Compact groups provide an environment to study the growth of galaxies amid multiple prolonged interactions. With their dense galaxy concentrations and relatively low velocity dispersions, compact groups mimic the conditions of hierarchical galaxy assembly. Compact group galaxies are known to show a bimodality in Spitzer IRAC infrared color space: galaxies are preferentially either quiescent with low specific star formation rates (SSFRs) or prolifically forming stars-galaxies with moderate levels of specific star formation are rare. Previous Spitzer IRAC studies identifying this "canyon" have been limited by small number statistics. We utilize whole-sky Wide-field Infrared Survey Explorer (WISE) data to study 163 compact groups, thereby tripling our previous sample and including more galaxies with intermediate mid-IR colors indicative of moderate SSFRs. We define a distinct WISE. mid-IR color space (log [f(12)/f(4.6)]) versus (log [f(22)/f(3.4)]) that we use to identify canyon galaxies from the larger sample. We confirm that compact group galaxies show a bimodal distribution in the mid-infrared and identify 37 canyon galaxies with reliable photometry and intermediate mid-IR colors. Morphologically, we find that the canyon harbors a large population of both Sa-Sbc and E/S0 type galaxies, and that they fall on the optical red sequence rather than the green valley. Finally, we provide a catalog of WISE. photometry for 567 of 652 galaxies selected from the sample of 163 compact groups.

Compact groups of galaxies provide insight into the role of low-mass, dense environments in galaxy evolution because the low velocity dispersions and close proximity of galaxy members result in frequent interactions that take place over extended time-scales. We expand the census of star formation in compact group galaxies by Tzanavaris et al. (2010) and collaborators with Swift UVOT, Spitzer IRAC and MIPS 24 mu m photometry of a sample of 183 galaxies in 46 compact groups. After correcting luminosities for the contribution from old stellar populations, we estimate the dust-unobscured star formation rate (SFRUV) using the UVOT uvw2 photometry. Similarly, we use the MIPS 24 mu m photometry to estimate the component of the SFR that is obscured by dust (SFRIR). We find that galaxies which are MIR-active (MIR-'red'), also have bluer UV colours, higher specific SFRs, and tend to lie in H I-rich groups, while galaxies that are MIR-inactive (MIR-'blue') have redder UV colours, lower specific SFRs, and tend to lie in H I-poor groups. We find the SFRs to be continuously distributed with a peak at about 1 M-circle dot yr(-1), indicating this might be the most common value in compact groups. In contrast, the specific SFR distribution is bimodal, and there is a clear distinction between star-forming and quiescent galaxies. Overall, our results suggest that the specific SFR is the best tracer of gas depletion and galaxy evolution in compact groups.

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The relation between central or mean metallicity and luminosity in elliptical galaxies is a well observed phenomenon. Theoretical explanations proposed for this relation include scenarios in which peak metallicities are determined either by the epoch at which the remaining gas is expelled from the galaxy by supernova-driven winds, or by the efficiency of star formation following a series of mergers by small stellar/gaseous subsystems. These explanations suggest that an investigation of the metallicity-luminosity relation for spiral galaxies might have implications for galaxy formation models and for the origin of SO galaxies. The existing evidence concerning SO's points to a relation between mean metallicity and total luminosity. The problem of measuring metal abundances in the nuclei of spiral galaxies is that the line strength variations due to metallicity changes must be distinguished from those due to a filling in of the lines by the continuum from a young population. This was accomplished by measuring absorption line indices for Mg b and for a CN band at λ3880. Nuclear spectra of twenty ellipticals, obtained with a reticon detector, show these two indices to be well correlated for pure old populations; models including the effects of young stars show a very different trajectory for age effects. A procedure is thus defined for determining the metallicity of the population and the fraction of light coming from the young component, and this procedure is applied to observations of 25 spiral galaxies. A comparison of the results of this analysis with detailed population syntheses for six galaxies confirms the correctness of the procedure. In order to obtain bulge luminosities and bulge-to-disk ratios, photographic plates of twenty-two of the spirals were obtained. This material was digitized and reduced to a series of radial luminosity profiles for each galaxy. A procedure was established for decomposing the profiles into disk and bulge contributions. In addition to the desired gross parameters of the bulge and disk, the inclinations and true bulge flattenings for some of the galaxies are accurately determined. A discussion of the results of this analysis deals with the nature of departures from the exponential fitting function for some disks, a decomposition of the Hubble sequence into quantitative parameters, and the implication of the distribution of true bulge flattenings. The metallicities and luminosities are then combined, and two tests indicate that in spiral galaxies, central metallicity and bulge luminosity follow the same relation seen in ellipticals. The implications of this result are twofold. First, galaxy formation models in which the disk material can affect the processes which determine the central metallicity in the bulge are ruled out. Specifically, it is likely that the disks of spiral galaxies are not undergoing vigorous star formation at the time the bulge ceases forming stars. A picture in which the disk material has not yet accreted on to the galaxy at this time is also quite consistent. A somewhat more straightforward implication comes from a comparison of the results of this study with similar studies of SO galaxies. It is concluded that, aside from the uncertain effects of radial gradients, the evidence is inconsistent with the theory that most SO's were at one time spiral galaxies.

