The Evolution of Galaxies in Compact Groups

We present an analysis of the spectra of 62 galaxies in 15 compact groups. The galaxies are classified into four activity classes: galaxies without emission, starburst galaxies, luminous AGNs (Seyfert and LINERs), and low-luminosity AGNs (LLAGNs). The star formation in the Hickson compact group (HCG) starbursts is more intense than in normal spirals, but comparable to that observed in starburst-nucleus galaxies (SBNGs) in the field. In general, the HCG starbursts have mean solar gas metallicity and do not follow the metallicity-luminosity relation traced by the early-type SBNGs in the field, suggesting that most of them are late-type SBNGs. This morphology preference, coupled with the observation that the HCG starbursts are predominantly located in the halos of the groups, is consistent with the idea that compact groups are embedded in sparser structures. The stellar metallicities of the nonstarburst galaxies are comparable to those observed in normal galaxies with similar morphologies, but are relatively high for their luminosities. In these galaxies, the metal absorption line equivalent widths are slightly narrower than normal, while the Balmer absorption lines are relatively strong. All these observations suggest the presence of a population of intermediate-age stars. These galaxies could be poststarburst, but at a very advanced stage of evolution, the last bursts having happened more than 2 Gyr in the past. Our observations support a scenario in which the cores of the groups are slowly collapsing evolved systems embedded in more extended structures. In the cores of the groups, the interactions were more frequent and the galaxies evolved at a more rapid rate than in their halos.