The Relation between Activity and Environment in Compact Groups of Galaxies

We present the results of the classification of spectral activity types for 193 galaxies from a new sample of 49 compact groups of galaxies in the southern hemisphere (SCGs). The SCGs represents a new sample of compact groups selected in automated fashion from a digitized galaxy catalog, covering an area of ∼5200 deg2 around the south Galactic pole. It is complete up to m ∼ 14.5 in bj for the brightest galaxy of the group. This sample is very well suited for statistical studies of compact groups properties as it is unaffected by the biases introduced by visual selection methods. The spectral analysis of the SCG galaxies confirms the results previously obtained from the observation of a smaller sample of 17 CGs in Hickson's sample (HCG). We confirm the luminosity-activity and morphology-activity relations previously observed in HCGs: (1) low-luminosity AGNs and AGNs are preferentially located in the most luminous galaxies in the groups, while non-emission–line galaxies and star-forming galaxies share a common distribution among the less luminous galaxies; (2) the non-emission–line galaxies, the low-luminosity AGNs and AGNs are nearly all in early-type galaxies, while the star-forming galaxies are preferentially located in late-type spirals. We also verified that the number of "evolved" galaxies (early-type non-emission–line galaxies) significantly increases with the number of members in the group. Finally, we confirm that AGNs (including low-luminosity or dwarf AGNs) are the most frequent (41%) activity type encountered in CGs. The SCGs contain more star-forming galaxies and less non-emission–line galaxies than HCGs. This difference is possibly related to the different selection criteria of SCGs with respect to HCGs. Being selected in an automated way, the SCGs probe a wider range of physical properties than the HCGs, which are biased toward more easily detectable, denser and therefore more evolved groups. The star-forming galaxies in SCGs are composed of few starburst galaxies and a higher number of H II nucleus galaxies, which generally have less intense star formation than starburst galaxies. As a consequence, the star formation activity in SCGs is remarkably low. At the same time, we found clear evidence for nuclear activity in all the star-forming galaxies. Adding up the percentage of AGNs and star-forming galaxies showing evidence of nuclear activity would result in more than 70% of the galaxies in SCGs having an active nucleus. If these results are further confirmed, CGs will be in the local universe the best location where to find AGNs. Curiously, however, this characteristic of CGs generally excludes Seyfert 1 galaxies.