spherical.
N-body simulations of galactic collisions are employed to investigate the formation of elliptical rings in disk galaxies. The relative inclination between disk and dwarf galaxies is studied with a fine step of five degrees. It is confirmed that the eccentricity of elliptical ring is linearly proportional to the inclination angle. Deriving from the simulational results, an analytic formula which expresses the eccentricity as a function of time and inclination angle is obtained. This formula shall be useful for the interpretations of the observations of ring systems, and therefore reveals the merging histories of galaxies.
The problem of impulsive heating of dust grains in cold, dense interstellar clouds is revisited theoretically, with the aim to better understand leading mechanisms of the explosive desorption of icy mantles. It is rigorously shown that if the heating of a reactive medium occurs within a sufficiently localized spot (e.g., heating of mantles by cosmic rays), then the subsequent thermal evolution is characterized by a single dimensionless number $\lambda$. This number identifies a bifurcation between two distinct regimes: When $\lambda$ exceeds a critical value (threshold), the heat equation exhibits the explosive solution, i.e., the thermal (chemical) explosion is triggered. Otherwise, thermal diffusion causes the deposited heat to spread over the entire grain -- this regime is commonly known as the whole-grain heating. The theory allows us to find a critical combination of the physical parameters that govern the explosion of icy mantles due to impulsive spot heating. In particular, the calculations suggest that heavy cosmic ray species (e.g., iron ions) colliding with dust are able to trigger the explosion. Based on the recently calculated local cosmic-ray spectra, the expected rate of the explosive desorption is estimated. The efficiency of the desorption, which affects all solid species independent of their binding energy, is shown to be comparable with other cosmic-ray desorption mechanisms typically considered in the literature. Also, the theory allows us to estimate maximum abundances of reactive species that may be stored in the mantles, which provides important constraints on available astrochemical models.
A new one-dimensional, dynamical model is proposed for geometrically thin,
self-gravitating viscous accretion discs. The vertically integrated equations
are simplified using the slow accretion limit and the monopole approximation
with a time-dependent central point mass to account for self-gravity and
accretion. It is shown that the system of partial differential equations can be
reduced to a single non-linear advection diffusion equation which describes the
time evolution of angular velocity.
In order to solve the equation three different turbulent viscosity
prescriptions are considered. It is shown that for these parametrizations the
differential equation allows for similarity transformations depending only on a
single non-dimensional parameter. A detailed analysis of the similarity
solutions reveals that this parameter is the initial power law exponent of the
angular velocity distribution at large radii. The radial dependence of the
self-similar solutions is in most cases given by broken power laws. At small
radii the rotation law always becomes Keplerian with respect to the current
central point mass. In the outer regions the power law exponent of the rotation
law deviates from the Keplerian value and approaches asymptotically the value
determined by the initial condition. It is shown that accretion discs with
flatter rotation laws at large radii yield higher accretion rates.
The methods are applied to self-gravitating accretion discs in active
galactic nuclei. Fully self-gravitating discs are found to evolve faster than
nearly Keplerian discs. The implications on supermassive black hole formation
and Quasar evolution are discussed.
Aims. We aim at finding candidates of potential survivors of high-redshift
compact galaxies in SDSS, as targets for more detailed follow-up observations.
Methods. From the virial theorem it is expected that for a given mass,
compact galaxies have stellar velocity dispersion higher than the mean due to
their smaller sizes. Therefore velocity dispersion coupled with size (or mass)
is an appropriate method to select relics, independent of the stellar
population properties. Based on these consideration we design a set of criteria
using distribution of early-type galaxies from SDSS on the
log$_{10}$(R$_{0}$)-log$_{10}$($\sigma_{0}$) plane to find the most extreme
objects on it.
Results. We find 76 galaxies at 0.05 < z < 0.2, which have properties similar
to the typical quiescent galaxies at high redshift. We study how well these
galaxies fit on well-known local universe relations of early-type galaxies such
as the fundamental plane, the red sequence or mass-size relations. As expected
from the selection criteria, the candidates are located in an extreme corner of
mass-size plane. However, they do not extend as deeply into the so-called zone
of exclusion as some of the red nuggets found at high redshift, being a factor
2-3 less massive at a given intrinsic scale size. We find that our candidates
are systematically offset on scaling relation compared to the average
early-type galaxies, and similar to the mass-size range expected for passive
evolution of the red nuggets from their high redshift to the present.
Conclusions. The 76 selected candidates form a well suited set of objects for
further follow-up observations. We argue that selecting a high velocity
dispersion is the best way to find analogues of compact high redshift galaxies
in the local universe.
In this work we present IRAM-30m telescope observations of a sample of bulge-dominated galaxies with large dust lanes, which have had a recent minor merger. We find these galaxies are very gas rich, with H2 masses between 4x10^8 and 2x10^10 Msun. We use these molecular gas masses, combined with atomic gas masses from an accompanying paper, to calculate gas-to-dust and gas-to-stellar mass ratios. The gas-to-dust ratios of our sample objects vary widely (between ~50 and 750), suggesting many objects have low gas-phase metallicities, and thus that the gas has been accreted through a recent merger with a lower mass companion. We calculate the implied minor companion masses and gas fractions, finding a median predicted stellar mass ratio of ~40:1. The minor companion likely had masses between ~10^7 - 10^10 Msun. The implied merger mass ratios are consistent with the expectation for low redshift gas-rich mergers from simulations. We then go on to present evidence that (no matter which star-formation rate indicator is used) our sample objects have very low star-formation efficiencies (star-formation rate per unit gas mass), lower even than the early-type galaxies from ATLAS3D which already show a suppression. This suggests that minor mergers can actually suppress star-formation activity. We discuss mechanisms that could cause such a suppression, include dynamical effects induced by the minor merger.
