Stellar populations, neutral hydrogen and ionised gas in field early-type galaxies

TL;DR

This study investigates how stellar populations, neutral hydrogen, and ionised gas relate in field early-type galaxies, revealing links between gas content, galaxy mass, and stellar rejuvenation, suggesting different formation processes.

Contribution

It provides new insights into the connection between HI content, stellar age, and gas dynamics in early-type galaxies, highlighting the role of gas-rich mergers in their evolution.

Findings

Centrally rejuvenated galaxies are mostly low-mass with less than 100 million solar masses of HI.

No clear trend between stellar population parameters and HI mass.

HI-rich galaxies often lack age gradients and show signs of high-angular momentum interactions.

Abstract

We present a study of the stellar populations of a sample of 39 local, field early-type galaxies whose HI properties are known from interferometric data. Our aim is to understand whether stellar age and chemical composition depend on the HI content of galaxies. As a by-product of our analysis, we also study their ionised gas content and how it relates to the neutral hydrogen gas. Stellar populations and ionised gas are studied from optical long-slit spectra. We determine stellar age, metallicity and alpha-to-iron ratio by analysing a set of Lick/IDS line-strength indices measured from the spectra after modelling and subtracting the ionised-gas emission. We do not find any trend in the stellar populations parameters with M(HI). However, we do find that, at stellar velocity dispersion below 230 km/s, 2/3 of the galaxies with less than 100 million solar masses of HI are centrally rejuvenated, while none of the HI-richer systems are. Furthermore, none of the more massive (velocity-dispersion>230 km/s) objects are centrally rejuvenated independently on their HI mass. Concerning the ionised gas, we detect emission in 60% of the sample. This is generally extended and always carachterised by LINER-like emission-line ratios at any radius. We find that a large HI mass is necessary (but not sufficient) for a galaxy to host bright ionised-gas emission. A plausible interpretation of our results is that gas-rich mergers play a significant role in E/S0 formation, especially at lower mass. Within this picture, HI-poor, centrally-rejuvenated objects could form in mergers where gas angular-momentum removal (and therefore inflow) is efficient; HI-rich galaxies with no significant age gradients (but possibly uniformly young) could be formed in interactions characterised by high-angular momentum gas.