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.

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 quantitative morphology measurements of a sample of optically selected group galaxies at 0.3 < z < 0.55 using the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) and the GIM2D surface brightness fitting software package. The group sample is derived from the Canadian Network for Observational Cosmology Field Galaxy Redshift Survey (CNOC2) and follow-up Magellan spectroscopy. We compare these measurements to a similarly selected group sample from the Millennium Galaxy Catalogue (MGC) at 0.05 < z < 0.12. We find that, at both epochs, the group and field fractional bulge luminosity (B/T) distributions differ significantly, with the dominant difference being a deficit of disc-dominated (B/T < 0.2) galaxies in the group samples. At fixed luminosity, z = 0.4 groups have similar to 5.5 +/- 2 per cent fewer disc-dominated galaxies than the field, while by z = 0.1 this difference has increased to similar to 19 +/- 6 per cent. Despite the morphological evolution we see no evidence that the group environment is actively perturbing or otherwise affecting the entire existing disc population. At both redshifts, the discs of group galaxies have similar scaling relations and show similar median asymmetries as the discs of field galaxies. We do find evidence that the fraction of highly asymmetric, bulge-dominated galaxies is 6 +/- 3 per cent higher in groups than in the field, suggesting there may be enhanced merging in group environments. We replicate our group samples at z = 0.4 and 0 using the semi-analytic galaxy catalogues of Bower et al. This model accurately reproduces the B/T distributions of the group and field at z = 0.1. However, the model does not reproduce our finding that the deficit of discs in groups has increased significantly since z = 0.4.

We present quantitative morphology measurements of a sample of optically selected group galaxies at 0.3 < z < 0.55 using the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) and the GIM2D surface brightness fitting software package. The group sample is derived from the Canadian Network for Observational Cosmology Field Galaxy Redshift Survey (CNOC2) and follow-up Magellan spectroscopy. We compare these measurements to a similarly selected group sample from the Millennium Galaxy Catalogue (MGC) at 0.05 < z < 0.12. We find that, at both epochs, the group and field fractional bulge luminosity (B/T) distributions differ significantly, with the dominant difference being a deficit of disc-dominated (B/T < 0.2) galaxies in the group samples. At fixed luminosity, z = 0.4 groups have similar to 5.5 +/- 2 per cent fewer disc-dominated galaxies than the field, while by z = 0.1 this difference has increased to similar to 19 +/- 6 per cent. Despite the morphological evolution we see no evidence that the group environment is actively perturbing or otherwise affecting the entire existing disc population. At both redshifts, the discs of group galaxies have similar scaling relations and show similar median asymmetries as the discs of field galaxies. We do find evidence that the fraction of highly asymmetric, bulge-dominated galaxies is 6 +/- 3 per cent higher in groups than in the field, suggesting there may be enhanced merging in group environments. We replicate our group samples at z = 0.4 and 0 using the semi-analytic galaxy catalogues of Bower et al. This model accurately reproduces the B/T distributions of the group and field at z = 0.1. However, the model does not reproduce our finding that the deficit of discs in groups has increased significantly since z = 0.4.

We present quantitative morphology measurements of a sample of optically selected group galaxies at 0.3 < z < 0.55 using the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) and the GIM2D surface brightness fitting software package. The group sample is derived from the Canadian Network for Observational Cosmology Field Galaxy Redshift Survey (CNOC2) and follow-up Magellan spectroscopy. We compare these measurements to a similarly selected group sample from the Millennium Galaxy Catalogue (MGC) at 0.05 < z < 0.12. We find that, at both epochs, the group and field fractional bulge luminosity (B/T) distributions differ significantly, with the dominant difference being a deficit of disc-dominated (B/T < 0.2) galaxies in the group samples. At fixed luminosity, z = 0.4 groups have similar to 5.5 +/- 2 per cent fewer disc-dominated galaxies than the field, while by z = 0.1 this difference has increased to similar to 19 +/- 6 per cent. Despite the morphological evolution we see no evidence that the group environment is actively perturbing or otherwise affecting the entire existing disc population. At both redshifts, the discs of group galaxies have similar scaling relations and show similar median asymmetries as the discs of field galaxies. We do find evidence that the fraction of highly asymmetric, bulge-dominated galaxies is 6 +/- 3 per cent higher in groups than in the field, suggesting there may be enhanced merging in group environments. We replicate our group samples at z = 0.4 and 0 using the semi-analytic galaxy catalogues of Bower et al. This model accurately reproduces the B/T distributions of the group and field at z = 0.1. However, the model does not reproduce our finding that the deficit of discs in groups has increased significantly since z = 0.4.

We present quantitative morphology measurements of a sample of optically selected group galaxies at 0.3 < z < 0.55 using the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) and the GIM2D surface brightness fitting software package. The group sample is derived from the Canadian Network for Observational Cosmology Field Galaxy Redshift Survey (CNOC2) and follow-up Magellan spectroscopy. We compare these measurements to a similarly selected group sample from the Millennium Galaxy Catalogue (MGC) at 0.05 < z < 0.12. We find that, at both epochs, the group and field fractional bulge luminosity (B/T) distributions differ significantly, with the dominant difference being a deficit of disc-dominated (B/T < 0.2) galaxies in the group samples. At fixed luminosity, z = 0.4 groups have similar to 5.5 +/- 2 per cent fewer disc-dominated galaxies than the field, while by z = 0.1 this difference has increased to similar to 19 +/- 6 per cent. Despite the morphological evolution we see no evidence that the group environment is actively perturbing or otherwise affecting the entire existing disc population. At both redshifts, the discs of group galaxies have similar scaling relations and show similar median asymmetries as the discs of field galaxies. We do find evidence that the fraction of highly asymmetric, bulge-dominated galaxies is 6 +/- 3 per cent higher in groups than in the field, suggesting there may be enhanced merging in group environments. We replicate our group samples at z = 0.4 and 0 using the semi-analytic galaxy catalogues of Bower et al. This model accurately reproduces the B/T distributions of the group and field at z = 0.1. However, the model does not reproduce our finding that the deficit of discs in groups has increased significantly since z = 0.4.

Using deep Chandra and optical spectroscopic observations, we investigate an intriguing young massive group, RX J1648.7+6109, at z = 0.376, and we combine these observations with previous measurements to fit the scaling relations of intermediate-redshift groups and poor clusters. RX J1648 appears to be in an early stage of formation; while it follows X-ray scaling relations, its X-ray emission is highly elongated, and it lacks a central, dominant BCG. Instead, RX J1648 contains a central string of seven bright galaxies, which have a smaller velocity dispersion, are on average brighter, and have less star formation [lower EW([O II]) and EW(H delta)] than other group galaxies. The four to five brightest galaxies in this string should sink to the center and merge through dynamical friction by z = 0, forming a BCG consistent with a system of RX J1648's mass even if 5%-50% of the light is lost to an intracluster light component. The L-X-T-X relation for intermediate-redshift groups/poor clusters is very similar to the low-redshift cluster relation and consistent with the low-redshift group relation. In contrast, the L-X-sigma(nu) and sigma(nu)-T-X relations reveal that intermediate-redshift groups/poor clusters have significantly lower velocity dispersions for their X-ray properties compared to low-redshift systems; however, the intermediate-redshift relations are currently limited to a small range in luminosity.