The history of star formation and mass assembly in early-type galaxies

TL;DR

This study examines the star formation history and mass assembly of over 14,000 early-type galaxies using SDSS data, revealing how galaxy properties vary with mass, environment, and radius, and estimating their assembly epoch.

Contribution

It provides a comprehensive analysis of stellar populations in ETGs, highlighting environmental and mass-dependent trends in age, metallicity, and gradients, with implications for galaxy evolution models.

Findings

Age, metallicity, and alpha-enhancement increase with galaxy mass.

Field ETGs are younger than cluster ETGs by ~2 Gyr.

Radial metallicity gradients are negative across all masses and environments.

Abstract

We define a volume limited sample of over 14,000 early-type galaxies (ETGs) selected from data release six of the Sloan Digital Sky Survey. The density of environment of each galaxy is robustly measured. By comparing narrow band spectral line indices with recent models of simple stellar populations (SSPs) we investigate trends in the star formation history as a function of galaxy mass (velocity dispersion), density of environment and galactic radius. We find that age, metallicity and alpha-enhancement all increase with galaxy mass and that field ETGs are younger than their cluster counterparts by ~2 Gyr. We find negative radial metallicity gradients for all masses and environments, and positive radial age gradients for ETGs with velocity dispersion over 180 km/s. Our results are qualitatively consistent with a relatively simple picture for ETG evolution in which the low-mass halos accreted by a proto-ETG contained not only gas but also a stellar population. This fossil population is preferentially found at large radii in massive ETGs because the stellar accretions were dissipationless. We estimate that the typical, massive ETG should have been assembled at z < 3.5. The process is similar in the cluster and the field but occurred earlier in dense environments.