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 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 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.

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.

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.

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 report the discovery of a small galaxy system in the vicinity of the Ne VIII absorber at z = 0.20701 toward HE0226 - 4110. The galaxy system consists of two 0.25 L(*) disk galaxies and a 0.05 L(*) galaxy all within Delta v < 300 km s(-1) and rho <= 200 h(-1) physical kpc of the absorber. We consider various scenarios for the origin of the Ne VIII absorption, including photo-ionized gas from an active galactic nucleus, a starburst-driven wind, a hot intragroup medium, hot gas in a galaxy halo, and a conductive front produced by cool clouds moving at high speed through a hot medium. We argue that the conductive front scenario is most likely responsible for producing the Ne VIII feature, because it is consistent with the observed galactic environment around the absorber and because it naturally explains the multi-phase nature of the gas and the kinematic signatures of the absorption profiles. Although our preferred scenario suggests that Ne VIII may not be directly probing the warm-hot intergalactic medium, it does imply the existence of an extended hot confining medium around a disk galaxy that may contain a significant reservoir of baryons in the form of hot gas.

We report the discovery of a small galaxy system in the vicinity of the Ne VIII absorber at z = 0.20701 toward HE0226 - 4110. The galaxy system consists of two 0.25 L(*) disk galaxies and a 0.05 L(*) galaxy all within Delta v < 300 km s(-1) and rho <= 200 h(-1) physical kpc of the absorber. We consider various scenarios for the origin of the Ne VIII absorption, including photo-ionized gas from an active galactic nucleus, a starburst-driven wind, a hot intragroup medium, hot gas in a galaxy halo, and a conductive front produced by cool clouds moving at high speed through a hot medium. We argue that the conductive front scenario is most likely responsible for producing the Ne VIII feature, because it is consistent with the observed galactic environment around the absorber and because it naturally explains the multi-phase nature of the gas and the kinematic signatures of the absorption profiles. Although our preferred scenario suggests that Ne VIII may not be directly probing the warm-hot intergalactic medium, it does imply the existence of an extended hot confining medium around a disk galaxy that may contain a significant reservoir of baryons in the form of hot gas.

We report the discovery of a small galaxy system in the vicinity of the Ne VIII absorber at z = 0.20701 toward HE0226 - 4110. The galaxy system consists of two 0.25 L(*) disk galaxies and a 0.05 L(*) galaxy all within Delta v < 300 km s(-1) and rho <= 200 h(-1) physical kpc of the absorber. We consider various scenarios for the origin of the Ne VIII absorption, including photo-ionized gas from an active galactic nucleus, a starburst-driven wind, a hot intragroup medium, hot gas in a galaxy halo, and a conductive front produced by cool clouds moving at high speed through a hot medium. We argue that the conductive front scenario is most likely responsible for producing the Ne VIII feature, because it is consistent with the observed galactic environment around the absorber and because it naturally explains the multi-phase nature of the gas and the kinematic signatures of the absorption profiles. Although our preferred scenario suggests that Ne VIII may not be directly probing the warm-hot intergalactic medium, it does imply the existence of an extended hot confining medium around a disk galaxy that may contain a significant reservoir of baryons in the form of hot gas.