Early-type galaxies at large galactocentric radii - II. Metallicity gradients, and the [Z/H]--mass, [alpha/Fe]--mass relations

TL;DR

This study investigates stellar population gradients in early-type galaxies, revealing how metallicity and abundance ratios relate to galaxy mass, and discusses implications for galaxy formation theories including collapse and hierarchical models.

Contribution

It provides new radial metallicity and abundance ratio profiles for low- and high-mass early-type galaxies, extending previous work and comparing results with galaxy formation models.

Findings

Metallicity gradients correlate with galaxy mass, with a slope change at 3.5×10^10 M_sun.

Low-mass galaxies show signs of early, extended star formation with outflows.

High-mass galaxies exhibit flatter gradients and signs of merger events.

Abstract

We present the results of a study of stellar population properties at large galactocentric radii of 14 low-mass early-type galaxies in the Fornax and Virgo clusters. We derive radial profiles of Age, total metallicity [Z/H], and [alpha/Fe] abundance ratios out to 1 - 3 effective radii by using nearly all of the Lick/IDS absorption-line indices in comparison to recent single stellar population models. We extend our study to higher galaxy mass via a novel literature compilation of 37 early-type galaxies, which provides stellar population properties out to one effective radius. We find that metallicity gradients correlate with galactic mass, and the relationship shows a sharp change in slope at a dynamical mass of 3.5 10^10 M_{sun}. The central and mean values of the stellar population parameters (measured in r < r_e/8, and at r = r_e, respectively) define positive mass trends. We suggest that the low metallicities, almost solar [alpha/Fe] ratios and the tight mass-metallicity gradient relation displayed by the low-mass galaxies are indicative of an early star-forming collapse with extended (i.e., > 1 Gyr), low efficiency star formation, and mass-dependent galactic outflows of metal-enriched gas. The flattening of metallicity gradients in high-mass galaxies, and the broad scatter of the relationship are attributed to merger events. The high metallicities and supersolar abundances shown by these galaxies imply a rapid, high efficiency star formation. The observed [Z/H]--mass and [alpha/Fe]--mass relationships can be interpreted as a natural outcome of an early star-forming collapse. However, we find that hierarchical galaxy formation models implementing mass-dependent star formation efficiency, varying IMF, energy feedback via AGN, and the effects due to merger-induced starbursts can also reproduce both our observed relationships.