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.

We present an imaging and spectroscopic survey of galaxies in fields around QSOs HE 0226-4110, PKS 0405-123, and PG 1216+069. The fields are selected to have ultraviolet echelle spectra available, which uncover 195 Ly alpha absorbers and 13 O VI absorbers along the three sightlines. We obtain robust redshifts for 1104 galaxies of rest-frame absolute magnitude M-R - 5 log h less than or similar to - 16 and at projected physical distances. rho less than or similar to 4 h(-1) Mpc from the QSOs. Hubble Space Telescope (HST)/WFPC2 images of the fields around PKS 0405-123 and PG 1216+069 are available for studying the optical morphologies of absorbing galaxies. Combining the absorber and galaxy data, we perform a cross-correlation study to understand the physical origin of Lya and O VI absorbers and to constrain the properties of extended gas around galaxies. The results of our study are: (1) both strong Ly alpha absorbers of log N(H I) >= 14 and O VI absorbers exhibit a comparable clustering amplitude as emission-line-dominated galaxies and a factor of approximate to 6 weaker amplitude than absorption-line-dominated galaxies on comoving projected distance scales of r(p) < 3 h(-1) Mpc; (2) weak Ly alpha absorbers of log N(H I) < 13.5 appear to cluster very weakly around galaxies; (3) none of the absorption-line-dominated galaxies at r(p) <= 250 h(-1) kpc has a corresponding O VI absorber to a sensitive upper limit of W(1031) less than or similar to 0.03 angstrom, while the covering fraction of O VI absorbing gas around emission-line-dominated galaxies is found to be kappa approximate to 64%; and (4) high-resolution images of five O VI absorbing galaxies show that these galaxies exhibit disk-like morphologies with mildly disturbed features on the edge. Together, the data indicate that O VI absorbers arise preferentially in gas-rich galaxies. In addition, tidal debris in groups/galaxy pairs may be principally responsible for the observed O VI absorbers, particularly those of W(1031) > 70 m angstrom.

We present new optical and near-infrared imaging for a sample of 98 spectroscopically selected galaxy groups at 0.25 < z < 0.55, most of which have velocity dispersions Sigma < 500 km s(-1). We use point spread function matched aperture photometry to measure accurate colours for group members and the surrounding field population. The sample is statistically complete above a stellar mass limit of approximately M = 1 x 10(10) M(circle dot). The overall colour distribution is bimodal in both the field and group samples; but, at fixed luminosity the fraction of group galaxies populating the red peak is larger, by similar to 20 +/- 7 per cent, than that of the field. In particular, group members with early-type morphologies, as identified in Hubble Space Telescope imaging, exhibit a tight red sequence, similar to that seen for more massive clusters. Using optical and near-infrared colours, including data from the Spitzer Space Telescope, we show that approximately 20-30 per cent of galaxies on the red sequence may be dust-reddened galaxies with non-negligible star formation and early-spiral morphologies. This is true of both the field and group samples, and shows little dependence on near-infrared luminosity. Thus, the fraction of bright ((0.4)M(K) < -22) group members with no sign of star formation or active galactic nuclei activity, as identified by their colours or [O ii] emission, is 54 +/- 6 per cent. Our field sample, which includes galaxies in all environments, contains 35 +/- 3 per cent of such inactive galaxies, consistent with the amount expected if all such galaxies are located in groups and clusters. This reinforces our earlier conclusions that dense environments at z less than or similar to 0.5 are associated with a premature cessation of star formation in some galaxies; in particular, we find no evidence for significantly enhanced star formation in these environments. Simple galaxy formation models predict a quenching of star formation in groups that is too efficient, overpopulating the red sequence. Attempts to fix this by increasing the time-scale of this quenching equally for all group members distort the colour distribution in a way that is inconsistent with observations.

X-ray properties of galaxy groups can unlock some of the most challenging research topics in modern extragalactic astronomy: the growth of structure and its influence on galaxy formation. Only with the advent of the Chandra and XMM-Newton facilities have X-ray observations reached the depths required to address these questions in a satisfactory manner. Here we present an X-ray imaging study of two patches from the CNOC2 spectroscopic galaxy survey using combined Chandra and XMM-Newton data. A state of the art extended source finding algorithm has been applied, and the resultant source catalog, including redshifts from a spectroscopic follow-up program, is presented. The total number of spectroscopically identified groups is 25 spanning a redshift range 0.04-0.79. Approximately 50% of CNOC2 spectroscopically selected groups in the deeper X-ray ( RA14h) field are likely X-ray detections, compared to 20% in the shallower ( RA21h) field. Statistical modeling shows that this is consistent with expectations, assuming an expected evolution of the LX-M relation. A significant detection of a stacked shear signal for both spectroscopic and X-ray groups indicates that both samples contain real groups of about the expected mass. We conclude that the current area and depth of X-ray and spectroscopic facilities provide a unique window of opportunity at z similar to 0.4 to test the X-ray appearance of galaxy groups selected in various ways. There is at present no evidence that the correlation between X-ray luminosity and velocity dispersion evolves significantly with redshift, which implies that catalogs based on either method can be fairly compared and modeled.

