The Properties of Poor Groups of Galaxies: I. Spectroscopic Survey and Results

Abstract

We use multi-fiber spectroscopy of 12 nearby, poor groups of galaxies to address whether the groups are bound systems or chance projections of galaxies along the line-of-sight, why the members of each group have not already merged to form a single galaxy, despite the groups' high galaxy densities, short crossing times, and likely environments for galaxy-galaxy mergers, and how galaxies might evolve in these groups, where the collisional effects of the intra-group gas and the tidal influences of the global potential are weaker than in rich clusters. We conclude the following. (1) The nine groups with diffuse X-ray emission (cf. Paper II) are bound systems with at least 20-50 group members. (2) Galaxies in each X-ray-detected group have not all merged together, because a significant fraction of the group mass lies outside of the galaxies and in a common halo. (3) Unlike cD galaxies in some rich clusters, the giant, brightest elliptical in each X-ray group lies in the center of the group potential, suggesting that such galaxies may form first in poor groups. (4) In some groups, the fraction and recent star formation histories of the early types are consistent with those in rich clusters, suggesting that the effects of cluster environment on these galaxies are relatively unimportant at the current epoch. (Abridged)