We investigate the galaxy populations in seven X-ray-selected, intermediate-redshift groups (0.2 < z < 0.6). Overall, the galaxy populations in these systems are similar to those in clusters at the same redshift; they have large fractions of early-type galaxies (f(e)similar to 70%) and small fractions of galaxies with significant star formation (f ([OII])similar to 30%). We do not observe a strong evolution in the galaxy populations from those seen in X-ray-luminous groups at low redshift. Both f(e) and f ([OII]) are correlated with radius but do not reach the field value out to similar to r(500). However, we find significant variation in the galaxy populations between groups, with some groups having fieldlike populations. Comparisons between the morphological and spectral properties of group galaxies reveal both gas-poor mergers and a population of passive spirals. Unlike low-redshift, X-ray-emitting groups, in some of these groups the brightest galaxy does not lie at the center of the X-ray emission, and in several of the groups that do have a central BGG, the BGG has multiple components. These groups appear to represent a range of evolutionary stages in the formation of the BGG. Some groups have relatively large central galaxy densities, and one group contains a string of seven bright galaxies within a radius of 200 kpc that have a lower velocity dispersion than the rest of the system. None of the central galaxies, including those with multiple components, have significant ([O II]) emission. These observations support a scenario in which BGGs are formed relatively late through gas-poor mergers.

We present a search for galaxy clusters in the fields of three bona fide short gamma-ray bursts ( 050709, 050724, and 051221a) and the putative short-burst GRB 050911, using multislit optical spectroscopy. These observations are part of a long-term program to constrain the progenitor age distribution based on the fraction of short GRBs in galaxy clusters and early-type galaxies. We find no evidence for cluster associations at the redshifts of the first three bursts, but we confirm the presence of the cluster EDCC 493 within the error circle of GRB 050911 and determine its redshift, z = 0.1646, and velocity dispersion, sigma approximate to 660 km s(-1). In addition, our analysis of Swift XRT observations of this burst reveals diffuse X-ray emission coincident with the optical cluster position, with luminosity L-X approximate to 4.9 x 10(42) ergs s(-1) and temperature kT approximate to 0.9 keV. The inferred mass of the cluster is 2.5 x 10(13) M-circle dot, and the probability of chance coincidence is about 0.1%-1%, indicating an association with GRB 050911 at the 2.6-3.2 sigma confidence level. A search for diffuse X-ray emission in coincidence with the 15 other short GRBs observed with XRT and Chandra reveals that, with the exception of the previously noted cluster ZwCl 1234.0+02916 likely associated with GRB 050509b, no additional associations are evident to a typical limit of 3 x 10(-14) ergs s(-1) cm(-2), or M less than or similar to 5 x 10(13) M-circle dot, assuming a typical z = 0.3. The estimated fraction of short GRBs hosted by galaxy clusters of about 5%-20% is in rough agreement with the fraction of stellar mass in clusters of sigma 10%-20%.

The most massive galaxies in the Universe are also the oldest. To overturn this apparent contradiction with hierarchical growth models, we focus on the group-scale haloes which host most of these galaxies. Our z similar to 0.4 group sample is selected in redshift space from the CNOC2 redshift survey. A stellar mass selected M* greater than or similar to 2 x 10(10) M-circle dot sample is constructed using IRAC observations. A sensitive Mid InfraRed (MIR) IRAC colour is used to isolate passive galaxies. It produces a bimodal distribution, in which passive galaxies (highlighted by morphological early-types) define a tight MIR colour sequence (Infrared Passive Sequence, IPS). This is due to stellar atmospheric emission from old stellar populations. Significantly offset from the IPS are galaxies where reemission by dust boosts emission at lambda(obs)=8 mu m. We term them InfraRed-Excess galaxies whether star formation and/or AGN activity are present. They include all known morphological late-types. The fraction of InfraRed Excess galaxies, f(IRE) drops with M* such that f(IRE) = 0.5 at a "crossover mass" of M-cr similar to 1.3 x 10(11) M-circle dot. Within our optically-defined group sample there is a strong and consistent deficit in f(IRE) at all masses, but most clearly at M* greater than or similar to 10(11) M-circle dot. Suppression of star formation must mainly occur in groups, and the observed trend of f(IRE) with M, can be explained if suppression of M* greater than or similar to 10(11) M-circle dot galaxies occurs primarily in the group environment.

