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

Aims. We present a study of the stellar populations of a sample of 39 local, field early-type galaxies whose H I properties are known from interferometric data. Our aim is to understand whether stellar age and chemical composition depend on the H I 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(H I). However, we do find that, at stellar velocity dispersion sigma <= 230 km s(-1), 2/3 of the galaxies with less than 10(8) M-circle dot of H I are centrally rejuvenated, while none of the H I-richer systems is. Furthermore, none of the more massive, sigma >= 230 km s(-1) -objects is centrally rejuvenated independently of their H I 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 H I 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, H I-poor, centrally-rejuvenated objects could form in mergers where gas angular-momentum removal (and therefore inflow) is efficient; H I-rich galaxies with no significant age gradients (but possibly uniformly young) could be formed in interactions characterised by high-angular momentum gas.