No description available.

A major question in galaxy formation is how the gas supply that fuels activity in galaxies is modulated by their environment. We use spectroscopy of a set of well-characterized clusters and groups at 0.4 < z < 0.8 from the ESO Distant Cluster Survey and compare it to identically selected field galaxies. Our spectroscopy allows us to isolate galaxies that are dominated by old stellar populations. Here we study a stellar-mass-limited sample (log(M*/M-circle dot) > 10.4) of these old galaxies with weak [O II] emission. We use line ratios and compare to studies of local early-type galaxies to conclude that this gas is likely excited by post-AGB stars and hence represents a diffuse gas component in the galaxies. For cluster and group galaxies the fraction with EW([O II]) > 5 angstrom is f([O II]) = 0.08(-0.03)(+0.02) and f([O II]) = 0.06(-0.04)(+0.07), respectively. For field galaxies we find f([O II]) = 0.2 (+0.07)(-0.06), representing a 2.8 sigma difference between the [O II] fractions for old galaxies between the different environments. We conclude that a population of old galaxies in all environments has ionized gas that likely stems from stellar mass loss. In the field galaxies also experience gas accretion from the cosmic web, and in groups and clusters these galaxies have had their gas accretion shut off by their environment. Additionally, galaxies with emission preferentially avoid the virialized region of the cluster in position-velocity space. We discuss the implications of our results, among which is that gas accretion shutoff is likely effective at group halo masses (log M/M-circle dot > 12.8) and that there are likely multiple gas removal processes happening in dense environments.

We examine the fraction of massive (M-* > 10(10)M(circle dot)) compact star-forming galaxies (cSFGs) that host an active galactic nucleus (AGN) at z similar to 2. These cSFGs are likely the direct progenitors of the compact quiescent galaxies observed at this epoch, which are the first population of passive galaxies to appear in large numbers in the early Universe. We identify cSFGs that host an AGN using a combination of Hubble WFC3 imaging and Chandra X-ray observations in four fields: the Chandra Deep Fields, the Extended Groth Strip, and the UKIDSS Ultra Deep Survey field. We find that 39.2(-3.6)(+3.9)% (65/166) of cSFGs at 1.4 < z < 3.0 host an X-ray detected AGN. This fraction is 3.2 times higher than the incidence of AGN in extended star-forming galaxies with similar masses at these redshifts. This difference is significant at the 6.2 sigma level. Our results are consistent with models in which cSFGs are formed through a dissipative contraction that triggers a compact starburst and concurrent growth of the central black hole. We also discuss our findings in the context of cosmological galaxy evolution simulations that require feedback energy to rapidly quench cSFGs. We show that the AGN fraction peaks precisely where energy injection is needed to reproduce the decline in the number density of cSFGs with redshift. Our results suggest that the first abundant population of massive quenched galaxies emerged directly following a phase of elevated supermassive black hole growth and further hints at a possible connection between AGN and the rapid quenching of star formation in these galaxies.

