The observational mass function of loose galaxy groups

We analyze the three catalogs of nearby loose groups compiled by A. M. Garc ia. She identified groups in a magnitude-limited redshift galaxy catalog, w hich covers about similar to 2/3 of sky within cz = 5500 km s(-1) using two methods, a percolation method and a hierarchical method. The free paramete rs of the group-selection algorithms were tuned to obtain similar catalogs of groups. The author also proposed a third catalog of groups, defined as a combination of the two. Each catalog contains almost 500 groups. In agreem ent with previous works on earlier catalogs, we find that groups can be des cribed as collapsing systems. Their sampled size is in general considerably larger than their expected virialized region. We compute the virial masses and correct them by taking into account the young dynamical status of thes e groups. We estimate corrected group masses, M, for two reference cosmolog ical models, a flat one with a matter density parameter Omega(0) = 1 and an open one with Omega(0) = 0.2. We calculate the mass function for each of t he three catalogs. We find that the amplitude of the mass function is not v ery sensitive to the choice of the group-identification algorithm. The numb er density of groups with M > 9 x 10(12) h(-1) M., which is the adopted lim it of sample completeness, ranges in the interval 1.3-1.9 x 10(-3) h(3) Mpc (-3) for Omega(0) = 1, and it is about a factor of 15% lower for Omega(0) = 0.2. The mass functions of the hierarchical and combined catalogs have ess entially the same shape, while the mass function of the percolation catalog shows a flattening toward large masses. However, the difference decreases if we do not consider the most massive groups, for which reliable results c ome from galaxy cluster studies. After having estimated the mass contained within the central, presumably virialized, regions of groups by adopting a reduction in mass of similar to 30%-40%, we make a comparison with the resu lts from the virial analysis of nearby rich clusters. All three group mass functions turn out to be a smooth extrapolation of the cluster mass functio n at M < 4 x 10(14) h(-1) M., which is the completeness Limit of the cluste r sample. The resulting optical virial mass function of galaxy systems, whi ch extends over 2 orders of magnitude, is fitted to a Schechter expression with a slope of similar to 1.5 and a characteristic mass of M* similar to 3 x 10(14) h(-1) M.. We also verify that our group mass function agrees reas onably well with the Press-Schechter predictions of models which at large m asses describe the virial mass function of clusters.