We present deep NH$_3$ observations of the L1495-B218 filaments in the Taurus molecular cloud covering over a 3 degree angular range using the K-band focal plane array on the 100m Green Bank Telescope. The L1495-B218 filaments form an interconnected, nearby, large complex extending over 8 pc. We observed NH$_3$ (1,1) and (2,2) with a spectral resolution of 0.038 km/s and a spatial resolution of 31$"$. Most of the ammonia peaks coincide with intensity peaks in dust continuum maps at 350 $\mu$m and 500 $\mu$m. We deduced physical properties by fitting a model to the observed spectra. We find gas kinetic temperatures of 8 $-$ 15 K, velocity dispersions of 0.05 $-$ 0.25 km/s, and NH$_3$ column densities of 5$\times$10$^{12}$ $-$ 1$\times$10$^{14}$ cm$^{-2}$. The CSAR algorithm, which is a hybrid of seeded-watershed and binary dendrogram algorithms, identifies a total of 55 NH$_3$ structures including 39 leaves and 16 branches. The masses of the NH$_3$ sources range from 0.05 M$_\odot$ to 9.5 M$_\odot$. The masses of NH$_3$ leaves are mostly smaller than their corresponding virial mass estimated from their internal and gravitational energies, which suggests these leaves are gravitationally unbound structures. 9 out of 39 NH$_3$ leaves are gravitationally bound and 7 out of 9 gravitationally bound NH$_3$ leaves are associated with star formation. We also found that 12 out of 30 gravitationally unbound leaves are pressure-confined. Our data suggest that a dense core may form as a pressure-confined structure, evolve to a gravitationally bound core, and undergo collapse to form a protostar.
We argue that the `changing look' AGN recently reported by LaMassa et al. could in fact be a luminous flare produced by the tidal disruption of a super-solar mass star passing just a few gravitational radii outside the event horizon of a $\sim 10^8 M_{\odot}$ nuclear black hole. This flare occurred in a massive, star forming galaxy at redshift $z=0.312$, which is robustly characterized thanks to repeated late-time photometric and spectroscopic observations. By taking difference-photometry of the well sampled multi-year SDSS Stripe-82 light-curve, we are able to probe the evolution of the nuclear spectrum over the course of the outburst. The tidal disruption event (TDE) interpretation is consistent with the very rapid rise and the decay time of the flare, which displays an evolution consistent with the well-known $t^{-5/3}$ behaviour, and would otherwise be inconsistent with a viscous draining of a large-scale accretion disc. Our analysis places strong constraints on the physical properties of the TDE, such as the putative disrupted star's mass and orbital parameters, as well as the size and temperature of the emitting material. Assuming standard (and conservative) bolometric corrections, this would be amongst the most luminous non-beamed tidal disruption flares discovered so far, and the only one observed from a black hole as massive as $\sim 10^8 M_{\odot}$. The properties of the broad and narrow emission lines observed in two epochs of SDSS spectra provide further constraints on the circum-nuclear structure, and could be indicative that the system hosted a moderate-luminosity AGN as recently as a few $10^4$ years ago. We discuss the complex interplay between tidal disruption events and gas accretion episodes in galactic nuclei, highlighting the implications for future TDE searches and for estimates of their intrinsic rates.
Electronic transitions of the title molecules were measured between 250 and 710 nm using a mass-resolved 1+1' resonant two-photon ionization technique at a resolution of 0.1 nm. Calculations at the B3LYP/aug-cc-pVQZ level of theory support the analyses. Because of their spectral properties, SiC$_2$, linear Si$_2$C$_2$, Si$_3$C, and SiC$_6$H$_4$ are interesting target species for astronomical searches in the visible spectral region. Of special relevance is the Si--C$_2$--Si chain, which features a prominent band at 516.4 nm of a strong transition ($f=0.25$). This band and one from SiC$_6$H$_4$ at 445.3 nm were also investigated at higher resolution (0.002 nm).
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As galaxy simulations increase in resolution more attention is being paid towards the evolution of dwarf galaxies and how the simulations compare to observations. Despite this increasing resolution we are however, far away from resolving the interactions of satellite dwarf galaxies and the hot coronae which surround host galaxies. We describe a new method which focuses only on the local region surrounding an infalling dwarf in an effort to understand how the hot baryonic halo will alter the chemodynamical evolution of dwarf galaxies. Using this method we examine how a dwarf, similar to Sextans dwarf spheroidal, evolves in the corona of a Milky Way like galaxy. We find that even at high perigalacticons the synergistic interaction between ram pressure and tidal forces transform a dwarf into a stream, suggesting that Sextans was much more massive in the past in order survive its perigalacticon passage. In addition the large confining pressure of the hot corona allows gas that was originally at the outskirts to begin forming stars, initially forming stars of low metallicity compared to the dwarf evolved in isolation. This increase in star formation eventually allows a dwarf galaxy to form more metal rich stars compared to one in isolation, but only if the dwarf retains gas for a sufficiently long period of time. In addition, dwarfs which formed substantial numbers of stars post-infall will have a slightly elevated [Mg/Fe] at high metallicity ([Fe/H] -1.5).
The classification of galaxy mergers and isolated disks is key for understanding the relative importance of galaxy interactions and secular evolution during the assembly of galaxies. The kinematic properties of galaxies as traced by emission lines have been used to suggest the existence of a significant population of high-z star-forming galaxies consistent with isolated rotating disks. However, recent studies have cautioned that post-coalescence mergers may also display disk-like kinematics. To further investigate the robustness of merger/disk classifications based on kinematic properties, we carry out a systematic classification of 24 local (U)LIRGs spanning a range of galaxy morphologies: from isolated spiral galaxies, ongoing interacting systems, to fully merged remnants. We artificially redshift the WiFeS observations of these local (U)LIRGs to z=1.5 to make a realistic comparison with observations at high-z, and also to ensure that all galaxies have the same spatial sampling of ~900 pc. Using both kinemetry-based and visual classifications, we find that the reliability of kinematic classification shows a strong trend with the interaction stage of galaxies. Mergers with two nuclei and tidal tails have the most distinct kinematic properties compared to isolated disks, whereas a significant population of the interacting disks and merger remnants are indistinguishable from isolated disks. The high fraction of late-stage mergers showing disk-like kinematics reflects the complexity of the dynamics during galaxy interactions. However, the exact fractions of misidentified disks and mergers depend on the definition of kinematic asymmetries and the classification threshold when using kinemetry-based classifications. Our results suggest that additional indicators such as morphologies traced by stars or molecular gas are required to further constrain the merger/disk classifications at high-z.
