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 σ v correlates with group axial ratio q is present and particularly strong also in the HCG + environment systems; (c) the velocity dispersion σ v 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/S0 galaxies in HCGs is similar to the corresponding one 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, albeit weakly, on the group's richness and on σ v ; (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.

The Evolution of Galaxies in X‐Ray–luminous Groups

“…Therefore, at least some X-rayYluminous, intermediate-redshift groups have central densities comparable to CGs. This result is consistent with the observation that many CGs at low redshift are actually associated with larger galaxy systems (Ramella et al 1994;Barton et al 1996;de Carvalho et al 1997;Ribeiro et al 1998;Tovmassian et al 2006). This is also consistent with the fact that many CGs are luminous in X-rays, although typically with lower luminosities than the groups studied here (Ponman et al 1996).…”

Ultra-diffuse galaxies outside clusters: clues to their formation and evolution

“…To do this, we have compiled in (Koda et al 2015), the velocity dispersion was taken from Girardi et al (1993), whereas for the Fornax cluster (Muñoz et al 2015) the velocity dispersion is obtained from Drinkwater et al (2001). Finally, for the groups presented in this paper, we have used the velocity dispersions given by Tovmassian et al (2006). Once we have the velocity dispersions, we follow Munari et al (2013) to get M 200 for these structures.…”

Comparing galaxy populations in compact and loose groups of galaxies

“…Brasseur et al (2009) found that only 30 per cent of the simulated groups are truly compact in three dimensions, which will lead to biases in any observational study unable to infer this lack of compactness. An even more radical scenario was proposed by Tovmassian et al (2006), who claimed that CGs are no different from LGs. However, the general consensus is that a significant fraction of the compact groups with accordant-velocity members are physically dense systems.…”