On the Radial Stellar Content of Early-Type Galaxies as a Function of Mass and Environment

TL;DR

This study investigates how the radial distribution of age and metallicity in early-type galaxies varies with mass and environment, revealing distinct gradient patterns in group versus field galaxies that inform galaxy formation models.

Contribution

It provides new insights into the environmental dependence of stellar population gradients in early-type galaxies, highlighting differences between group and field galaxies.

Findings

Group galaxies have positive age gradients and steeper metallicity gradients.

Field galaxies show flatter gradients, with age gradients becoming positive at higher masses.

Gradients suggest different formation and evolutionary processes in different environments.

Abstract

Using optical-optical and optical-NIR colors, we analyze the radial dependence of age and metallicity inside massive (M* > 10^10.5 MSun), low-redshift (z<0.1), early-type galaxies (ETGs), residing in both high-density group regions and the field. On average, internal color gradients of ETGs are mainly driven by metallicity, consistent with previous studies. However, we find that group galaxies feature positive age gradients, Nabla_t, i.e. a younger stellar population in the galaxy center, and steeper metallicity gradients, compared to the field sample, whose Nabla_t ranges from negative in lower mass galaxies, to positive gradients at higher mass. These dependencies yield new constraints to models of galaxy formation and evolution. We speculate that age and metallicity gradients of group ETGs result from (either gas-rich or minor-dry) mergers and/or cold-gas accretion, while field ETGs exhibit the characteristic flatter gradients expected from younger, more metal-rich, stars formed inside--out by later gas-cooling.