New spectral line observations, obtained with the Jansky Very Large Array (VLA), of a sample of 34 galaxies in 17 close pairs are presented in this paper. The sample of galaxy pairs is selected to contain galaxies in close, major interactions (i.e., projected separations $<$30 kpc/h, and mass ratios less extreme than 4:1), while still having a sufficiently large angular separation that the VLA can spatially resolve both galaxies in the pair. Of the 34 galaxies, 17 are detected at $> 3\sigma$. We compare the HI gas fraction of the galaxies with the triggered star formation present in that galaxy. When compared to the star formation rates (SFRs) of non-pair galaxies matched in mass, redshift, and local environment, we find that the star formation enhancement is weakly positively correlated ($\sim 2.5\sigma$) with HI gas fraction. In order to help understand the physical mechanisms driving this weak correlation, we also present results from a small suite of binary galaxy merger simulations with varying gas fractions. The simulated galaxies indicate that larger initial gas fractions are associated with lower levels of interaction-triggered star formation (relative to an identical galaxy in isolation), but also show that high gas fraction galaxies have higher absolute SFRs prior to an interaction. We show that when interaction-driven SFR enhancements are calculated relative to a galaxy with an average gas fraction for its stellar mass, the relationship between SFR and initial gas fraction dominates over the SFR enhancements driven by the interaction. Simulated galaxy interactions that are matched in stellar mass but not in gas fraction, like our VLA sample, yield the same general positive correlation between SFR enhancement and gas fraction that we observe.
We explore the quenching of low-mass galaxies (10^4 < Mstar < 10^8 Msun) as a function of lookback time using the star formation histories (SFHs) of 38 Local Group dwarf galaxies. The SFHs were derived from analyzing color-magnitude diagrams of resolved stellar populations in archival Hubble Space Telescope/Wide Field Planetary Camera 2 imaging. We find: (1) Lower mass galaxies quench earlier than higher mass galaxies; (2) Inside of virial radius there is no correlation between a satellite's current proximity to a massive host and its quenching epoch; (3) There are hints of systematic differences in quenching times of M31 and Milky Way (MW) satellites, although the sample sample size and uncertainties in the SFHs of M31 dwarfs prohibit definitive conclusions. Combined with literature results, we qualitatively consider the redshift evolution (z=0-1) of the quenched galaxy fraction over ~7 dex in stellar mass (10^4 < Mstar < 10^11.5 Msun). The quenched fraction of all galaxies generally increases toward the present, with both the lowest and highest mass systems exhibiting the largest quenched fractions at all redshifts. In contrast, galaxies between Mstar ~ 10^8-10^10 Msun have the lowest quenched fractions. We suggest that such intermediate-mass galaxies are the least efficient at quenching. Finally, we compare our quenching times with predictions for infall times of low-mass galaxies associated with the MW. We find that some of the lowest-mass satellites (e.g., CVn II, Leo IV) may have been quenched before infall while higher mass satellites (e.g., Leo I, Fornax) typically quench ~1-4 Gyr after infall.
Active Galactic Nuclei (AGN) represent the growth phases of the supermassive black holes in the center of almost every galaxy. Powerful, highly ionized winds, with velocities $\sim 0.1- 0.2c$ are a common feature in X--ray spectra of luminous AGN, offering a plausible physical origin for the well known connections between the hole and properties of its host. Observability constraints suggest that the winds must be episodic, and detectable only for a few percent of their lifetimes. The most powerful wind feedback, establishing the $M -\sigma$ relation, is probably not directly observable at all. The $M - \sigma$ relation signals a global change in the nature of AGN feedback. At black hole masses below $M-\sigma$ feedback is confined to the immediate vicinity of the hole. At the $M-\sigma$ mass it becomes much more energetic and widespread, and can drive away much of the bulge gas as a fast molecular outflow.
Direct imaging searches have revealed many very low-mass objects, including a small number of planetary mass objects, as wide-orbit companions to young stars. The formation mechanism of these objects remains uncertain. In this paper we present the predictions of the disc fragmentation model regarding the properties of the discs around such low-mass objects. We find that the discs around objects that have formed by fragmentation in discs hosted by Sun-like stars (referred to as 'parent' discs and 'parent' stars) are more massive than expected from the ${M}_{\rm disc}-M_*$ relation (which is derived for stars with masses $M_*>0.2 {\rm M}_{\odot}$). Accordingly, the accretion rates onto these objects are also higher than expected from the $\dot{M}_*-M_*$ relation. Moreover there is no significant correlation between the mass of the brown dwarf or planet with the mass of its disc nor with the accretion rate from the disc onto it. The discs around objects that form by disc fragmentation have larger than expected masses as they accrete gas from the disc of their parent star during the first few kyr after they form. The amount of gas that they accrete and therefore their mass depend on how they move in their parent disc and how they interact with it. Observations of disc masses and accretion rates onto very low-mass objects are consistent with the predictions of the disc fragmentation model. Future observations (e.g. by ALMA) of disc masses and accretion rates onto substellar objects that have even lower masses (young planets and young, low-mass brown dwarfs), where the scaling relations predicted by the disc fragmentation model diverge significantly from the corresponding relations established for higher-mass stars, will test the predictions of this model.
Arp 270 (NGC 3395 and NGC 3396) is the system of two actively star-forming late-type galaxies in contact, which already have experienced at least one close encounter in the past. We performed long-slit observations of peripheric regions of this merging system with the 6-m telescope of SAO RAS. Line-of-sight velocity distribution along the slits was obtained for gas and stellar population. We found that the stellar component of NGC 3395 differs by its velocity from the emission gas component in the extended region in the periphery, which evidences a spatial separation of stars and gas in the tidally disturbed galaxy. Gas abundances obtained by different methods demonstrate that both galaxies are mildly underabundant (log(O/H) $\approx 8.4$) without significant variations of metallicity along the slits. By comparing stellar and gaseous masses of galaxies we came to conclusion that the chemical evolution of gas is badly described by the closed box model. It allows us to admit that the significant part of interstellar gas was swept out of galaxies during the preceding encounter(s). A special attention was paid to the extended kpc-size island of star formation between the galaxies. We have not found neither noticeable kinematic decoupling of this region from the adjacent areas, nor any peculiarities of its emission spectra, which evidences that it was formed recently from the gas of NGC 3395 in the transition region between the colliding galaxies.
