Origins of Compact Elliptical Galaxies in Different Local Environments

We present the stellar populations of compact elliptical galaxies (cEs) in different local environments. Following the conventional selection criteria of cEs with low luminosity (M_g > -18.7 mag), small effective radius (R_eff < 600 pc), and high-velocity dispersion (> 60 km s^-1), we selected 138 cE candidates in the redshift range of z < 0.05 using the Sloan Digital Sky Survey (SDSS) DR12. We divided our cEs into two subsamples, namely cEs with (cE_w) and without (cE_w/o) a host galaxy, which are inside and outside of one virial radius of the nearby bright (M_r < -21 mag) galaxy, respectively. We investigated the stellar population properties (age, metallicity, and [α/Fe]) of cEs based on the Lick line indices extracted from SDSS spectra. The metallicity distributions between the two subsamples show a strong difference, in which cE_w has a systematically higher mean metallicity than cE_w/o. In the mass-metallicity distribution, cE_w deviates from the relation observed for early-type galaxies at a given stellar mass, whereas cE_w/o conforms to the relation. We also observed a hint of a difference in the [α/Fe] distributions between cE_w and cE_w/o, in which cE_w has a slightly higher mean value of [α/Fe] than cE_w/o. In the case of age distribution, no statistically significant difference was found between cE_w and cE_w/o. Based on the different features in the stellar populations of cE_w and cE_w/o, we can propose two different cE formation channels tracing different original masses of the progenitors. cE_w would be the remnant cores of the massive progenitor galaxies whose outer parts are tidally stripped by a massive neighboring galaxy (i.e., nurture origin). In contrast, cE_w/o are likely the faint end of early-type galaxies maintaining in-situ evolution in an isolated environment with no massive galaxy nearby (i.e., nature origin). Our results reinforce the propositions that cEs comprise a mixture of galaxies with two types of origins depending on their local environment.