Truncated star formation in compact groups of galaxies: A stellar population study

We present the results of a study comparing the stellar populations in the elliptical galaxies of Hickson compact groups (HCGs) with those in low-density environments. We analyze a sample of intermediate-resolution (4.25 8 FWHM) spectra of 22 galaxies in HCGs and 12 in the field or in loose groups. Three different population-synthesis models and stellar population analyses are used to make the results more robust. Great care has been taken to correct for the emission contamination of the Balmer lines, used to determine the mean luminosity-weighted ages of the galaxies. We find that, on average, galaxies in HCGs are systematically older and more metal-poor than in the field, in agreement with previous works. The most interesting finding is that the low-sigma galaxies in HCGs show an enhanced [Mg/Fe] ratio and depleted metallicity [Z/H] with respect to their counterparts in the field. This anomalous behavior is interpreted as evidence for the action of a mechanism that truncates the star formation (SF). Hydrodynamic simulations of galaxy mergers (Di Matteo et al. 2005) support this interpretation by predicting the quenching of SF soon after the merger event. Combining this scenario and the evidence presented here, the HCGs, generally considered to be ideal environments for galaxy-galaxy interactions, become ideal places for SF truncation.