11.2 Galaxy Formation and Baryonic Halos
Many of the results explored in the previous sections have major implications for the process of galaxy formation. As mentioned in Section 10, if the Universe is dominated by non-baryonic material, then pure hot DM models dictate that clusters form before galaxies, whereas cold DM and seeded hot DM leads to galaxies forming hierarchically from smaller sub-units. However, there is also an important link between astrophysical processes during galaxy formation and the nature of the DM.
A general picture of galaxy formation has developed in which gas collapses within a pre-existing dark halo and eventually forms stars. Gas will dissipate only if it can cool within the age of the Universe, and the gas cooling time turns out to be a function of halo mass (Rees and Ostriker 1977; Silk 1977; Binney 1977). It has been shown that this scenario explains the observed mass range of galaxies and accounts for the angular momentum of spiral disks (White and Rees 1978; Fall and Efstathiou 1980; Blumenthal et al. 1984). However, in the context of this picture, the increase of the DM fraction with decreasing luminosity of disk galaxies is, at first sight, a surprise.
If the Universe is dominated by non-baryonic material, then the initial ratio of gas mass to halo mass in protogalaxies just reflects the ratio of baryonic to non-baryonic matter. In this case, the dark-to-luminous mass ratio in spiral galaxies should be a constant given by this universal value. The fact that it is not leaves two possibilities. The first option is that protogalactic gas is prevented from cooling within low-mass halos, perhaps because it is ejected from the protogalaxy by supernova explosions (Dekel and Silk 1986). Alternatively, some protogalactic gas could form baryonic DM, the effect being largest in the least massive halos (Ashman 1990a).
Some support for baryonic dark halos was provided by the evidence that cooling flows in clusters of galaxies may form significant amounts of baryonic DM in the form of brown dwarfs (Fabian, Nulsen and Canizares 1984 and Section 10 above). The gas in these flows is at high pressure and is cooling quasistatically. Ashman and Carr (1988) showed that similar quasistatic flows can occurr at pregalactic and protogalactic epochs. However, in order for a cosmologically interesting density of baryons to cool quasistatically, gas must be reheated until massive galaxies form (Thomas and Fabian 1990; White 1990; Ashman and Carr 1991). In the absence of reheating, most gas in the Universe cools rapidly on subgalactic scales.
An alternative to producing baryonic DM in quasistatic flows is to assume that this rapid cooling regime is more suitable for baryonic DM formation (Ashman 1990a). This scheme has certain advantages such as forming dark halos before spiral disks, thereby explaining the angular momentum of such disks even in purely baryonic models. It has also been shown that the increase in the dark-to-luminous mass ratio of spirals described in Section 6.2 arises naturally in this scenario (Ashman 1990a).