Neutral hydrogen in nearby elliptical and lenticular galaxies: the continuing formation of early-type galaxies

Abstract

We present the results of deep Westerbork Synthesis Radio Telescope observations of neutral hydrogen in 12 nearby elliptical and lenticular galaxi es. The selected objects come from a representative sample of nearby galaxies earlier studied at optical wavelengths with the integral-field spectrograph SAURON. They are field galaxies, or (in two cases) located in poor group environments. We detect H I - both in regular discs as well as in clouds and tails offset from the host galaxy - in 70% of the galaxies. This detection rate is much higher than in previous, shallower single-dish surveys, and is similar to that for the ionised gas. The results suggest that at faint detection levels the presence of H I is a relatively common characteristic of field early-type galaxies, confirming what was suggested t wenty years ago by Jura based on IRAS observations. The observed total H I masses range between a few times 10 6 M⊙ to just over 10 9 M⊙. The presence of regular disc-like structures is a situatio n as common as H I in offset clouds and tails around early-type galaxies. All g alaxies where H I isdetected also contain ionised gas, whereas no H I is found around galaxies without ionised gas. Galaxies with regular H I discs tend to have strong emission from ionised gas. In these cases, the similar kinematics of the neutral hydrogen and ionised gas suggest that they form one structure. The kinematical axis of the stellar component is nearly always misaligned with respect to that of the gas. We do not find a clear trend between the presence of H I and the global age of the stellar population or the global dynamical characteristics of t he galaxies. More specifically, H I detections are uniformly spread through the (V/σ, ǫ) diagram. If fast and slow rotators - galaxies with high and low specific angular momentum - represent th e relics of different formation paths, this does not appear in the presence and characterist ics of the H I. Our observations support the idea that gas accretion is common and does not happen exclusively in peculiar early-type galaxies. The links observed between the large-scale gas and the characteristics on the nuclear scale (e.g., the presence of kinematically de coupled cores, radio continuum emission etc.), suggest that for the majority of the cases th e gas is acquired through merging, but the lack of correlation with the stellar population age s uggests that smooth, cold accretion could be an alternative scenario, at least in some galaxies. In either cases, the data suggest that early-type galaxies continue to build their mass up to the pr esent.