The rate of tidal disruption events (TDEs), R-TDE, is predicted to depend on stellar conditions near the super-massive black hole (SMBH), which are on difficult-to-measure sub-parsec scales. We test whether R-TDE depends on kpc-scale global galaxy properties, which are observable. We concentrate on stellar surface mass density, Sigma M-*, and velocity dispersion, sigma(nu), which correlate with the stellar density and velocity dispersion of the stars around the SMBH. We consider 35 TDE candidates, with and without known X-ray emission. The hosts range from star-forming to quiescent to quiescent with strong Balmer absorption lines. The last (often with post-starburst spectra) are overrepresented in our sample by a factor of 35(-17)(+21) or 18(-7)(+8), depending on the strength of the H delta absorption line. For a subsample of hosts with homogeneous measurements, Sigma M-* = 10(9)-10(10) M-circle dot/kpc(2), higher on average than for a volume-weighted control sample of Sloan Digital Sky Survey galaxies with similar redshifts and stellar masses. This is because (1) most of the TDE hosts here are quiescent galaxies, which tend to have higher Sigma M-* than the star-forming galaxies that dominate the control, and (2) the star-forming hosts have higher average Sigma M-* than the star-forming control. There is also a weak suggestion that TDE hosts have lower sigma(nu) than for the quiescent control. Assuming that R-TDE infinity Sigma M-*(alpha) x sigma(beta)(nu), and applying a statistical model to the TDE hosts and control sample, we estimate (alpha) over cap = 0.9 +/- 0.2 and (beta) over cap = -1.0 +/- 0.6. This is broadly consistent with RTDE being tied to the dynamical relaxation of stars surrounding the SMBH.

The first billion years of the Universe has been a pivotal time: stars, black holes (BHs), and galaxies formed and assembled, sowing the seeds of galaxies as we know them today. Detecting, identifying, and understanding the first galaxies and BHs is one of the current observational and theoretical challenges in galaxy formation. In this paper we present a population synthesis model aimed at galaxies, BHs, and active galactic nuclei (AGNs) at high redshift. The model builds a population based on empirical relations. The spectral energy distribution of galaxies is determined by age and metallicity, and that of AGNs by BH mass and accretion rate. We validate the model against observations, and predict properties of galaxies and AGN in other wavelength and/or luminosity ranges, estimating the contamination of stellar populations (normal stars and high-mass X-ray binaries) for AGN searches from the infrared to X-rays, and vice versa for galaxy searches. For high-redshift galaxies with stellar ages <1 Gyr, we find that disentangling stellar and AGN emission is challenging at restframe UV/optical wavelengths, while high-mass X-ray binaries become more important sources of confusion in X-rays. We propose a color-color selection in the James Webb Space Telescope bands to separate AGN versus star-dominated galaxies in photometric observations. We also estimate the AGN contribution, with respect to massive, hot, and metal-poor stars, at driving high-ionization lines, such as C IV and He II. Finally, we test the influence of the minimum BH mass and occupation fraction of BHs in low-mass galaxies on the restframe UV/near-IR and X-ray AGN luminosity function.

The formation of stars from interstellar gas is the cornerstone of galaxy evolution. This thesis represents work undertaken in order to characterise the role of cool interstellar gas, and its relation to star formation, in galaxy evolution across cosmic time. In particular, it concentrates on star forming galaxies at the extremes of the galaxy assembly spectrum - extremely faint dwarfs, and extremely luminous starbursts - in an attempt to test the limits of galaxy evolution models. The thesis falls into two complimentary halves, addressing topics in the low redshift and high redshift Universe respectively. In the low redshift Universe, I discuss multi-wavelength studies of large samples of z rv O galaxies, which include extremely faint dwarf galaxies in the Local Volume. Using these samples, it is possible to derive a multitude of physical parameters (including star formation rates, stellar masses, and gas masses) which allow the interrelationship between star formation and gas content to be assessed in a statistically significant manner. In particular, modern wide field surveys (combined with deep, volume-limited data) allow trends to be analysed across many orders of magnitude in galaxy mass and star formation rate, shedding light on the global properties of galaxies in the local Universe. Moving to higher redshift, I discuss targeted observations of molecular gas in extreme star forming galaxies in the early Universe. These 'sub-millimetre' galaxies number amongst the most luminous objects ever discovered, and molecular gas observations have the power to uncover many of their physical properties, including their morphologies, kinematics, and star formation behaviour. I begin by presenting high-resolution observations of a small number of these galaxies at z rv 2, and discussing the implications for galaxy evolution studies. The final chapter of this thesis consists of the results of a survey for molecular gas in sub-millimetre galaxies conducted over the last decade, which represents the largest single study of molecular gas in the early Universe to date.