12.3.4. Dark Matter in Dwarf Galaxies
Although most of the galaxies that we see are of relatively high luminosity, it is not yet clear that they are in fact good tracers of the mass distribution. It is already known that low-luminosity galaxies are found to cluster less strongly than higher-luminosity ones (see Section 12.7.2). Since the number density of galaxies still rises at low luminosities, it is critical to understand whether the presence of dark matter - the so-called "missing mass" - is required to the same extent in the low-luminosity but numerous dwarf galaxies. In particular, we need to have some idea of whether these objects contribute more, less, or the same amount to the matter distribution as do galaxies of higher luminosity.
Dwarf counterparts of elliptical, lenticular, and irregular galaxies exist and are in fact the dominant population, by number, of the Virgo cluster. The presence of dwarf spiral galaxies is controversial, and an absence of low-mass spirals is predicted by some theories of the origin of spiral structure. While the dE's may be the most common galaxy in Virgo, dI's may represent the most widely distributed objects in the universe (see Section 12.7.2). Although dI's are typically of low optical luminosity and surface brightness, they also contain significant fractions of their total mass in the form of HI (Fisher and Tully 1975). Their global 21-cm profiles are often Gaussian rather than two-horned, indicating that random motions are significant and possibly dominant.
Recent synthesis observations have been made to investigate the HI distribution and velocity field of a number of very-low-luminosity dI's (Sargent and Lo 1986). The HI distributions vary from annular to totally without organized structure, but are generally larger than the stellar distributions. In all of the mapped dI's, the amplitude of the rotation curve is comparable to or smaller than the random velocity dispersion, and several of the objects show no sign of systematic velocity fields. While the kinematics of the brighter dI's (Mpg < - 14) tends to be dominated by rotation, the chaotic motions are more important at lower luminosities. While there is a trend that the HI fraction of the mass of the dI's rises with decreasing optical luminosity, the gas only rarely dominates the total mass. Still, in no case is the rotation curve observed to turn over at large r; in most cases, the rotation curve is linear and still rising (solid-body rotation) even outside the optical image. It is difficult, if not impossible, to estimate the inclination of a dI, unless its velocity field is regular enough that the inclination can be derived from the best-fit kinematic model. Despite the resulting uncertainties in mass estimates, it is clear that in all cases, the total mass-to-light ratio MT / L is high, in the range of 10 to 30: dI's have dark halos also.
Although true dwarf spirals do indeed seem rare, at least one low-luminosity Scd galaxy, UGC 2259, shows a symmetrical HI distribution and a regular velocity field indicating a flat rotation curve similar to that seen in more luminous spirals (Carignan et al. 1987). The interpretation of the mass distribution for this galaxy is model dependent, but even the assumption of a minimum-mass dark halo implies that at the optical (Holmberg) radius, the luminous disk and the dark halo contribute equal amounts of mass. Thus it appears that the ratio of nonluminous to luminous matter is independent of the Hubble type.