Annu. Rev. Astron. Astrophys. 1997. 35: 357-88
Copyright © 1997 by Annual Reviews Inc. All rights reserved

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The space distribution and environment of compact groups provides important clues to their nature. The median redshift of the HCGs is z = 0.030, placing most of them at distances well beyond the Virgo Cluster (Hickson et al. 1992). A cursory inspection reveals that they are fairly uniformly distributed and show no preference for rich clusters. This is at least partly due to the isolation criterion. However, galaxies in rich clusters have rather different kinematical and morphological properties than do those in compact groups, so one might justifiably argue that small clumps of galaxies within clusters are not compact groups.

A natural question is whether or not compact groups are associated with loose groups. Rood & Struble (1994) observed that 70% of the HCGs are located within the bounds of cataloged loose groups and clusters. Studies of the distribution of galaxies in redshift space (Vennik et al. 1993, Ramella et al. 1994, Sakai et al. 1994, Garcia 1995, Barton et al. 1996) indicate that compact sub-condensations do occur within loose groups and filaments. Vennik et al. (1993) and Ramella et al. (1994) find that most HCGs are indeed associated with loose groups.

While the above studies show that compact groups trace large-scale structure, it is also clear that they prefer low-density environments. Sulentic (1987), Rood & Williams (1989), Kindl (1990), and Palumbo et al. (1995) have examined the surface density of galaxies surrounding the groups. They generally agree that about two-thirds groups show no statistically significant excess of nearby neighbors. This is not inconsistent with the redshift-space results because most of the HCG associations identified by Ramella et al. (1996) contain fewer than 5 excess galaxies. Thus while compact groups are associated with loose groups and filaments, these tend to be low-density and sparsely-populated systems.

Are the galaxies in compact groups in any way distinct from those in their immediate environments? Rood & Williams (1989) and Kindl (1990) both found that compact groups, including those in rich environments, contain a significantly smaller fraction of late-type (spiral and irregular) galaxies than do their neighborhoods. This result is of particular importance to the question of the physical nature of compact groups, discussed below in Section 6. In addition, many independent studies have found significant differences between galaxies in compact groups and those in other environments. These are examined in Section 5.

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