### Galaxy Groups in Cold + Hot and CDM Universes: Comparison with CfA

#### Abstract

This letter presents results of new high resolution $\Omega=1$
Cold + Hot Dark Matter (CHDM) and Cold Dark Matter (CDM)
simulations. Properties of groups in these simulations
reflect the lower small-scale velocities 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 very robust statistic, median group rms velocity
$v_{\rm gr}(f_{\rm gr})$ as a function of the fraction
$f_{\rm gr}$ of galaxies in groups. Using this statistic, we
compare ``observed'' simulations to CfA1 data in redshift
space in a careful and consistent way. We find that CHDM
remains a promising model, with for example $v_{\rm gr}(0.45)
\approx 125 \pm 25 \kms$ in agreement with the CfA data,
while CDM with bias b=1.0 (COBE-compatible) or b=1.5, both
giving $v_{\rm gr}(0.45) \approx 400 \pm 25 \kms$, can be
virtually ruled out. Using median $M/L$, the observed value of
$\Omega$ is $0.10$ (CHDM) to $0.38$ (CDM).

Reference: ApJ Lett 422, L45 (1994)

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