STELLAR POPULATIONS OF ELLIPTICAL GALAXIES IN THE LOCAL UNIVERSE

We study the stellar populations of 1923 elliptical galaxies at z < 0.05 selected from the Sloan Digital Sky Survey as a function of velocity dispersion, sigma, and environment. Our sample constitutes among the largest high-fidelity samples of elliptical galaxies with uniform imaging and optical spectroscopy assembled to date. Confirming previous studies, we find that elliptical galaxies dominate at high luminosities (greater than or similar to L*), and that the highest-sigma ellipticals favor high-density environments. We construct average, high signal-to-noise spectra in bins of sigma and environment and find the following: (1) lower-sigma galaxies have a bluer optical continuum and stronger (but still weak) emission lines; (2) at fixed sigma, field ellipticals have a slightly bluer stellar continuum, especially at wavelengths less than or similar to 4000 angstrom, and have stronger (but still weak) emission lines compared with their group counterparts, although this environmental dependence is strongest for low-sigma ellipticals and the highest-sigma ellipticals are much less affected. Based on Lick indices measured from both the individual and average spectra, we find that (1) at a given sigma, elliptical galaxies in groups have systematically weaker Balmer absorption than their field counterparts, although this environmental dependence is most pronounced at low sigma; (2) there is no clear environmental dependence of < Fe >, while the alpha-element absorption indices such as Mg b are only slightly stronger in galaxies belonging to rich groups. An analysis based on simple stellar populations (SSPs) reveals that more massive elliptical galaxies are older, more metal-rich, and more strongly alpha-enhanced. We also find that (1) the SSP-equivalent ages of galaxies in rich groups are, on average, similar to 1Gyr older than in the field, although once again this effect is strongest at low sigma; (2) galaxies in rich groups have slightly lower [Fe/H] and are marginally more strongly alpha-enhanced; and (3) there is no significant environmental dependence of total metallicity, [Z/H]. Our results are generally consistent with stronger low-level recent star formation in field ellipticals at low sigma, similar to recent results based on ultraviolet and infrared observations. We conclude with a brief discussion of our results in the context of recent theoretical models of elliptical galaxy formation.