As groups today contain similar to 60% of the galaxy population [1], and are the first step in the hierarchical growth tree which dominates structure formation, these environments must have a critical influence on the evolution of star formation in the Universe as a whole. Indeed their dynamics make them the ideal environments to foster galaxy galaxy interactions and mergers, leading to a dramatic transformation of galaxy properties. To study the evolution of galaxies in groups requires highly complete, targetted, deep spectroscopic surveys. At intermediate redshift, the only such is our sample of 26 groups at 0.3 < z < 0.55, selected from the CNOC2 redshift survey [2], with additional targetted spectroscopy using the Magellan 6.5m and VLT telescopes providing a complete kinematic description to a depth of similar to M-*. + 3 at z = 0.4. [3]. Our full multiwavelength dataset will include HST-ACS, GALEX UV. Chandra, XMM and Spitzer imaging, with the power to ultimately reveal the importance of the group environment in controlling the evolutionary fate of a galaxy. In this contribution, we present some of the more recent and illuminating analysis, revealing evolution in the group environment and the dependence of starformation and galaxy morphologies upon environment and stellar mass. Finally we discuss the important role Spitzer will play in revealing the processes actively transforming galaxies in the group environment.

We have obtained near-infrared (NIR) imaging of 58 galaxy groups, in the redshift range 0.1 < z < 0.6, from the William Herschel Telescope and from the Spitzer telescope Infrared Array Camera (IRAC) data archive. The groups are selected from the CNOC2 redshift survey, with additional spectroscopy from the Baade telescope (Magellan). Our group samples are statistically complete to K-Vega = 17.7 (INGRID) and [3.6 mu m](AB) = 19.9 (IRAC). From these data we construct NIR luminosity functions, for groups in bins of velocity dispersion, up to 800 km s(-1), and redshift. The total amount of NIR luminosity per group is compared with the dynamical mass, estimated from the velocity dispersion, to compute the mass-to-light ratio, M-200/L-K. We find that the M-200/L-K values in these groups are in good agreement with those of their statistical descendants at z = 0, with no evidence for evolution beyond that expected for a passively evolving population. There is a trend of M-200/L-K with group mass, which increases from M-200/L-K approximate to 10 for groups with sigma < 250 km s(-1) to M-200/L-K approximate to 100 for 425 km s(-1) < sigma < 800 km s(-1). This trend is weaker, but still present, if we estimate the total mass from weak lensing measurements. In terms of stellar mass, stars make up greater than or similar to 2 per cent of the mass in the smallest groups, and less than or similar to 1 per cent in the most massive groups. We also use the NIR data to consider the correlations between stellar populations and stellar masses, for group and field galaxies at 0.1 < z < 0.6. We find that fewer group galaxies show strong [O (II)] emission, compared with field galaxies of the same stellar mass and at the same redshift. We conclude that most of the stellar mass in these groups was already in place by z similar to 0.4, with little environment-driven evolution to the present day.

We investigate the galaxy populations in seven X-ray-selected, intermediate-redshift groups (0.2 < z < 0.6). Overall, the galaxy populations in these systems are similar to those in clusters at the same redshift; they have large fractions of early-type galaxies (f(e)similar to 70%) and small fractions of galaxies with significant star formation (f ([OII])similar to 30%). We do not observe a strong evolution in the galaxy populations from those seen in X-ray-luminous groups at low redshift. Both f(e) and f ([OII]) are correlated with radius but do not reach the field value out to similar to r(500). However, we find significant variation in the galaxy populations between groups, with some groups having fieldlike populations. Comparisons between the morphological and spectral properties of group galaxies reveal both gas-poor mergers and a population of passive spirals. Unlike low-redshift, X-ray-emitting groups, in some of these groups the brightest galaxy does not lie at the center of the X-ray emission, and in several of the groups that do have a central BGG, the BGG has multiple components. These groups appear to represent a range of evolutionary stages in the formation of the BGG. Some groups have relatively large central galaxy densities, and one group contains a string of seven bright galaxies within a radius of 200 kpc that have a lower velocity dispersion than the rest of the system. None of the central galaxies, including those with multiple components, have significant ([O II]) emission. These observations support a scenario in which BGGs are formed relatively late through gas-poor mergers.

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.

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.

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.