Galaxy groups at intermediate redshift

Abstract

Galaxy groups likely to be virialized are identified within the CNOC2 intermediate-redshift galaxy survey. The resulting groups have a median velocity dispersion, 1200 km s-1. The virial masstolight ratios, using k-corrected and evolution-compensated luminosities, have medians in the range of 150250 h M/L, depending on group definition details. The numbervelocity dispersion relation at 1200 km s-1 is in agreement with the low-mass extrapolation of the cluster-normalized Press-Schechter model. Lower velocity dispersion groups are deficient relative to the Press-Schechter model. The two-point group-group autocorrelation function has r0=6.8±0.3 h-1 Mpc, which is much larger than the correlations of individual galaxies, but about as expected from biased clustering. The mean number density of galaxies around group centers falls nearly as a power law with r-2.5 and has no well-defined core. The projected velocity dispersion of galaxies around group centers is either flat or slowly rising outward. The combination of a steeper than isothermal density profile and the outward rising velocity dispersion implies that the mass-to-light ratio of groups rises with radius if the velocity ellipsoid is isotropic but could be nearly constant if the galaxy orbits are nearly circular. Such strong tangential anisotropy is not supported by other evidence. Although the implication of a rising M/L must be viewed with caution, it could naturally arise through dynamical friction acting on the galaxies in a background of "classical" collisionless dark matter.