Ram-pressure stripping of galactic gas is generally assumed to be inefficient in galaxy groups due to the relatively low density of the intragroup medium (IGM) and the small velocity dispersions of groups. To test this assumption, we obtained Chandra X-ray data of the starbursting spiral NGC 2276 in the NGC 2300 group of galaxies, a candidate for a strong galaxy interaction with hot intragroup gas. The data reveal a shock-like feature along the western edge of the galaxy and a low surface brightness tail extending to the east, similar to the morphology seen in other wavebands. Spatially resolved spectroscopy shows that the data are consistent with intragroup gas being pressurized at the leading western edge of NGC 2276 due to the galaxy moving supersonically through the IGM at a velocity similar to 850 km s(-1). Detailed modelling of the gravitational potential of NGC 2276 shows that the resulting ram pressure could significantly affect the morphology of the outer gas disc but is probably insufficient to strip large amounts of cold gas from the disc. We estimate the mass-loss rates due to turbulent viscous stripping and starburst outflows being swept back by ram pressure, showing that both mechanisms could plausibly explain the presence of the X-ray tail. Comparison to existing H I measurements shows that most of the gas escaping the galaxy is in a hot phase. With a total mass-loss rate of similar to 5 M-circle dot yr(-1), the galaxy could be losing its entire present H I supply within a Gyr. This demonstrates that the removal of galactic gas through interactions with a hot IGM can occur rapidly enough to transform the morphology of galaxies in groups. Implications of this for galaxy evolution in groups and clusters are briefly discussed.

We use a cosmological numerical simulation to study the tidal features produced by a minor merger with an elliptical galaxy. We find that the simulated tidal features are quantitatively similar to the red tidal features, i.e., dry tidal features, recently found in deep images of elliptical galaxies at intermediate redshifts. The minor merger in our simulation does not trigger star formation due to active galactic nuclei heating. Therefore, both the tidal features and the host galaxy are red, i.e., a dry minor merger. The stellar mass of the infalling satellite galaxy is about 10(10) M-circle dot, and the tidal debris reach the surface brightness of mu(R) similar to 27 mag arcsec(-2). Thus, we conclude that tidal debris from minor mergers can explain the observed dry tidal features in elliptical galaxies at intermediate redshifts, although other mechanisms (such as major dry mergers) may also be important.

We have undertaken a multiwavelength project to study the relatively unknown properties of groups and poor clusters of galaxies at intermediate redshifts. In this paper, we describe the XMM-Newton observations of six X-ray selected groups with 0.2 < z < 0: 6. The X-ray properties of these systems are generally in good agreement with the properties of low-redshift groups. They appear to follow the scaling relations between luminosity, temperature, and velocity dispersion defined by low-redshift groups and clusters. The X-ray emission in four of the six groups is also centered on a dominant early-type galaxy. The lack of a bright elliptical galaxy at the peak of the group X-ray emission is rare at low redshifts, and the other two groups may be less dynamically evolved. We find indications of excess entropy in these systems over self-similar predictions out to large radii. We also confirm the presence of at least one X-ray-luminous AGN associated with a group member galaxy and find several other potential group AGNs.

X-ray observations of hot, intergalactic gas in galaxy groups provide a useful means of characterizing the global properties of groups. However, X-ray studies of large group samples have typically involved very shallow X-ray exposures or have been based on rather heterogeneous samples. Here we present the first results of the XI (XMM/IMACS) Groups Project, a study targeting, for the first time, a redshift-selected, statistically unbiased sample of galaxy groups using deep X-ray data. Combining this with radio observations of cold gas and optical imaging and spectroscopy of the galaxy population, the project aims to advance the understanding of how the properties and dynamics of group galaxies relate to global group properties. Here, X-ray and optical data of the first four galaxy groups observed as part of the project are presented. In two of the groups we detect diffuse emission with a luminosity of L-X approximate to 10(41) erg s(-1), among the lowest found for any X-ray detected group thus far, with a comparable upper limit for the other two. Compared to typical X-ray selected groups of similar velocity dispersion, these four systems are all surprisingly X-ray faint. We discuss possible explanations for the lack of significant X-ray emission in the groups, concluding that these systems are most likely collapsing for the first time. Our results strongly suggest that, unlike our current optically selected sample, previous X-ray selected group samples represented a biased picture of the group population. This underlines the necessity of a study of this kind, if one is to reach an unbiased census of the properties of galaxy groups and the distribution of baryons in the Universe.

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.