We use Chandra observations of 13 nearby groups of galaxies to investigate the hot gas content of their member galaxies. We find that a large fraction of near-IR-bright, early-type galaxies in groups have extended X-ray emission, indicating that they retain significant hot gas halos even in these dense environments. In particular, we detect hot gas halos in similar to 80% of L-K > L* galaxies. We do not find a significant difference in the L-K-L-X relation for detected group and cluster early-type galaxies. However, we detect X-ray emission from a significantly higher fraction of galaxies brighter than L* in groups compared to clusters, indicating that a larger fraction of galaxies in clusters experience significant stripping of their hot gas. In addition, group and cluster galaxies appear to be X-ray-faint compared to field galaxies, although a Chandra-based field sample is needed to confirm this result. The near-IR-bright late-type galaxies in clusters and groups appear to follow the L-K-L-X relation for early-type galaxies, while near-IR- fainter late-type galaxies are significantly more X-ray luminous than this relation likely due to star formation. Finally, we find individual examples of ongoing gas stripping of group galaxies. One galaxy shows a 40-50 kpc X-ray tail, and two merging galaxy systems show tidal bridges/tails of X-ray emission. Therefore, stripping of hot galactic gas through both ram pressure and tidal forces does occur in groups and clusters, but the frequency or efficiency of such events must be moderate enough to allow hot gas halos in a large fraction of bright galaxies to survive even in group and cluster cores.

The most massive galaxies in the universe are also the oldest. To overturn this apparent contradiction with hierarchical growth models we focus on the group-scale halos that host most of these galaxies. Our z similar to 0.4 group sample is selected in redshift space from the CNOC2 redshift survey. A stellar mass-selected M* greater than or similar to 2 x 10(10) M-circle dot sample is constructed using IRAC observations. A sensitive mid-infrared (MIR) IRAC color is used to isolate passive galaxies. It produces a bimodal distribution, in which passive galaxies (highlighted by morphological early types) define a tight MIR color sequence (infrared passive sequence, IPS). This is due to stellar atmospheric emission from old stellar populations. Significantly offset from the IPS are galaxies where reemission by dust boosts emission at lambda(obs) 8 mu m. We term them infrared excess galaxies, whether star formation and/or AGN activity are present. They include all known morphological late types. Comparison with EW[O II] shows that MIR color is highly sensitive to low levels of activity and allows us to separate dusty active from passive galaxies at high stellar mass. The fraction of infrared excess galaxies, f (IRE), drops with M*, such that f (IRE) 0.5 at a "crossover mass" of M-cr similar to 1.3 x 10(11) M-circle dot. Within our optically defined group sample there is a strong and consistent deficit in f (IRE) at all masses, but most clearly at M-* greater than or similar to 10(11) M-circle dot. Suppression of star formation must mainly occur in groups. In particular, the observed trend of f (IRE) with M* can be explained if suppression of M* greater than or similar to 10(11) M-circle dot galaxies occurs primarily in the group environment. This is confirmed using a mock galaxy catalog derived from the millenium simulation. In this way, the mass-dependent evolution in f (IRE) (downsizing) can be driven solely by structure growth in the universe, as more galaxies are accreted into group-sized halos with cosmic time.

We present results from a systematic investigation of the X-ray properties of a sample of moderate-redshift (0.3 < z < 0.6) galaxy groups. These groups were selected not by traditional X-ray or optical search methods, but rather by an association, either physical or along the line of sight, with a strong gravitational lens. We calculate the properties of seven galaxy groups in the fields of six lens systems. Diffuse X-ray emission from the intragroup medium is detected in four of the groups. All of the detected groups have X-ray luminosities greater than 10(42) h(-2) ergs s(-1) and lie on the LX-sigma(v) relations defined by local groups and clusters. The upper limits for the nondetections are also consistent with the local LX-sigma(v) relationships. Although the sample size is small and deeper optical and X-ray data are needed, these results suggest that lens-selected groups are similar to X-ray-selected samples and thus are more massive than the typical poor-group environments of local galaxies.

We present quantitative morphology measurements of a sample of optically selected group galaxies at 0.3 < z < 0.55 using the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) and the GIM2D surface brightness fitting software package. The group sample is derived from the Canadian Network for Observational Cosmology Field Galaxy Redshift Survey (CNOC2) and follow-up Magellan spectroscopy. We compare these measurements to a similarly selected group sample from the Millennium Galaxy Catalogue (MGC) at 0.05 < z < 0.12. We find that, at both epochs, the group and field fractional bulge luminosity (B/T) distributions differ significantly, with the dominant difference being a deficit of disc-dominated (B/T < 0.2) galaxies in the group samples. At fixed luminosity, z = 0.4 groups have similar to 5.5 +/- 2 per cent fewer disc-dominated galaxies than the field, while by z = 0.1 this difference has increased to similar to 19 +/- 6 per cent. Despite the morphological evolution we see no evidence that the group environment is actively perturbing or otherwise affecting the entire existing disc population. At both redshifts, the discs of group galaxies have similar scaling relations and show similar median asymmetries as the discs of field galaxies. We do find evidence that the fraction of highly asymmetric, bulge-dominated galaxies is 6 +/- 3 per cent higher in groups than in the field, suggesting there may be enhanced merging in group environments. We replicate our group samples at z = 0.4 and 0 using the semi-analytic galaxy catalogues of Bower et al. This model accurately reproduces the B/T distributions of the group and field at z = 0.1. However, the model does not reproduce our finding that the deficit of discs in groups has increased significantly since z = 0.4.

