Cosmological density fluctuations produced by sources of radiation at z > 100

It is suggested that the formation of galaxies and galaxy clusters was not a direct result of primordial fluctuations, but was caused by a release of energy from pregalactic sources of radiation, such as massive stars or quasars, at a cosmological epoch t ≲ 10⁷ yr. An approximate theory is developed, using the standard description of perturbations in radiation and ionized gas coupled by Thomson scattering, which shows that such sources automatically produce white noise matter–density fluctuations on scales 10¹²−10¹⁶M_⊙ which are identical to the fluctuations postulated in hierarchical theories of galaxy formation and clustering. The radiation Jeans mass $$M_{\text J \enspace\gamma} \cong 10^{16}(\Omega h^2)^{-2}M_\odot$$ emerges in this theory as the natural maximum scale of structure and of binding energy. Tests of this hypothesis are proposed, based on observations of the amplitude and angular spectrum of anisotropy in the microwave background at angular scales of a few degrees. The essential difference of the model from the standard scenarios is that the fluctuations emerging from the fireball phase can be much smaller, and may in fact be negligible on galactic scales. A simple model is used to illustrate the possibility of triggering gas collapse and star formation over large regions ≫ 10⁶M_⊙ using radiation from primordial ‘seeds’ as small as ≅ 10M_⊙. A number of scenarios are outlined in which the only primordial fluctuations are seeds on this scale which form as a result of a phase transition at T ≳ GeV.