Ages, Metallicities, and α-Element Enhancement for Galaxies in Hickson Compact Groups

Central velocity dispersions and eight line-strength Lick indices have beendetermined from 1.3${\rm \AA}$ resolution long-slit spectra of 16 ellipticalgalaxies in Hickson compact groups. These data were used to determine galaxyproperties (ages, metallicities and $\alpha$-element enhancements) and alloweda comparison with the parameters determined for a sample of galaxies in lowerdensity environments, studied by Gonz\'alez (1993). The stellar populationparameters were derived by comparison to single stellar population models ofThomas et al. (2003) and to a new set of SSP models for the indices Mg$_2$,Fe5270 and Fe5335 based on synthetic spetra. These models, based on an updateversion of the fitting functions presented in Barbuy et al. (2003), are fullydescribed here. Our main results are: (1) the two samples have similar meanvalues for the metallicities and [$\alpha$/Fe] ratios, (2) the majority of thegalaxies in compact groups seem to be old (median age of 14 Gyr for eightgalaxies for which ages could be derived), in agreement with recent work byProctor et al. (2004). These findings support two possible scenarios: compactgroups are either young systems whose members have recently assembled and hadnot enough time to experience any merging yet or, instead, they are old systemsthat have avoided merging since their time of formation.Comment: Accepted for publication in A