Optical and radio survey of southern compact groups of galaxies

Abstract

Context. Multi-wavelength observations of Hickson's Compact Groups (HCGs) have shown that many of these groups are physical bound structures and are in different stage of evolution, from spiral-dominated systems to almost merged objects. Very few studies have analysed the Southern Compact Groups (SCGs) sample, which is thought to be younger that HCGs, due to an on average higher number of spiral galaxies. We present here the first results from optical and radio observations on a pilot sample of SCGs. Aims. Previous HI studies of HCGs seems to corroborate an evolutionary sequence from low velocity dispersion, spiral dominated, young and star-forming groups to higher velocity dispersion, mostly early-type, X-ray bright groups. In this scheme, the dominant parameter is most likely the ratio between the galaxy mass vs. the available gas mass. A second parameter, namely the merging history of the group, however might have an important role in the evolution of the intra-group medium. Powerful merging events and/or interactions can contribute significantly to the heating of the intra-group medium by means of supernova explosions or by triggering an AGN. We propose to investigate whether the evolutionary scheme of the intragroup medium found for HCGs also holds in a different compact group sample. In addition to this, we start in this paper to investigate which is the influence of the merging history of groups on their evolution. Methods. Optical observations of SCGs obtained with ESO telescopes and radio data from the ATCA allow us to probe the distribution of the warm intra-group medium and the evolutionary stage of each group, by means of morphological studies and via measurements of star formation and other types of nuclear activity. Results. We present here results from a pilot sample of SCGs: we find an evolutionary trend, based on the properties of the warm intra-group medium, similar to what has been found for HCGs. Both galaxies with Sy2 nuclei in our sample are members of groups in the late stages of their dynamical evolution. However while one group is strongly HI deficient and shows, at the same time a strong ongoing merging, the other does not show any HI deficiency and the galaxies are only interacting with each other.