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

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  properties are known from interferometric data. Our aim is to understand whether stellar age and chemical composition depend on the  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(). However, we do find that, at stellar velocity dispersion  km s^-1, 2/3 of the galaxies with less than 10⁸  of  are centrally rejuvenated, while none of the -richer systems is. Furthermore, none of the more massive,  km s^-1-objects is centrally rejuvenated independently of their  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  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 σ. Within this picture, -poor, centrally-rejuvenated objects could form in mergers where gas angular-momentum removal (and therefore inflow) is efficient; -rich galaxies with no significant age gradients (but possibly uniformly young) could be formed in interactions characterised by high-angular momentum gas.