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

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 [α/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 × 1010 M⊙. The central and mean values of the stellar population parameters (measured in r≤re/8 and at r=re, respectively) define positive mass trends. We suggest that the low metallicities, almost solar [α/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 [α/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 initial mass function, energy feedback via active galactic nucleus and the effects due to merger‐induced starbursts can also reproduce both our observed relationships.

The effects of AGN feedback on present-day galaxy properties in cosmological simulations

“…When AGN feedback is included, a number of galaxies move towards the observed relations for earlytype galaxies (Kuntschner et al 2010;Spolaor et al 2010;Johansson et al 2012, note that all simulated galaxies are displayed). For log σ 2.1, the relation is reversed when AGN is included, more in keeping with observations.…”

“…Observational data show a tight relation between [α/Fe] and galaxy mass for both central values (Thomas et al 2003;Johansson et al 2012) and values within Re (Kuntschner et al 2010;Spolaor et al 2010), although the slope and absolute values depend on the analysis of the observed absorption lines. The aperture effect should be negligible as the observed radial gradient of [α/Fe] is very small.…”

Chemoarchaeological downsizing in a hierarchical universe: impact of a top-heavy IGIMF

“…Since then, a large amount of work has been done measuring alphaelement abundances to study this trend in all sort of environments (e.g. Worthey et al 1992;Trager et al 2000;Kuntschner 2000;Bernardi et al 2006;Sánchez-Blázquez et al 2006;Spolaor et al 2010; Thomas et al 2010).…”

The metal enrichment of passive galaxies in cosmological simulations of galaxy formation

“…This process explains the trend in the [α/Fe]-σ relation at σ 100 km s −1 . For less massive galaxies with σ 100 km s −1 , which are not affected by AGN feedback, [α/Fe] is nearly constant; this behaviour might be inconsistent with the observations which suggest that the [α/Fe]-σ trend exists at least for σ 10 1.7 km s −1 (Spolaor et al 2010;Johansson et al 2012).…”

“…Below σ ∼ 100 km s −1 , [α/Fe] becomes nearly constant, while the observational trend continues at least down to σ ∼ 10 1.7 km s −1 (e.g. Spolaor et al 2010;Johansson et al 2012). The star-forming galaxies, in the reference simulation, exist below σ ∼ 10 2.1 km s −1 and have slightly lower [α/Fe] than the passive galaxies.…”