Early-type galaxies in different environments : an H i view

Abstract

We present an analysis of deep Westerbork Synthesis Radio Telescope observations of the neutral hydrogen in 33 nearby early-type galaxies selected from a representative sample studied earlier at optical wavelengths with the SAURON integral-field spectrograph. This is the deepest homogeneous set of H i imaging data available for this class of objects. The sample covers both field environments and the Virgo cluster. Our analysis shows that gas accretion plays a role in the evolution of field early-type galaxies, but less so for those in clusters. The H i properties of SAURON early-type galaxies strongly depend on environment. For detection limits of a few times 106 M⊙, H i is detected in about 2/3 of the field galaxies, while <10 per cent of the Virgo objects are detected. In about half of the detections, the H i forms a regularly rotating disc or ring. In many galaxies unsettled tails and clouds are seen. All H i discs have counterparts of ionized gas, and inner H i discs are also detected in molecular gas. The cold interstellar medium (ISM) in the central regions is dominated by molecular gas (). Assuming our sample is representative, we conclude that accretion of H i is very common for field early-type galaxies, but the amount of material involved is usually small and the effects on the host galaxy are, at most, subtle. Cluster galaxies appear not to accrete H i, or the accreted material gets removed quickly by environmental effects. The relation between H i and stellar population is complex. The few galaxies with a significant young sub-population all have inner gas discs, but for the remaining galaxies there is no trend between stellar population and H i properties. A number of early-type galaxies are very gas rich, but only have an old population. The stellar populations of field galaxies are typically younger than those in Virgo. This is likely related to differences in accretion history. There is no obvious overall relation between gas H i content and global dynamical characteristics except that the fastest rotators all have an H i disc. This confirms that if fast and slow rotators are the result of different evolution paths, this is not strongly reflected in the current H i content. In about 50 per cent of the galaxies we detect a central radio continuum source. In many objects this emission is from a low-luminosity active galactic nucleus (AGN), and in some it is consistent with the observed star formation. Galaxies with H i in the central regions are more likely detected in continuum. This is due to a higher probability for star formation to occur in such galaxies and not to H i-related AGN fuelling.