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

4.2. Population III as Dark Matter Producers

We have seen that it is possible that most of the baryons were processed through a first generation of pregalactic or protogalactic stars and henceforth the term "Population III" is used specifically in this sense. However, it should be stressed that the cosmological interest in Population III stars is not confined to the dark matter issue. They would also be expected to produce radiation, explosions, and nucleosynthesis products, and each of these could have important cosmological consequences (Carr et al 1984). Although there are no observations which unambiguously demand that most of the baryons were processed through Population III stars, there are theoretical reasons for anticipating their formation. This is because the existence of galaxies and clusters of galaxies implies that there must have been density fluctuations in the early Universe and, in many scenarios, these fluctuations would also give rise to a population of pregalactic stars. The precise way in which this occurs depends on the nature of the fluctuations and the nature of the dominant dark matter, as we now discuss.

In a baryon-dominated universe with isothermal or isocurvature density fluctuations, the first bound objects usually have a mass corresponding to the baryonic Jeans mass at decoupling. This is M_Jb 10⁶ _b^-1/2 M, where _b is the baryon density parameter, and clouds of this mass would bind at a redshift ~ 100, depending on the form of the spectrum of fluctuations at decoupling. Larger bound objects - like galaxies and clusters of galaxies - would then build up through a process of hierarchical clustering (Peebles & Dicke 1968). Regions smaller than M_Jb, even though their initial overdensity might be higher, would not begin to collapse until they were larger than the Jeans length and by then they would generally have been erased either by viscous damping prior to decoupling or by nonlinear processes during the oscillatory period after decoupling (Carr & Rees 1984). However, more exotic possibilities arise if the fluctuation spectrum is sufficiently steep for the fluctuations to be highly nonlinear on smaller scales because, in this case, very small regions could collapse well before recombination (Hogan 1978). Indeed, this is expected in the primordial isocurvature baryon-dominated model (Hogan 1993).

In the Cold Dark Matter scenario, in which the density of the Universe is dominated by cold particle relics, structure also builds up hierarchically (Blumenthal et al 1984). In this case, one expects bound clumps of the particles to form down to very small scales (Hogan & Rees 1988), but baryons would only fall into the potential wells, forming bound clouds, on baryon scales above M_Ja 10⁶ _{b a}^-3/2 M, where _a is the cold particle density (Carr & Rees 1984, de Araujo & Opher 1990). In fact, the formation of the pregalactic clouds is even easier in this case because the cold particle fluctuations grow by an extra factor of 10_a between the time when the cold particles dominate the density and decoupling.

In a baryon-dominated universe with adiabatic density fluctuations, the first objects to form are pancakes of cluster size (Zeldovich 1970) because adiabatic fluctuations are erased by photon diffusion for M < 10¹³ _b^-5/4 M (Silk 1968). Galaxies and smaller scale structures therefore have to form as a result of fragmentation. This scenario appears to be excluded by CMB anistropy constraints but a similar picture applies if one has adiabatic fluctuations in a Hot Dark Matter scenario, in which the Universe's mass is dominated by a particle like the neutrino. In this case, the fluctuations are erased by neutrino free-streaming for M < 10^{15 -2} M (Bond et al 1980), so the first objects to form are pancakes of supercluster scale. In both scenarios one expects the pancakes to initially fragment into clumps of mass 10⁸ M; these clumps must then cluster in order to form galaxies. Even in this case, therefore, one might expect pregalactic clouds to form, albeit at a relatively low redshift (z < 10).

All of these scenarios would be modified if the Universe contained topological relics such as strings or textures (Cen et al 1991). Such relics could induce the formation of smaller scale bound regions than usual. For example, Silk & Stebbins (1993) find that in the CDM picture with strings, up to 10^-3 of the mass of the Universe could go into cold dark matter clumps at the time of matter-radiation equilibrium. These clumps would then accrete baryonic halos, forming globular-cluster type objects.

In the explosion scenario (Ostriker & Cowie 1981, Ikeuchi 1981), the first objects to form are explosive seeds (stars or clusters of stars). These generate shocks which sweep up vast shells of gas; when the shells overlap, most of the gas gets compressed into thin sheets (Carr & Ikeuchi 1985). The sheets then fragment either directly into galaxies or into lower-mass systems, depending on the cooling mechanism (Bertschinger 1983, Wandel 1985). Although the explosion scenario was originally invoked to explain large-scale structure, this now seems to be incompatible with the upper limit on the y-parameter permitted by FIRAS. However, one can still envisage this as a mechanism for amplifying the fraction of the gas going into stars - an idea applicable in models with or without nonbaryonic dark matter (Scherrer 1992).