Astronomy & Astrophysics

Table of contents

A&A 483, 57-69 (2008)
DOI: 10.1051/0004-6361:20078954

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

P. Serra¹, S. C. Trager¹, T. A. Oosterloo^{1, 2}, and R. Morganti^{1, 2}

¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
e-mail: pserra@astro.rug.nl
² Netherlands Foundation for Research in Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands

(Received 29 October 2007 / Accepted 7 December 2007)

Abstract
Aims. We present a study of the stellar populations of a sample of 39 local, field early-type galaxies whose $\ion$ properties are known from interferometric data. Our aim is to understand whether stellar age and chemical composition depend on the $\ion$ content of galaxies. We also study their ionised gas content and how it relates to the neutral hydrogen gas.
Methods. 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.
Results. We do not find any trend in the stellar populations parameters with M( $\ion$ ). However, we do find that, at stellar velocity dispersion $\sigma\leq 230$ km s^-1, 2/3 of the galaxies with less than 10⁸ $M_{\odot}$ of $\ion$ are centrally rejuvenated, while none of the $\ion$ -richer systems is. Furthermore, none of the more massive, $\sigma\geq 230$ km s^-1-objects is centrally rejuvenated independently of their $\ion$ mass. Concerning the ionised gas, we detect emission in 60% of the sample. This is generally extended and always characterised by LINER-like emission-line ratios at any radius. We find that a large $\ion$ mass is necessary (but not sufficient) for a galaxy to host bright ionised-gas emission.
Conclusions. A plausible interpretation of our results is that gas-rich mergers play a significant role in E/S0 formation, especially at lower $\sigma$ . Within this picture, $\ion$ -poor, centrally-rejuvenated objects could form in mergers where gas angular-momentum removal (and therefore inflow) is efficient; $\ion$ -rich galaxies with no significant age gradients (but possibly uniformly young) could be formed in interactions characterised by high-angular momentum gas.

Key words: galaxies: elliptical and lenticular, cD -- galaxies: evolution -- galaxies: interactions -- galaxies: stellar content -- galaxies: ISM