GALAXY GROUPS IN COLD PLUS HOT DARK-MATTER AND COLD DARK-MATTER UNIVERSES - COMPARISON WITH CFA DATA

This Letter presents results of new high-resolution OMEGA = 1 cold + h ot dark matter (CHDM) and cold dark matter (CDM) simulations. Properti es of groups in these simulations reflect the lower small-scale veloci ties and the greater tendency to form distinct filaments on both small and large scales in CHDM as compared to CDM. The fraction of galaxies in groups and the median group rms velocity are found to be powerful discriminators between models. We combine these two features into a ve ry robust statistic, median group rms velocity v(gr)(f(gr)) as a funct ion of the fraction f(gr) of galaxies in groups. Using this statistic, we compare ''observed'' simulations to CfA data in redshift space in a careful and consistent way. We find that CHDM remains a promising mo del, with for example v(gr)(0.45) almost-equal-to 125 +/- 25 km s-1 in agreement with the CfA data, while CDM with bias b = 1.0 (COBE-compat ible) or b = 1.5, both giving v(gr)(0.45) almost-equal-to 400 +/- 25 k m s-1, can be virtually ruled out. Using median M/L, the observed valu e of OMEGA is 0.10 (CHDM) to 0.38 (CDM).