We have obtained a deep 8-field XMM-Newton mosaic of M33 covering the galaxy out to the D$_{25}$ isophote and beyond to a limiting 0.2--4.5 keV unabsorbed flux of 5$\times$10$^{-16}$ erg cm$^{-2}$ s$^{-1}$ (L${>}$4$\times$10$^{34}$ erg s$^{-1}$ at the distance of M33). These data allow complete coverage of the galaxy with high sensitivity to soft sources such as diffuse hot gas and supernova remnants. Here we describe the methods we used to identify and characterize 1296 point sources in the 8 fields. We compare our resulting source catalog to the literature, note variable sources, construct hardness ratios, classify soft sources, analyze the source density profile, and measure the X-ray luminosity function. As a result of the large effective area of XMM-Newton below 1 keV, the survey contains many new soft X-ray sources. The radial source density profile and X-ray luminosity function for the sources suggests that only $\sim$15% of the 391 bright sources with L${>}$3.6$\times$10$^{35}$ erg s$^{-1}$ are likely to be associated with M33, and more than a third of these are known supernova remnants. The log(N)--log(S) distribution, when corrected for background contamination, is a relatively flat power-law with a differential index of 1.5, which suggests many of the other M33 sources may be high-mass X-ray binaries. Finally, we note the discovery of an interesting new transient X-ray source, which we are unable to classify.
A local dwarf galaxy, NGC 5253, has a young super star cluster that may provide an example of highly efficient star formation. Here we report the detection and imaging, with the Submillimeter Array, of the J= 3-2 rotational transition of CO at the location of the massive cluster associated with the supernebula. The gas cloud is hot, dense, quiescent, and extremely dusty. Its gas-to-dust ratio is lower than the Galactic value, which we attribute to dust enrichment by Wolf-Rayet stars within the embedded star cluster. Its star formation efficiency exceeds 50%, ten times higher than clouds in the Milky Way: this cloud is a factory of stars and soot. We suggest that high efficiency results from the force-feeding of star formation by a streamer of gas falling into the galaxy.
We report laboratory measurements of the absorption coefficient of solid para-H2, within the wavelength range from 1 to 16.7 micron, at high spectral resolution. In addition to the narrow rovibrational lines of H2 which are familiar from gas phase spectroscopy, the data manifest double transitions and broad phonon branches that are characteristic specifically of hydrogen in the solid phase. These transitions are of interest because they provide a spectral signature which is independent of the impurity content of the matrix. We have used our data, in combination with a model of the ultraviolet absorptions of the H2 molecule, to construct the dielectric function of solid para-H2 over a broad range of frequencies. Our results will be useful in determining the electromagnetic response of small particles of solid hydrogen. The dielectric function makes it clear that pure H2 dust would contribute to IR extinction predominantly by scattering starlight, rather than absorbing it, and the characteristic IR absorption spectrum of the hydrogen matrix itself will be difficult to observe.
We present a photometric catalogue of star cluster candidates in Hickson compact groups (HCGs) 7, 31, 42, 59, and 92, based on observations with the Advanced Camera for Surveys and the Wide Field Camera 3 on the Hubble Space Telescope. The catalogue contains precise cluster positions (right ascension and declination), magnitudes, and colours in the BVI filters. The number of detected sources ranges from 2200 to 5600 per group, from which we construct the high-confidence sample by applying a number of criteria designed to reduce foreground and background contaminants. Furthermore, the high-confidence cluster candidates for each of the 16 galaxies in our sample are split into two sub-populations: one that may contain young star clusters and one that is dominated by globular older clusters. The ratio of young star cluster to globular cluster candidates varies from group to group, from equal numbers to the extreme of HCG 31 which has a ratio of 8 to 1, due to a recent starburst induced by interactions in the group. We find that the number of blue clusters with $M_V < -9$ correlates well with the current star formation rate in an individual galaxy, while the number of globular cluster candidates with $M_V < -7.8$ correlates well (though with large scatter) with the stellar mass. Analyses of the high-confidence sample presented in this paper show that star clusters can be successfully used to infer the gross star formation history of the host groups and therefore determine their placement in a proposed evolutionary sequence for compact galaxy groups.
In an attempt to place an explicit constraint on dark matter models, we define and estimate a mean surface density of a dark halo within a radius of maximum circular velocity, which is derivable for various galaxies with any dark-matter density profiles. We find that this surface density is generally constant across a wide range of maximum circular velocities of $\sim$ 10 to 400 km s$^{-1}$, irrespective of different density distribution in each of the galaxies. This common surface density at high halo-mass scales is found to be naturally reproduced by both cold and warm dark matter (CDM and WDM) models, even without employing any fitting procedures. However, the common surface density at dwarf-galaxy scales, for which we have derived from the Milky Way and Andromeda dwarf satellites, is reproduced only in a massive range of WDM particle masses, whereas CDM provides a reasonable agreement with the observed constancy. This is due to the striking difference between mass-concentration relations for CDM and WDM halos at low halo-mass scales. In order to explain the universal surface density of dwarf-galaxy scales in WDM models, we suggest that WDM particles need to be heavier than 3 keV.
