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.

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.

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.

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.

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 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 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 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.