HCG 16 Revisited: Clues about Galaxy Evolution in Groups

We present new spectroscopic observations of five galaxies, members of the unusually active compact group HCG 16, observed using the Palomar 5 m telescope. The high signal-to-noise ratios (S/N ∼ 70) of the spectra allow us to study the variation of the emission-line characteristics and the stellar populations in the nucleus and the circumnuclear regions of the galaxies. The emission-line characteristics of these galaxies are complex, varying between Seyfert 2's and LINERs or between LINERs and starbursts. All of the galaxies show traces of intermediate-age stellar populations, which supports our previous result that poststarburst galaxies are common in compact groups. The galaxies HCG 16-4 and HCG 16-5 show double nuclei and therefore could be two cases of recent merger. Our observations support a scenario in which HCG 16 was formed by the successive merger of metal-poor, low-mass galaxies. The galaxies HCG 16-1 and HCG 16-2, which are more evolved, form the old core of the group. Galaxies HCG 16-4 and HCG 16-5 are two more recent additions that are still in a merging phase. Galaxy HCG 16-5 is a starburst galaxy that is just beginning to fall into the core. If HCG 16 is representative of compact groups in their early stage, the whole set of observations implies that the formation of compact groups is the result of hierarchical galaxy formation. HCG 16 could be one example of this process operating in the local universe.