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(*).

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.