The Structure of Compact Groups of Galaxies

Abstract

Compact groups are small close isolated associations of galaxies, High space densities (≤10⁴/Mpc³) and short crossing times (~0.1 H⁻¹) make them particularly susceptible to dynamical evolution. Several well known groups contain galaxies with very discrepant redshifts, casting doubt upon the physical reality of compact groups in general or on the cosmological redshift hypothesis. The small population of these groups, typically less than ten galaxies, makes the interpretation of the structure and dynamics of an individual group difficult at best. It is often more informative to investigate the statistical properties of a homogeneous sample of groups. Galaxies in such groups are generally too faint to appear in standard galaxy catalogs, however, so they cannot be found by automated techniques used to study loose groups (Turner and Gott, 1976, Geller, this volume). For this reason a survey was undertaken of the Palomar Observatory Sky Survey (POSS) prints for groups satisfying three selection criteria of population, isolation, and mean surface brightness. The resulting catalog of 100 groups (Hickson, 1982) forms our study sample.