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