We present an automated method to detect populations of groups in galaxy
redshift catalogs. This method uses both analysis of the redshift distribution
along lines of sight in fixed cells to detect elementary structures and a
friend-of-friend algorithm to merge these elementary structures into physical
structures. We apply this method to the SSRS2 galaxy redshift catalog. The
groups detected with our method are similar to group catalogs detected with
pure friend-of-friend algorithms. They have similar mass distribution, similar
abundance versus redshift, similar 2-point correlation function and the same
redshift completeness limit, close to 5000 km/s. If instead of SSRS2, we use
catalogs of new generation, it would lead to a completeness limit of
z∼0.7. We model the luminosity function for nearby galaxy groups by a
Schechter function with parameters M*=(-19.99+/-0.36)+5logh and alpha=-1.46 +/-
0.17 to compute the mass to light ratio. The median value of the mass to light
ratio is 360 h M/L and we deduce a relation between mass to light ratio and
velocity dispersion sigma (M/L=3.79 +/- 0.64)sigma -(294 +/- 570)). The more
massive the group, the higher the mass to light ratio, and therefore, the
larger the amount of dark matter inside the group. Another explanation is a
significant stripping of the gas of the galaxies in massive groups as opposed
to low mass groups. This extends to groups of galaxies the mild tendency
already detected for rich clusters of galaxies. Finally, we detect a barely
significant fundamental plane for these groups but much less narrow than for
clusters of galaxies.Comment: 8 pages, 5 figures, accepted in A&A, shortened abstrac
