Compact elliptical galaxies in different local environments: a mixture of galaxies with different origins?

TL;DR

This study analyzes the stellar populations of 138 compact elliptical galaxies in the local universe, revealing two distinct formation channels linked to their environment and progenitor mass, based on spectral analysis from SDSS data.

Contribution

It identifies two different origins for compact elliptical galaxies, supported by stellar population differences and environmental context, advancing understanding of their formation mechanisms.

Findings

cE(w)s have higher metallicity and alpha enhancement than cE(w/o)s.

No significant age difference between cE(w)s and cE(w/o)s.

cE(w)s deviate from the typical mass-metallicity relation, unlike cE(w/o)s.

Abstract

We present the stellar populations of 138 compact elliptical galaxies (cEs) in the redshift range of $z < 0.05$ using the Sloan Digital Sky Survey (SDSS) DR12. Our cEs are divided into those with [cE(w)] and without [cE(w/o)] a bright ($M_{r} < -21$ mag) host galaxy. We investigated the stellar population properties of cEs based on the Lick line indices extracted from SDSS spectra. cE(w)s show [Z/H] and \afe\ distributions skewed toward higher values compared to those of the cE(w/o)s. No statistically significant difference in age distribution was found between the cE(w)s and cE(w/o)s. In the mass-metallicity distribution, cE(w)s deviate from the relation observed for early-type galaxies at a given stellar mass, whereas cE(w/o)s conform to the relation. Based on the different features in the stellar populations of cE(w)s and cE(w/o)s, we can propose two different cE formation channels tracing different original masses of the progenitors. cE(w)s 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)s 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.