Using deep Chandra and optical spectroscopic observations, we investigate an intriguing young massive group, RX J1648.7+6109, at z = 0.376, and we combine these observations with previous measurements to fit the scaling relations of intermediate-redshift groups and poor clusters. RX J1648 appears to be in an early stage of formation; while it follows X-ray scaling relations, its X-ray emission is highly elongated, and it lacks a central, dominant BCG. Instead, RX J1648 contains a central string of seven bright galaxies, which have a smaller velocity dispersion, are on average brighter, and have less star formation [lower EW([O II]) and EW(H delta)] than other group galaxies. The four to five brightest galaxies in this string should sink to the center and merge through dynamical friction by z = 0, forming a BCG consistent with a system of RX J1648's mass even if 5%-50% of the light is lost to an intracluster light component. The L-X-T-X relation for intermediate-redshift groups/poor clusters is very similar to the low-redshift cluster relation and consistent with the low-redshift group relation. In contrast, the L-X-sigma(nu) and sigma(nu)-T-X relations reveal that intermediate-redshift groups/poor clusters have significantly lower velocity dispersions for their X-ray properties compared to low-redshift systems; however, the intermediate-redshift relations are currently limited to a small range in luminosity.

The detection and characterization of the afterglow emission and host galaxies of short-hard gamma-ray bursts (SHBs) is one of the most exciting recent astronomical discoveries. In particular, indications that SHB progenitors belong to old stellar populations, in contrast to the long-soft GRBs, provide a strong clue about the physical nature of these systems. Definitive conclusions are currently limited by the small number of SHBs with known hosts available for study. Here, we present our investigation of SHBs previously localized by the interplanetary network (IPN). We show that the brightest galaxy within the error box of SHB 000607, at z = 0.1405, is the probable host galaxy of this event, expanding the sample of SHBs with known hosts and distances. We find a spatial association of the bright SHB 790613 and the cataloged position of the rich galaxy cluster Abell 1892. However, we are unable to verify the reality of this cluster via spectroscopy or multicolor imaging, and we conclude that this association may well be spurious. In addition, we rule out the existence of galaxy overdensities (down to approximate to 21 mag, i.e., approximate to 0.1 L-* at z-0.2) near the locations of two other SHBs and set a lower limit on their probable redshift. We combine our SHB sample with a complete sample of events discovered by the Swift and HETE-2 missions and investigate the properties of the extended sample. We show that the progenitors of SHBs appear to be older than those of Type Ia SNe, on average, suggesting a typical lifetime of several Gyr. The low typical redshift of SHBs leads to a significant increase in the local SHB rate and bodes well for the detection of gravitational radiation from these events, should they result from compact binary mergers, with forthcoming facilities.

The low-redshift universe (z similar to 0.5) is not a dull place. Processes leading to the suppression of star formation and morphological transformation are prevalent: this is particularly evident in the dramatic upturn in the fraction of S0-type galaxies in clusters. However, until now, the process and environment of formation remained unidentified. We present a morphological analysis of galaxies in the optically-selected (spectroscopic friends-of-friends) group and field environments at z similar to 0.4. Groups contain a much higher fraction of S0s at fixed luminosity than the lower density field, with >99.999% confidence. Indeed, the S0 fraction in groups is at least as high as in z similar to 0.4 clusters and X-ray-selected groups, which have more luminous intragroup medium (IGM). An excess of S0s at >= 0.3h(75)(-1) Mpc from the group center with respect to the inner regions, existing with 97% confidence at fixed luminosity, tells us that formation is not restricted to, and possibly even avoids, the group cores. Interactions with a bright X-ray-emitting IGM cannot be important for the formation of the majority of S0s in the universe. In contrast to S0s, the fraction of elliptical galaxies in groups at fixed luminosity is similar to the field, while the brightest ellipticals are strongly enhanced toward the group centers (greater than 99.999% confidence within >= 0.3h(75)(-1) Mpc). Interestingly, while spirals are altogether less common in groups than in the field, there is also an excess of faint, Sc+ type spirals within >= 0.3h(75)(-1) Mpc of the group centers (99.953% confidence). We conclude that the group and subgroup environments must be dominant for the formation of S0 galaxies, and that minor mergers, galaxy harassment, and tidal interactions are the most likely responsible mechanisms. This has implications not only for the inferred preprocessing of cluster galaxies, but also for the global morphological and star formation budget of galaxies: as hierarchical clustering progresses, more galaxies will be subject to these transformations as they enter the group environment.