Physical properties of Hickson compact groups and of the loose groups within which they are embedded

Abstract

Using new data and an enlarged group sample we verify some of our previously published results and present a number of new facts that suggest that compact groups could be casual concentrations in prolate-like looser groups, and thus the nature of compact and ordinary poor groups is probably the same. To this end we used 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). We also used known member redshifts of looser groups in the environment of 7 other HCGs. From this sample of 22 HCGs 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 σv correlates with group axial ratio q is present and particularly strong also in the HCG + environment systems; (c) the velocity dispersion σv of the HCG + environment systems as well as of ordinary poor groups, depends only weakly on the group richness, i.e. on the mass; (d) the mean absolute K-band magnitude of E/SO galaxies in HCGs is similar to the corresponding one in ordinary poor groups and is brighter than that of isolated E/SO's, indicating that they were formed by the merging of two galaxies of similar luminosity; (e) the fraction of E/SO galaxies in these HCGs depends, albeit weakly, on the group's richness and on σv; (f) the fraction of AGNs is similar in the HCGs and their close environment, while the fraction of starburst galaxies is significantly higher in the HCGs; (g) the fraction of active galaxies (AGNs and starbursts) is anti-correlated with the velocity dispersion of the HCG + environment systems. The combination of all the above facts constitutes a picture in which compact groups are condensations within looser prolate-like, elongated systems, and they appear to be compact when their member galaxies, moving in radial orbits along the group elongation, happen to come close to each other (in which case dynamical interactions among these galaxies become even more probable) or when the group is oriented close to the line of sight, so that many of its members are projected over a small solid angle. The probability of either case is small, so the number of CGs should be much smaller than that of ordinary groups, as observed. 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. © ESO 2006.