Groups of galaxies in the Center for Astrophysics redshift survey

We apply an objective group identification algorithm described by Huchra and Geller to the Center for Astrophysics redshift survey complete to mB (0) _ = 15. 5 over the right ascension range 8ʰ^ ~30% of the groups with three or four members are probably an artifact of the geometry. The median velocity dispersion for the 36 groups in the "statistical sample" with five or more members is σᵥ_ = 228 km s^-1^, and the median M/LB (0) _ is 178h Mₛun_/Lₛun_, where the Hubble constant H₀_ is 100h km s^-1^ Mpc^-1^ (we take h = 1 unless otherwise indicated). The median parameters for the 92 group sample are similar. The sample contains seven Abell clusters; the physical properties of these clusters overlap substantially with those of groups. In fact, the distinction between groups and clusters is not generally apparent on the basis of selection of systems in redshift space. Comparison of the distribution of group centers with the distribution of all of the galaxies in the survey shows qualitatively that groups trace the large-scale structure in the region. The physical properties of groups may be related to the details of the large-scale structure. Groups reextracted from the earlier CfA survey complete to mB (0) _ = 14. 5 have a significantly lower median velocity dispersion, σᵥ_ = 131 km s^-1^, than the groups in the 15. 5 survey. About 58% of the groups in the mB (0) _ <= 15. 5 survey contains three or more galaxies brighter than L^*^, the characteristic luminosity in the Schechter form of the luminosity function; in contrast, 77% of the groups in the 14. 5 survey have fewer than three members brighter than L^*^. The difference in the group catalogs is probably largely a result of the properties of large-scale structures and their location relative to the survey limits.