Ages and metallicities of Hickson compact group galaxies

ABSTRACT Hickson Compact Groups (HCGs) constitute an interesting extreme in the range ofenvironments in which galaxies are located, as the space density of galaxies in thesesmall groups are otherwise only found in the centres of much larger clusters. Thework presented here uses Lick indices to make a comparison of ages and chemicalcompositions of galaxies in HCGs with those in other environments (clusters, loosegroups and the field). The metallicity and relative abundance of ‘α-elements’ showstrong correlations with galaxy age and central velocity dispersion, with similar trendsfound in all environments. However, we show that the previously reported correlationbetween α-element abundance ratios and velocity dispersion disappears when a fullaccount is taken of the the abundance ratio pattern in the calibration stars. Thiscorrelation is thus found to be an artifact of incomplete calibration to the Lick system.Variations are seen in the ranges and average values of age, metallicity and α-element abundance ratios for galaxies in different environments. Age distributionssupport the hierarchicalformation prediction that field galaxiesareon averageyoungerthan their cluster counterparts. However, the ages of HCG galaxies are shown to bemore similar to those of cluster galaxies than those in the field, contrary to the expec-tations of current hierarchical models. A trend for lower velocity dispersion galaxies tobe younger was also seen. This is again inconsistent with hierarchical collapse models,but is qualitatively consistent with the latest N-body-SPH models based on monolithiccollapse in which star formation continues for many Gyr in low mass halos.Key words: galaxies: interactions - galaxies: elliptical - galaxies: evolution