Galaxy groups at intermediate redshift

Galaxy groups likely to be virialized are identified within the CNOC2 inter mediate-redshift galaxy survey. The resulting groups have a median velocity dispersion, sigma (1) similar or equal to 200 km s(-1). The virial mass-to -light ratios, using k-corrected and evolution-compensated luminosities, ha ve medians in the range of 150-250 h M./L., depending on group definition d etails. The number-velocity dispersion relation at sigma (1) greater than o r similar to 200 km s(-1) is in agreement with the low-mass extrapolation o f the cluster-normalized Press-Schechter model. Lower velocity dispersion g roups are deficient relative to the Press-Schechter model, The two-point gr oup-group autocorrelation function has r(0) = 6.8 +/- 0.3 h(-1) Mpc, which is much larger than the correlations of individual galaxies, but about as e xpected 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-defi ned core. The projected velocity dispersion of galaxies around group center s is either hat or slowly rising outward. The combination of a steeper than isothermal density profile and the outward rising velocity dispersion impl ies that the mass-to-light ratio of groups rises with radius if the velocit y 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 wit h caution, it could naturally arise through dynamical friction acting on th e galaxies in a background of "classical" collisionless dark matter.