Some Comments on Dark Matter

Kormendy (1990) has shown how the characteristic central densities of the dark halos of galaxies scale with their absolute magnitudes. For different kinds of galaxies, the central densities of the dark halos are estimated by different methods (rotation curves for the larger spirals and irregulars, hydrostatic equilibrium of the HI component for the smallest irregulars, and stellar dynamics for the nearby dwarf spheroidal galaxies). Despite the diversity of methods, a recent study of this scaling law (Kormendy & Freeman 1996) shows that the dynamically derived halo densities are tightly correlated with the absolute magnitudes. The halo densities range from about M pc for the brightest spirals with M to a very high value of about 1 M pc for the faintest dwarfs with M. For comparison, the total density of the galactic disk near the sun is about 0.1 M pc. What is the reason for these very high densities ( M pc) of the dark matter components in the faintest dwarfs ? Maybe the dark matter is dissipative, as Pfenniger et al. (1994) have suggested. If it is not dissipative, then maybe we should look to the early universe just after recombination, at a time when the density of the universe itself was M pc. The smallest halos, with masses M, may then be small bound objects from this early epoch. The lower densities of the larger halos may then reflect the processes of aggregation and tidal disruption by which they were built up.

In this context, I should mention the recent results of the MACHO survey (Alcock et al.\ 1996). This survey has now detected 8 secure microlensing events towards the LMC, and the properties of these events are consistent with a massive galactic dark halo for which a fraction of about 0.6 by mass is made of compact objects with individual masses of about 0.5 M. These numbers still have significant uncertainties, and both numbers are likely to increase as the survey proceeds (because more massive lensing objects can be detected as the photometric time series get longer). If these numbers turn out to be correct, then what could these objects be ? They are unlikely to be normal stars, because very faint stellar photometry in the Hubble Deep Field (eg Elson et al. 1996) indicates that less than 20% of the halo dark matter can be in the form of low mass luminous stars. Another possibility is that they are old white dwarfs: from the discussion of dwarf galaxies in the previous paragraph, these white dwarfs could be the stellar remnants from a very active pregalactic burst of star formation not long after recombination. This possibility brings with it some unresolved problems of chemical enrichment associated with the mass loss from the progenitors of these white dwarfs as they evolved. Another possibility is that they are primordial black holes, formed in the first sec of the universe. Primordial black holes could have a very wide range of masses, however, and it is not clear why their masses should be similar to those of normal stars.

© Copyright Astronomical Society of Australia 1997