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.

With a large, unique spectroscopic survey in the fields of 28 galaxy-scale strong gravitational lenses, we identify groups of galaxies in the 26 adequately sampled fields. Using a group-finding algorithm, we find 210 groups with at least 5 member galaxies; the median number of members is 8. Our sample spans redshifts of 0.04 <= z(grp) <= 0.76 with a median of 0.31, including 174 groups with 0.1 < z(grp) < 0.6 The groups have radial velocity dispersions of 60 <= sigma(grp) <= 1200 km s(-1) with a median of 350 km s(-1). We also discover a supergroup in field B0712+472 at z = 0.29 that consists of three main groups. We recover groups similar to similar to 85% of those previously reported in these fields within our redshift range of sensitivity and find 187 new groups with at least five members. The properties of our group catalog, specifically, (1) the distribution of sgrp, (2) the fraction of all sample galaxies that are group members, and (3) the fraction of groups with significant substructure, are consistent with those for other catalogs. The distribution of group virial masses agrees well with theoretical expectations. Of the lens galaxies, 12 of 26 (46%) (B1422+231, B1600+434, B2114+022, FBQS J0951+2635, HE0435-1223, HST J14113+5211, MG0751+2716, MGJ1654+1346, PG 1115+080, Q ER 0047-2808, RXJ1131-1231, and WFI J2033-4723) are members of groups with at least five galaxies, and one more (B0712+472) belongs to an additional, visually identified group candidate. There are groups not associated with the lens that still are likely to affect the lens model; in six of 25 (24%) fields (excluding the supergroup), there is at least one massive (sigma(grp) >= 500 km s(-1)) group or group candidate projected within 2' of the lens.

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.

Strong gravitational lensing provides an independent measurement of the Hubble parameter (H-0). One remaining systematic is a bias from the additional mass due to a galaxy group at the lens redshift or along the sightline. We quantify this bias for more than 20 strong lenses that have well-sampled sightline mass distributions, focusing on the convergence kappa and shear gamma. In 23% of these fields, a lens group contributes >= 1% convergence bias; in 57%, there is a similarly significant line-of-sight group. For the nine time-delay lens systems, H-0 is overestimated by 11(-2)(+3)% on average when groups are ignored. In 67% of fields with total kappa >= 0.01, line-of-sight groups contribute greater than or similar to 2x more convergence than do lens groups, indicating that the lens group is not the only important mass. Lens environment affects the ratio of four (quad) to two (double) image systems; all seven quads have lens groups while only 3 of 10 doubles do, and the highest convergences due to lens groups are in quads. We calibrate the gamma-kappa relation: log(kappa(tot)) = (1.94 +/- 0.34)log(gamma(tot)) + (1.31 +/- 0.49) with an rms scatter of 0.34 dex. Although shear can be measured directly from lensed images, unlike convergence, it can be a poor predictor of convergence; for 19% of our fields, kappa is greater than or similar to 2 gamma. Thus, accurate cosmology using strong gravitational lenses requires precise measurement and correction for all significant structures in each lens field.

Using Hubble Space Telescope Cosmic Origins Spectrograph observations of 89 QSO sightlines through the Sloan Digital Sky Survey footprint, we study the relationships between C IV absorption systems and the properties of nearby galaxies, as well as the large-scale environment. To maintain sensitivity to very faint galaxies, we restrict our sample to 0.0015 < z < 0.015, which defines a complete galaxy survey to L (SIC) 0.01 L-* or stellar mass M-* (SIC) 10(8) M-circle dot. We report two principal findings. First, for galaxies with impact parameter rho < 1 r(vir), C IV detection strongly depends on the luminosity/stellar mass of the nearby galaxy. C IV is preferentially associated with galaxies with M-* > 10(9.5) M-circle dot; lower-mass galaxies rarely exhibit significant C IV absorption (covering fraction f(C) = 9(-6)(+12)% for 11 galaxies with M-* < 10(9.5) M-circle dot.). Second, C IV detection within the M-* > 10(9.5) M-circle dot. population depends on environment. Using a fixed-aperture environmental density metric for galaxies with rho < 160 kpc at z < 0.055, we find that 57(-13)(+12)% (8/14) of galaxies in low-density regions (regions with fewer than seven L > 0.15 L* galaxies within 1.5 Mpc) have affiliated C IV absorption; however, none (0/7) of the galaxies in denser regions show C IV. Similarly, the C IV detection rate is lower for galaxies residing in groups with dark matter halo masses of M-halo > 10(12.5) M-circle dot. In contrast to C IV, H. I is pervasive in the circumgalactic medium without regard to mass or environment. These results indicate that C IV absorbers with log N(C IV). (SIC) 13.5 cm(-2) trace the halos of M-* > 10(9.5) M-circle dot galaxies but also reflect larger-scale environmental conditions.

This dissertation presents an algorithm for identifying galaxy groups, describes the effects of galaxy groups in the environments of strong gravitational lenses and elsewhere along their sightlines, and investigates the properties of brightest group galaxies. We develop an algorithm to identify galaxy groups and apply it to a large spectroscopic survey in the fields of 26 strong gravitational lenses. We identify 210 groups with at least five member galaxies having velocity dispersions of 60 ≤ σ grp ≤ 1200 km s −1 over a redshift range of 0.04 ≤ z grp ≤ 0.76. Using the group catalog defined by this algorithm, we study the environments and line-of-sight structures of 26 strong gravitational lenses. Using these systems to measure cosmological parameters requires an understanding of possible systematic errors as well as the large samples to combat random uncertainties that will be discovered by future surveys. We determine the impact of ignoring lens environments and groups elsewhere along the lens sightlines on H 0 . Lens groups that would bias H 0 high by ≥ 1% exist in 23% of our fields and similarly significant line-of-sight groups in 57%. For lens systems to be used for precision cosmology, the lens environments and line-of-sight groups must be considered to avoid the systematic biases they would cause if ignored. We also study the properties of brightest group galaxies. We compare their morphological, spectroscopic, photometric, and kinematic properties to those of other group galaxies and to a sample of brightest cluster galaxies (BCGs) from Tempel et al. (2014). There is a population (38%) of elliptical, quiescent BGGs as expected from local group and cluster samples. However, our sample also includes a diversity of BGG properties, including disks, disturbed morphologies, AGN, and star formation. BGG luminosities and colors are similar to those of BCGs. However,16 BGG colors show an intermediate amount of scatter between that of BCGs and other group galaxies. BGGs and other group galaxies also have similar phase space distributions. These diverse BGG properties suggest they are still evolving.

Since the discovery of the large (2 × 109 M ) intergalactic cloud known as the Leo Ring in the 1980s, the origin of this object has been the center of a lively debate. Determining the origin of this object is still important as we develop a deeper understanding of the accretion and feedback processes that shape galaxy evolution. We present Hubble Space Telescope/Cosmic Origins Spectrograph observations of three sightlines near the ring, two of which penetrate the high column density neutral hydrogen gas visible in 21 cm observations of the object. These observations provide the first direct measurement of the metallicity of the gas in the ring, an important clue to its origin. Our best estimate of the metallicity of the ring is 10% Z , higher than expected for primordial gas but lower than expected from an interaction. We discuss possible modifications to the interaction and primordial gas scenarios that would be consistent with this metallicity measurement.