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%.

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%.

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%.

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.

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.

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.

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 a cosmological chemodynamical simulation to study how the group environment impacts the star formation ( SF) properties of disk galaxies. The simulated group has a total mass of M similar to 8 x 10(12) M-circle dot and a total X-ray, luminosity of L-x similar to 10(41) erg s(-1). Our simulation suggests that ram pressure is not sufficient in this group to remove the cold disk gas from a V-rot similar to 150 km s galaxy. However, the majority of the hot gas in the galaxy is stripped over a timescale of approximately 1 Gyr. Since the cooling of the hot-gas component provides a source for new cold gas, the stripping of the hot component effectively cuts off the supply of cold gas. This in turn leads to a quenching of SF. The galaxy maintains the disk component after the cold gas is consumed, which may lead to a galaxy similar to an S0. Our self-consistent simulation suggests that this strangulation mechanism works even in low-mass groups, providing an explanation for the lower SF rates in group galaxies relative to galaxies in the field.

We present a detailed analysis of the intergalactic metal-line absorption systems in the archival HST STIS and FUSE ultraviolet spectra of the low-redshift quasar PKS 1302 - 102 ( z(QSO) 0: 2784). We supplement the archive data with CLOUDY ionization models and a survey of galaxies in the quasar field. There are 15 strong Ly proportional to absorbers with column densities log N-H (I) > 14. Of these, six are associated with at least C III lambda 977 absorption [ log N( C++) > 13]; this implies a redshift density dN(CIII) / dz = 36(-9)(+13) (68% confidence limits) for the five detections with rest equivalent width W-r > 50 m angstrom. Two systems show O VI lambda lambda 1031, 1037 absorption in addition to C III [log N(O+5) > 14]. One is a partial Lyman limit system (log N-HI = 17) with associated C III, O VI, andSi III lambda 1206 absorption. There are three tentative O VI systems that do not have C III detected. For one O VI doublet with both lines detected at 3 sigma withW(r) > 50m angstrom, dN(O) (VI) /dz = 7(-4)(.)(+9) We also search for O VI doublets without Ly alpha absorption but identify none. From CLOUDY modeling, these metal- line systems have metallicities spanning the range - 4 less than or similar to[M/H] less than or similar to - 0.3. The two O vi systems with associated C III absorption cannot be single- phase, collisionally ionized media based on the relative abundances of the metals and kinematic arguments. From the galaxy survey, we discover that the absorption systems are in a diverse set of galactic environments. Each metal- line system has at least one galaxy within 500 km s(-1) and 600 h(75)(-1) kpc with L > 0.1L(*).