Observations of strong gravitational lensing, stellar kinematics, and mass tracers on larger scales enable accurate measures of the distribution of dark matter and baryons in massive early-type galaxies (ETGs). While such techniques have previously been applied to galaxy-scale and cluster-scale lenses, the paucity of intermediate-mass systems with high-quality data has precluded a uniform analysis of mass-dependent trends. With the aim of bridging this gap, we present new observations and analyses of 10 group-scale lenses at <z>=0.36 characterized by Einstein radii 2.5"-5.1" and a mean halo mass of M_200 = 10^14.0 Msol. For these groups, we find a mean halo concentration c_200 = 5.0 +- 0.8 consistent with unmodified cold dark matter halos and recent simulations of galaxy formation. By combining our data with other lens samples in the literature, we analyze the mass structure of ETGs in halos spanning the mass range 10^13-10^15 Msol using homogeneous methods and data. We show that the slope of the total density profile gamma_tot within the effective radius depends on the stellar surface density, as demonstrated previously, but also on the halo mass. We analyze these trends using halo occupation models and resolved stellar kinematics with the goal of testing the universality of the dark matter profile within ETGs of various masses. Whereas the central galaxies of clusters require a shallow inner dark matter density profile, group-scale lenses are consistent with an unmodified Navarro-Frenk-White profile or one that is slightly contracted. We conclude that the net effect of baryons on the dark matter distribution may not be universal, but more likely varies with halo mass due to underlying trends in star formation efficiency and assembly history.
We analyzed near-infrared data of the nearby galaxy IC5063 taken with the Very Large Telescope SINFONI instrument. IC5063 is an elliptical galaxy that has a radio jet nearly aligned with the major axis of a gas disk in its center. The data reveal multiple signatures of molecular and atomic gas that has been kinematically distorted by the jet passage within an area of ~1 kpc^2. Concrete evidence that the impact of jet plasma upon gas causes the gas to accelerate comes from outflows detected near four different bending points of the jet: at the two bright radio lobes, near a diverted plasma stream close to the north lobe, and near the tip of a plasma stream in the narrow-line region. Gas moving with a velocity excess of 600 km/s to 1200 km/s with respect to ordered motions is detected in [FeII], Paa, and H2 lines. Around these regions, gas is scattered in different directions. Near the north lobe, the highly blueshifted and the highly redshifted [FeII] emission is offset by 240 pc. The (scattered or not) plasma and its cocoon drive a diffuse outflow that extends >700 pc parallel and perpendicular to the jet trail. This diffuse outflow has two main observational signatures: its emission unfolds around the jet trail and away from the nucleus with increasing velocity, and it forms a biconical shape that is centered 220 pc away from the nucleus and that is oriented perpendicularly to the jet trail. Overall, the highest gas line-of-sight velocities are attained near the jet trail and bending points. High H2 (1-0) S(1)/S(3) flux ratios indicate non-thermal excitation of gas in the diffuse outflow.
We calculate Lyman Alpha Emitter (LAE) angular correlation functions (ACFs) at $z\simeq6.6$ and the fraction of lifetime (for the 100 Myrs preceding $z\simeq6.6$) galaxies spend as Lyman Break Galaxies (LBGs) with/without Lyman Alpha (Ly\alpha) emission using a model that combines SPH cosmological simulations (GADGET-2), dust attenuation and a radiative transfer code (pCRASH). The ACFs are a powerful tool that significantly narrows the 3D parameter space allowed by LAE Ly$\alpha$ and UV luminosity functions (LFs) alone. With this work, we simultaneously constrain the escape fraction of ionizing photons $f_{esc}=0.05-0.5$, the mean fraction of neutral hydrogen in the intergalactic medium (IGM) $<\chi_{HI}>\leq 0.01$ and the dust-dependent ratio of the escape fractions of Ly$\alpha$ and UV continuum photons $f_\alpha/f_c=0.6-1.2$. Our results show that reionization has the largest impact on the amplitude of the ACFs, and its imprints are clearly distinguishable from those of $f_{esc}$ and $f_\alpha/f_c$. We also show that galaxies with a critical stellar mass of $M_* = 10^{8.5} (10^{9.5}) M_\odot$ produce enough luminosity to stay visible as LBGs (LAEs). Finally, the fraction of time during the past 100 Myrs prior to z=6.6 a galaxy spends as a LBG with (without) Lya emission increases with the UV magnitude (and $M_*$): considering observed (dust and IGM attenuated) luminosities, the fraction of time a galaxy spends as a LBG (LAE) increases from 65% to 100% (0-100%) as $M_{UV}$ decreases from $M_{UV} = -18.0$ to $-23.5$ ($M_*$ increases from $10^8-10^{10.5} M_\odot$). Thus in our model the brightest (most massive) LBGs most often show Ly$\alpha$ emission.
Recent work has demonstrated the potential of gravitationally lensed quasars to extend measurements of black hole spin out to high-redshift with the current generation of X-ray observatories. Here we present an analysis of a large sample of 27 lensed quasars in the redshift range 1.0<z<4.5 observed with Chandra, utilizing over 1.6 Ms of total observing time, focusing on the rest-frame iron K emission from these sources. Although the X-ray signal-to-noise (S/N) currently available does not permit the detection of iron emission from the inner accretion disk in individual cases in our sample, we find significant structure in the stacked residuals. In addition to the narrow core, seen almost ubiquitously in local AGN, we find evidence for an additional underlying broad component from the inner accretion disk, with a clear red wing to the emission profile. Based on simulations, we find the detection of this broader component to be significant at greater than the 3-sigma level. This implies that iron emission from the inner disk is relatively common in the population of lensed quasars, and in turn further demonstrates that, with additional observations, this population represents an opportunity to significantly extend the sample of AGN spin measurements out to high-redshift.
A growing body of evidence suggests that part of, if not all, scattering regions of active galactic nuclei (AGNs) are clumpy. Hence. in this paper, we run radiative transfer models in the optical/UV for a variety of AGN reprocessing regions with different distributions of clumpy scattering media. We use the latest version of the Monte Carlo code STOKES presented in the first two papers of this series to model AGN reprocessing regions of increasing morphological complexity. We replace previously uniform-density media with up to thousands of constant-density clumps. We couple a continuum source to fragmented equatorial scattering regions, polar outflows, and toroidal, obscuring dust regions and investigate a wide range of geometries. We also consider different levels of fragmentation in each scattering region to evaluate importance of fragmentation for the net polarization of the AGN. We find that, in comparison with uniform-density models, equatorial distributions of gas and dust clouds result in grayer spectra, and show a decrease of the net polarization percentage at all lines of sight. The resulting polarization position angle depends on the morphology of the clumpy structure, with extended tori favoring parallel polarization while compact tori produce orthogonal polarization position angles. In the case of polar scattering regions, fragmentation increases the net polarization unless the cloud filling factor is small. A complete set of AGN models constructed from the individual, fragmented regions is investigated. Our modeling shows that the introduction of fragmented dusty tori significantly alters the resulting net polarization of an AGN. Comparison of our models to polarization observations of large AGN samples greatly favors geometrically compact clumpy tori over extended ones.
