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, , 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 ( L * ), and that the highest- ellipticals favor high-density environments.We construct average, high signal-to-noise spectra in bins of and environment and find the following: (1) lower- galaxies have a bluer optical continuum and stronger (but still weak) emission lines;(2) at fixed , field ellipticals have a slightly bluer stellar continuum, especially at wavelengths 4000 , and have stronger (but still weak) emission lines compared with their group counterparts, although this environmental dependence is strongest for low- ellipticals and the highest- ellipticals are much less affected.Based on Lick indices measured from both the individual and average spectra, we find that (1) at a given , elliptical galaxies in groups have systematically weaker Balmer absorption than their field counterparts, although this environmental dependence is most pronounced at low ; (2) there is no clear environmental dependence of Fe , while the -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 -enhanced.We also find that (1) the SSP-equivalent ages of galaxies in rich groups are, on average, 1 Gyr older than in the field, although once again this effect is strongest at low ; (2) galaxies in rich groups have slightly lower Fe/H and are marginally more strongly -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 , 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.