Hierarchical Evolution in Poor Groups of Galaxies

To examine the evidence for hierarchical evolution on mass scales of 10^13-10^14 solar masses, we apply a statistic that measures correlations between galaxy velocity and projected position (Dressler & Shectman 1988) to data for six poor groups of galaxies, HCG 42, HCG 62, NGC 533, NGC 2563, NGC 5129, and NGC 741. Each group has more than 30 identified members (Zabludoff & Mulchaey 1998ab). The statistic is sensitive to clumps of galaxies on the sky whose mean velocity and velocity dispersion deviate from the kinematics of the group as a whole. The kinematics of galaxies within about 0.1 Mpc of the group center do not deviate from the global values, supporting our earlier claim that the group cores are close to virialization or virialized. We detect significant substructure (at 99.9% confidence) in the two groups with the most confirmed members, HCG 62 and NGC 741, that is attributable mostly to a subgroup lying 0.3-0.4 Mpc outside of the core. We conclude that at least some poor groups, like rich clusters, are evolving via the accretion of smaller structures from the field. With larger poor group surveys, the incidence of such accretion and the distribution of subgroup masses are potential constraints of cosmological models on mass scales of less than 10^13-10^14 solar masses and on physical scales of less than 0.5 Mpc.