We present the first uniform treatment of long duration gamma-ray burst (GRB) host galaxy detections and upper limits over the redshift range 3<z<5, a key epoch for observational and theoretical efforts to understand the processes, environments, and consequences of early cosmic star formation. We contribute deep imaging observations of 13 GRB positions yielding the discovery of eight new host galaxies. We use this dataset in tandem with previously published observations of 31 further GRB positions to estimate or constrain the host galaxy rest-frame ultraviolet (UV; 1600 A) absolute magnitudes M_UV. We then use the combined set of 44 M_UV estimates and limits to construct the M_UV luminosity function (LF) for GRB host galaxies over 3<z<5 and compare it to expectations from Lyman break galaxy (LBG) photometric surveys with the Hubble Space Telescope. Adopting standard prescriptions for the luminosity dependence of galaxy dust obscuration (and hence, total star formation rate), we find that our LF is compatible with LBG observations over a factor of 600x in host luminosity, from M_UV = -22.5 mag to >-15.6 mag, and with extrapolations of the assumed Schechter-type LF well beyond this range. We review proposed astrophysical and observational biases for our sample, and find they are for the most part minimal. We therefore conclude, as the simplest interpretation of our results, that GRBs successfully trace UV metrics of cosmic star formation over the range 3<z<5. Our findings suggest GRBs are providing an accurate picture of star formation processes from z ~3 out to the highest redshifts.
We present a method for modelling star-forming clouds that combines two different models of the thermal evolution of the interstellar medium (ISM). In the combined model, where the densities are low enough that at least some part of the spectrum is optically thin, a model of the thermodynamics of the diffuse ISM is more significant in setting the temperatures. Where the densities are high enough to be optically thick across the spectrum, a model of flux limited diffusion is more appropriate. Previous methods either model the low-density interstellar medium and ignore the thermal behaviour at high densities (e.g. inside collapsing molecular cloud cores), or model the thermal behaviour near protostars but assume a fixed background temperature (e.g. approximately 10 K) on large-scales. Our new method treats both regimes. It also captures the different thermal evolution of the gas, dust, and radiation separately. We compare our results with those from the literature, and investigate the dependence of the thermal behaviour of the gas on the various model parameters. This new method should allow us to model the ISM across a wide range of densities and, thus, develop a more complete and consistent understanding of the role of thermodynamics in the star formation process.
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We present the discovery of a faint Milky Way satellite, Laevens 2/Triangulum II, found in the Panoramic Survey Telescope And Rapid Response System (Pan-STARRS 1) 3 pi imaging data and confirmed with follow-up wide-field photometry from the Large Binocular Cameras. The stellar system, with an absolute magnitude of M_V=-1.8 +/-0.5, a heliocentric distance of 30 +2/-2 kpc, and a half-mass radius of 34 +9/-8 pc, shows remarkable similarity to faint, nearby, small satellites such as Willman 1, Segue 1, Segue 2, and Bo\"otes II. The discovery of Laevens 2/Triangulum II further populates the region of parameter space for which the boundary between dwarf galaxies and globular clusters becomes tenuous. Follow-up spectroscopy will ultimately determine the nature of this new satellite, whose spatial location hints at a possible connection with the complex Triangulum-Andromeda stellar structures.
The central image of a strongly lensed background source places constraints on the foreground lens galaxy's inner mass profile slope, core radius and mass of its nuclear supermassive black hole. Using high-resolution long-baseline Atacama Large Millimeter/submillimeter Array (ALMA) observations and archival $Hubble~Space~Telescope$ ($HST$) imaging, we model the gravitational lens H-ATLAS J090311.6+003906 (also known as SDP.81) and search for the demagnified central image. There is central continuum emission from the lens galaxy's active galactic nucleus (AGN) but no evidence of the central lensed image in any molecular line. We use the CO $J$=5-4 map to determine the flux limit of the central image excluding the AGN continuum. We predict the flux density of the central image and use the limits from the ALMA data to constrain the inner mass distribution of the lens. For the core radius of $0.15"$ measured from $HST$ photometry of the lens galaxy assuming that the central flux is completely attributed to the AGN, we find that a black hole mass of $\mathrm{\log(M_{BH}/M_{\odot})} \gtrsim 8.4$ is preferred. Deeper observations with a detection of the central image will significantly improve the constraints of the inner mass distribution of the lens galaxy.
We investigate the environmental dependence of the mass-metallicity relation at z=2 with MOSFIRE/Keck as part of the ZFIRE survey. Here, we present the chemical abundance of a Virgo-like progenitor at z=2.095 that has an established red sequence. We identified 43 cluster ($<z>=2.095\pm0.004$) and 74 field galaxies ($<z>=2.195\pm0.083$) for which we can measure metallicities. For the first time, we show that there is no discernible difference between the mass-metallicity relation of field and cluster galaxies to within 0.02dex. Both our field and cluster galaxy mass-metallicity relations are consistent with recent field galaxy studies at z~2. We present hydrodynamical simulations for which we derive mass-metallicity relations for field and cluster galaxies. We find at most a 0.1dex offset towards more metal-rich simulated cluster galaxies. Our results from both simulations and observations are suggestive that environmental effects, if present, are small and are secondary to the ongoing inflow and outflow processes that are governed by galaxy halo mass.
