Properties of Hickson Compact Groups and of the Loose Groups within which they are Embedded

Abstract

We use the Sloan Digital Sky Survey (SDSS) redshift catalogue to look for galaxies with accordant redshifts in the nearby environment (up to ~2 Mpc) of 15 Hickson Compact Groups (HCG) and 7 more HCG+environment from the literature. We find that: (a) HCG's tend to be aligned with the overall galaxy distribution in their ~1 Mpc environment, (b) the well-established orientation effect by which the group velocity dispersion correlates with group axial ratio, is present also in the HCG+environment systems, (c) the radial velocity dispersion of the HCG+environment systems and of ordinary poor groups only weakly depends on the group richness, (d) the mean absolute K-band magnitude of E/S0 galaxies in HCGs is similar to that in ordinary poor groups, and is brighter than that of isolated E/S0's, indicating that they were formed by the merging of two galaxies of similar luminosity, (e) the fraction of E/S0 galaxies in these HCGs depends only weakly on the group richness and velocity dispersion, (f) the fraction of AGNs is similar in the HCGs and their close environment, while that of starbursts is significantly higher in the HCGs, (g) the fraction of AGNs and starbursts is anti-correlated with the velocity dispersion of the HCG+environment systems. Furthermore, the observed fractions of early-type and active galaxies as well as their correlations with the group velocity dispersion suggests a picture by which nuclear activity and galaxy transformation by merging is instigated by effective gravitational interactions in the low-velocity dispersion groups, which then dynamically evolve via virialization processes to higher velocity dispersion groups, which thus have a higher fraction of early-type galaxies.

arXiv commentaccepted for publication in AA

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