Dark Matter in Early-Type Galaxies

Abstract

The quest for dark matter in galaxies has been an active field of research for more than a decade. (See, for example, the classic review by FABER and GALLAGHER [1] and the proceedings of IAU Symposium 117, edited by KORMENDY and KNAPP [2].) Most of the results, however, pertain to late-type galaxies and only in the last year or two has evidence for dark matter in early-type galaxies begun to emerge. This imbalance has hindered our understanding of the formation and evolution of galaxies. Many questions regarding the initial collapse, chemical enrichment, tidal interactions, and so on are affected by whether early-type galaxies do or ever did possess massive halos. Another important problem concerns the number and splitting angles of gravitational lenses, which depend sensitively on the projected density in all forms of matter. The evidence for dark matter in early-type galaxies comes from polar rings, shells, and X-rays. Unfortunately, only a few galaxies have been studied in detail and the observations are not always free of ambiguity. My purpose in this article is to collect together the fragmentary results that are now available.