The arm structure of the Milky Way remains somewhat of an unknown, with observational studies hindered by our location within the Galactic disc. In the work presented here we use smoothed particle hydrodynamics (SPH) and radiative transfer to create synthetic longitude-velocity observations. Our aim is to reverse-engineer a top down map of the Galaxy by comparing synthetic longitude-velocity maps to those observed. We set up a system of N-body particles to represent the disc and bulge, allowing for dynamic creation of spiral features. Interstellar gas, and the molecular content, is evolved alongside the stellar system. A 3D-radiative transfer code is then used to compare the models to observational data. The resulting models display arm features that are a good reproduction of many of the observed emission structures of the Milky Way. These arms however are dynamic and transient, allowing for a wide range of morphologies not possible with standard density wave theory. The best fitting models are a much better match than previous work using fixed potentials. They favour a 4-armed model with a pitch angle of approximately 20 degrees, though with a pattern speed that decreases with increasing Galactic radius. Inner bars are lacking however, which appear required to fully reproduce the central molecular zone.
We study the dynamics of stellar wind from one of the bodies in the binary system, where the other body interacts only gravitationally. We focus on following three issues: (i) we explore the origin of observed periodic variations in maser intensity; (ii) we address the nature of bipolar molecular outflows; and (iii) we show generation of baroclinicity in the same model setup. From direct numerical simulations and further numerical modelling, we find that the maser intensity along a given line of sight varies periodically due to periodic modulation of material density. This modulation period is of the order of the binary period. Another feature of this model is that the velocity structure of the flow remains unchanged with time in late stages of wind evolution. Therefore the location of the masing spot along the chosen sightline stays at the same spatial location, thus naturally explaining the observational fact. This also gives an appearance of bipolar nature in the standard position-velocity diagram, as has been observed in a number of molecular outflows. Remarkably, we also find the generation of baroclinicity in the flow around binary system, offering another site where the seed magnetic fields could possibly be generated due to the Biermann battery mechanisms, within galaxies.
Tip of the red giant branch measurements based on Hubble Space Telescope and ground-based imaging have resulted in accurate distances to 29 galaxies in the nearby Centaurus A Group. All but two of the 29 galaxies lie in either of two thin planes roughly parallel with the supergalactic equator. The planes are only slightly tilted from the line-of-sight, leaving little ambiguity regarding the morphology of the structure. The planes have characteristic r.m.s. long axis dimensions of ~300 kpc and short axis dimensions of ~60 kpc, hence axial ratios ~0.2, and are separated in the short axis direction by 303 kpc.
A satellite galaxy or dark matter subhalo that passes through a stellar disk may excite coherent oscillations in the disk perpendicular to its plane. We determine the properties of these modes for various self-gravitating plane symmetric systems (Spitzer sheets) using the matrix method of Kalnajs. In particular, we find an infinite series of modes for the case of a barotropic fluid. In general, for a collisionless system, there is a double series of modes, which include normal modes and/or Landau-damped oscillations depending on the phase space distribution function of the stars. Even Landau-damped oscillations may decay slowly enough to persist for several hundred Myr. We discuss the implications of these results for the recently discovered vertical perturbations in the kinematics of solar neighborhood stars and for broader questions surrounding secular phenomena such as spiral structure in disk galaxies.
We propose a new method of pushing $Herschel$ to its faintest detection limits using universal trends in the redshift evolution of the far infrared over 24$\mu$m colours in the well-sampled GOODS-North field. An extension to other fields with less multi-wavelength information is presented. This method is applied here to raise the contribution of individually detected $Herschel$ sources to the cosmic infrared background (CIRB) by a factor 5 close to its peak at 250$\mu$m and more than 3 in the 350$\mu$m and 500$\mu$m bands. We produce realistic mock $Herschel$ images of the deep PACS and SPIRE images of the GOODS-North field from the GOODS-$Herschel$ Key Program and use them to quantify the confusion noise at the position of individual sources, i.e., estimate a "local confusion noise". Two methods are used to identify sources with reliable photometric accuracy extracted using 24$\mu$m prior positions. The clean index (CI), previously defined but validated here with simulations, which measures the presence of bright 24$\mu$m neighbours and the photometric accuracy index (PAI) directly extracted from the mock $Herschel$ images. After correction for completeness, thanks to our mock $Herschel$ images, individually detected sources make up as much as 54% and 60% of the CIRB in the PACS bands down to 1.1 mJy at 100$\mu$m and 2.2 mJy at 160$\mu$m and 55, 33, and 13% of the CIRB in the SPIRE bands down to 2.5, 5, and 9 mJy at 250$\mu$m, 350$\mu$m, and 500$\mu$m, respectively. The latter depths improve the detection limits of $Herschel$ by factors of 5 at 250$\mu$m, and 3 at 350$\mu$m and 500$\mu$m as compared to the standard confusion limit. Interestingly, the dominant contributors to the CIRB in all $Herschel$ bands appear to be distant siblings of the Milky Way ($z$$\sim$0.96 for $\lambda$$<$300$\mu$m) with a stellar mass of $M_{\star}$$\sim$9$\times$10$^{10}$M$_{\odot}$.
Feedback in massive galaxies generally involves quenching of star formation, a favored candidate being outflows from a central supermassive black hole. At high redshifts however, explanation of the huge rates of star formation often found in galaxies containing AGN may require a more vigorous mode of star formation than attainable by simply enriching the gas content of galaxies in the usual gravitationally-driven mode that is associated with the nearby Universe. Using hydrodynamical simulations, we demonstrate that AGN-pressure-driven star formation potentially provides the positive feedback that may be required to generate the accelerated star formation rates observed in the distant Universe.
We report the discovery of a gravitationally lensed hyperluminous infrared galaxy (L_IR~10^13 L_sun) with strong radio emission (L_1.4GHz~10^25 W/Hz) at z=2.553. The source was identified in the citizen science project SpaceWarps through the visual inspection of tens of thousands of iJKs colour composite images of Luminous Red Galaxies (LRGs), groups and clusters of galaxies and quasars. Appearing as a partial Einstein ring (r_e~3") around an LRG at z=0.2, the galaxy is extremely bright in the sub-millimetre for a cosmological source, with the thermal dust emission approaching 1 Jy at peak. The redshift of the lensed galaxy is determined through the detection of the CO(3-2) molecular emission line with the Large Millimetre Telescope's Redshift Search Receiver and through [OIII] and H-alpha line detections in the near-infrared from Subaru/IRCS. We have resolved the radio emission with high resolution (300-400 mas) eMERLIN L-band and JVLA C-band imaging. These observations are used in combination with the near-infrared imaging to construct a lens model, which indicates a lensing magnification of ~10x. The source reconstruction appears to support a radio morphology comprised of a compact (<250 pc) core and more extended component, perhaps indicative of an active nucleus and jet or lobe.