We have performed a systematic search for X-ray cavities in the hot gas of 51 galaxy groups with Chandra archival data. The cavities are identified based on two methods: subtracting an elliptical beta-model fitted to the X-ray surface brightness, and performing unsharp masking. Thirteen groups in the sample (similar to 25%) are identified as clearly containing cavities, with another 13 systems showing tentative evidence for such structures. We find tight correlations between the radial and tangential radii of the cavities, and between their size and projected distance from the group center, in quantitative agreement with the case for more massive clusters. This suggests that similar physical processes are responsible for cavity evolution and disruption in systems covering a large range in total mass. We see no clear association between the detection of cavities and the current 1.4 GHz radio luminosity of the central brightest group galaxy, but there is a clear tendency for systems with a cool core to be more likely to harbor detectable cavities. To test the efficiency of the adopted cavity detection procedures, we employ a set of mock images designed to mimic typical Chandra data of our sample, and find that the model-fitting approach is generally more reliable than unsharp masking for recovering cavity properties. Finally, we find that the detectability of cavities is strongly influenced by a few factors, particularly the signal-to-noise ratio of the data, and that the real fraction of X-ray groups with prominent cavities could be substantially larger than the 25%-50% suggested by our analysis.

We present the first mid-IR study of galaxy groups in the nearby universe based on Spitzer MIPS observations of a sample of nine redshift-selected groups from the XMM-IMACS project, at z = 0.06. We find that on average the star-forming (SF) galaxy fraction in the groups is about 30% lower than the value in the field and 30% higher than in clusters. The SF fractions do not show any systematic dependence on group velocity dispersion, total stellar mass, or the presence of an X-ray emitting intragroup medium, but a weak anti-correlation is seen between SF fraction and projected galaxy density. However, even in the densest regions, the SF fraction in groups is still higher than that in cluster outskirts, suggesting that preprocessing of galaxies in group environments is not sufficient to explain the much lower SF fraction in clusters. The typical specific star formation rates (SFRs/M(*)) of SF galaxies in groups are similar to those in the field across a wide range of stellar mass (M(*) > 10(9.6) M(circle dot)), favoring a quickly acting mechanism that suppresses star formation to explain the overall smaller fraction of SF galaxies in groups. If galaxy -galaxy interactions are responsible, then the extremely low starburst galaxy fraction (<1%) implies a short timescale (similar to 0.1 Gyr) for any merger-induced starburst stage. Comparison to two rich clusters shows that clusters contain a population of massive SF galaxies with very low SFR (14% of all the galaxies with M(*) > 10(10) M(circle dot)), possibly as a consequence of ram pressure stripping being less efficient in removing gas frommoremassive galaxies.

We use Chandra and XMM-Newton to study the hot gas content in a sample of field early-type galaxies. We find that the L(X)-L(K) relationship is steeper for field galaxies than for comparable galaxies in groups and clusters. The low hot gas content of field galaxies with L(K) less than or similar to L(star) suggests that internal processes such as supernovae-driven winds or active galactic nucleus feedback expel hot gas from low-mass galaxies. Such mechanisms may be less effective in groups and clusters where the presence of an intragroup or intracluster medium can confine outflowing material. In addition, galaxies in groups and clusters may be able to accrete gas from the ambient medium. While there is a population of L(K) less than or similar to L(star) galaxies in groups and clusters that retain hot gas halos, some galaxies in these rich environments, including brighter galaxies, are largely devoid of hot gas. In these cases, the hot gas halos have likely been removed via ram pressure stripping. This suggests a very complex interplay between the intragroup/intracluster medium and hot gas halos of galaxies in rich environments, with the ambient medium helping to confine or even enhance the halos in some cases and acting to remove gas in others. In contrast, the hot gas content of more isolated galaxies is largely a function of the mass of the galaxy, with more massive galaxies able to maintain their halos, while in lower mass systems the hot gas escapes in outflowing winds.

We examine the star formation properties of group and field galaxies in two surveys, Sloan Digital Sky Survey (at z similar to 0.08) and Group Environment Evolution Collaboration (GEEC; at z similar to 0.4). Using ultraviolet imaging from the Galaxy Evolution Explorer space telescope, along with optical and, for GEEC, near-infrared photometry, we compare the observed spectral energy distributions to large suites of stellar population synthesis models. This allows us to accurately determine star formation rates and stellar masses. We find that star-forming galaxies of all environments undergo a systematic lowering of their star formation rate between z = 0.4 and 0.08 regardless of mass. None the less, the fraction of passive galaxies is higher in groups than the field at both redshifts. Moreover, the difference between the group and field grows with time and is mass dependent, in the sense the difference is larger at low masses. However, the star formation properties of star-forming galaxies, as measured by their average specific star formation rates, are consistent within the errors in the group and field environment at fixed redshift. The evolution of passive fraction in groups between z = 0.4 and 0 is consistent with a simple accretion model, in which galaxies are environmentally affected 3 Gyr after falling into a similar to 1013 M-circle dot group. This long time-scale appears to be inconsistent with the need to transform galaxies quickly enough to ensure that star-forming galaxies appear similar in both the group and field, as observed.