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(*).

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.

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.

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.

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.

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 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.