The stellar mass content of distant galaxy groups

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 um] 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 /LK. We find that the M 200 /LK 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 /LK with group mass, which increases from M 200 /LK ≈ 10 for groups with a < 250km S -1 to M 200 /LK ≈ 100 for 425 km s -1 < a < 800km 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 ≥2 per cent of the mass in the smallest groups, and ≤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 [On] 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 ∼ 0.4, with little environment-driven evolution to the present day.