H I in Low-luminosity Early-type Galaxies

We discuss the properties of the H I in low-luminosity earlytype galaxies. The morphology of the H I is more regular than that of the H I in many more-luminous early-type galaxies. The H I is always distributed in a disk and is more centrally concentrated. The central H I surface densities are higher than in luminous early-type galaxies and are high enough for star formation to occur. 1. Why study H I in low-luminosity early-type galaxies A few decades ago, elliptical galaxies appeared to form a homogeneous group. We know now that in reality they constitute a very varied family of galaxies of which many of the detailed characteristics vary from galaxy to galaxy. For many of these characteristics, elliptical galaxies can be divided into two groups, of which we list a few. Stellar rotation vs anisotropy. Some elliptical galaxies are purely ’pressure supported’ systems, i.e. the random motions of the stars support the shape of the galaxy, while for others the rotation of the stellar component is important for understanding the dynamical structure of the galaxy. Boxy vs Disky. For many ellipticals, the isophotes are not perfect ellipses. For some, the deviations are boxy-shaped, while for others they are disky. For the disky galaxies, there is a continuum in bulge-to-disk ratio, from genuine spiral galaxies all the way to ’no disk’ (e.g., Kormendy and Bender, 1996). Core properties. Imaging the centres of elliptical galaxies with HST has shown that the density distribution in the centre of ellipticals is quite different from galaxy to galaxy. Some ellipticals have relatively shallow central cusps, while others show a steep cusp. The shape of this cusp appears to depend on the luminosity of the galaxy (e.g., Lauer 1997). AGN. The radio continuum emission from the nuclei of elliptical galaxies varies strongly from galaxy to galaxy. The fraction of ellipticals with a radio core depends strongly on the luminosity, although for a given optical luminosity