Annu. Rev. Astron. Astrophys. 1994. 32: 531-590 Copyright © 1994 by Annual Reviews. All rights reserved

2.4. Dwarf Galaxies

Some of the dwarf irregulars are extremely gas-rich, which means that their HI rotation curves can be traced to many optical scale lengths. Many of them seem to have much higher dark mass fractions than bright spirals, with their dark halos dominating even within the optical regions. Particularly striking examples are DDO 154 (Carignan & Freeman 1988), for which the dark-to-luminous mass ratio exceeds 10 at the last measured point of the rotation curve, GR 8 (Carignan et al 1990), and DDO 170 (Lake et al 1990). Dwarf spheroidals also seem to have dark halos (Lin & Faber 1983, Aaronson 1983, Aaronson & Olszewski 1987). This claim is based on measurements of velocity dispersions and tidal radii for the six dwarf spheroidals within the Local Group. Originally the dispersions had to be inferred from the individual velocities of only a dozen or so objects per galaxy, but higher resolution velocity measurements now provide much better data (Mateo et al 1991) and seem to confirm the results of the earlier work. The presence of dark matter in dwarf galaxies is crucial in the present context because it requires that halos consist either of baryonic or cold nonbaryonic dark matter. Lake (1990) has argued that the observations are more consistent with the first possibility: If the formation of the halos were dissipationless, their central densities imply that the galaxies need to form at a redshift exceeding 30, whereas they should form at a redshift of 10 in the Cold Dark Matter (CDM) scenario.