How large were the first pregalactic objects?

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Abstract
If the amplitude of the fluctuations at decoupling decreases with increasing mass, the first systems to bind could be much smaller than galaxies. We calculate their characteristic mass Mmin. In a hot baryon-dominated universe with isothermal fluctuations, Mmin would be 104–106M⊙. In a neutrino-dominated universe, the formation of such objects would be inhibited; on the other hand, in a universe dominated by ‘cold’ non-baryonic matter (e.g. primordial black holes or axions), gas clouds of ≲ 106M⊙ could be the first astrophysically interesting objects to condense out even if the initial conditions were purely adiabatic. In a tepid universe with a primordial photon-to-baryon ratio S below the ‘hot’ value of ∼ 109, subgalactic systems could form from purely adiabatic initial conditions if S < 105. Even smaller pregalactic objects, such as primordial black holes, might form spontaneously at some sort of cosmological phase transition. Statistical fluctuations associated with these could be large enough to produce the Mmin objects.

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