Recent studies have presented evidence that the Milky Way global potential may be nonspherical. In this case, the assembling process of the Galaxy may have left long lasting stellar halo kinematic fossils due to the shape of the dark matter halo, potentially originated by orbital resonances. We further investigate such possibility, considering now potential models further away from $\Lambda$CDM halos, like scalar field dark matter halos, MOND, and including several other factors that may mimic the emergence and permanence of kinematic groups, such as, a spherical and triaxial halo with an embedded disk potential. We find that regardless of the density profile (DM nature), kinematic groups only appear in the presence of a triaxial halo potential. For the case of a MOND like gravity theory no kinematic structure is present. We conclude that the detection of these kinematic stellar groups could confirm the predicted triaxiality of dark halos in cosmological galaxy formation scenarios.
We derived elemental abundances in 27 Cepheids, the great majority situated within a zone of Galactocentric distances ranging from 5 to 7 kpc. One star of our sample, SU Sct, has a Galactocentric distance of about 3 kpc, and thus falls in a poorly investigated region of the inner thin disc. Our new results, combined with data on abundances in the very central part of our Galaxy taken from literature, show that iron, magnesium, silicon, sulfur, calcium and titanium LTE abundance radial distributions, as well as NLTE distribution of oxygen reveal a plateau-like structure or even positive abundance gradient in the region extending from the Galactic center to about 5 kpc.
Dielectronic recombination (DR) is the dominant recombination process for most heavy elements in photoionized clouds. Accurate DR rates for a species can be predicted when the positions of autoionizing states are known. Unfortunately such data are not available for most third and higher-row elements. This introduces an uncertainty that is especially acute for photoionized clouds, where the low temperatures mean that DR occurs energetically through very low-lying autoionizing states. This paper discusses S$^{2+} \rightarrow$ S$^+$ DR, the process that is largely responsible for establishing the [S~III]/[S~II] ratio in nebulae. We derive an empirical rate coefficient using a novel method for second-row ions, which do have accurate data. Photoionization models are used to reproduce the [O~III] / [O~II] / [O~I] / [Ne~III] intensity ratios in central regions of the Orion Nebula. O and Ne have accurate atomic data and can be used to derive an empirical S$^{2+} \rightarrow$ S$^+$ DR rate coefficient at $\sim 10^{4}$~K. We present new calculations of the DR rate coefficient for S$^{2+} \rightarrow$ S$^+$ and quantify how uncertainties in the autoionizing level positions affect it. The empirical and theoretical results are combined and we derive a simple fit to the resulting rate coefficient at all temperatures for incorporation into spectral synthesis codes. This method can be used to derive empirical DR rates for other ions, provided that good observations of several stages of ionization of O and Ne are available.
The observed high covering fractions of neutral hydrogen (HI) with column densities above $\sim 10^{17} \rm{cm}^{-2}$ around Lyman-Break Galaxies (LBGs) and bright quasars at redshifts z ~ 2-3 has been identified as a challenge for simulations of galaxy formation. We use the EAGLE cosmological, hydrodynamical simulation, which has been shown to reproduce a wide range of galaxy properties and for which the subgrid feedback was calibrated without considering gas properties, to study the distribution of HI around high-redshift galaxies. We predict the covering fractions of strong HI absorbers ($N_{\rm{HI}} \gtrsim 10^{17} \rm{cm}^{-2}$) inside haloes to increase rapidly with redshift but to depend only weakly on halo mass. For massive ($M_{200} \gtrsim 10^{12} {\rm M_{\odot}}$) halos the covering fraction profiles are nearly scale-invariant and we provide fitting functions that reproduce the simulation results. While efficient feedback is required to increase the HI covering fractions to the high observed values, the distribution of strong absorbers in and around halos of a fixed mass is insensitive to factor of two variations in the strength of the stellar feedback. In contrast, at fixed stellar mass the predicted HI distribution is highly sensitive to the feedback efficiency. The fiducial EAGLE simulation reproduces both the observed global column density distribution function of HI and the observed radial covering fraction profiles of strong HI absorbers around LBGs and bright quasars.
In the effort to understand the link between the structure of galaxy clusters and their galaxy populations, we focus on MACS J1206.2-0847, at z~0.44, probing its substructure in the projected phase space through the spectrophotometric properties of a large number of galaxies from the CLASH-VLT survey. Our analysis is mainly based on an extensive spectroscopic dataset of 445 member galaxies, mostly acquired with VIMOS@VLT as part of our ESO Large Programme, sampling the cluster out to a radius ~2R200 (4 Mpc). We classify 412 galaxies as: passive, with strong Hdelta absorption (red and blue ones), and with emission lines from weak to very strong ones. A number of tests for substructure detection is applied to analyze the galaxy distribution in the velocity space, in the 2D space, and in the (3D) projected phase-space. Studied in its entirety, the cluster appears as a large-scale relaxed system with a few, secondary, minor overdensities in 2D distribution. We detect no velocity gradient or evidence of deviations in local mean velocities. The main feature is the WNW-ESE elongation. The analysis of galaxy populations per spectral class highlights a more complex scenario. The passive and red strong Hdelta galaxies trace the cluster center and the WNW-ESE elongated structure. The red strong Hdelta galaxies also mark a secondary, dense peak ~2 Mpc at ESE. The emission line galaxies cluster in several loose structures, mostly outside R200. The observational scenario agrees with MACS J1206.2-0847 having WNW-ESE as the direction of the main cluster accretion, traced by passive and red strong Hdelta galaxies. The latter ones, interpreted as poststarburst galaxies, date a likely important event 1-2 Gyr before the epoch of observation. The emission line galaxies trace a secondary, ongoing infall where groups are accreted along several directions.
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