The radiative feedback of the first cosmological objects

In hierarchical models of structure formation, an early cosmic UV background (UVB) is produced by the small (Tvir ≲ 10^4 K) halos that collapse before reionization. The UVB at energies below 13.6 eV suppresses the formation of stars or black holes inside small halos by photodissociating their only cooling agent, molecular H2. We self-consistently compute the buildup of the early UVB in Press-Schechter models, coupled with H2 photodissociation both in the intergalactic medium (IGM) and inside virialized halos. We find that the intergalactic H2 has a negligible effect on the UVB, both because its initial optical depth is small (≲0.1) and because it is photodissociated at an early stage. If the UV sources in the first collapsed halos are stars, then their UV flux suppresses further star formation inside small halos. This results in a pause in the buildup of the UVB, and reionization is delayed until larger halos (Tvir ≳ 10^4 K) collapse. If the small halos host miniquasars with hard spectra extending to ~1 keV, then their X-rays balance the effects of the UVB, the negative feedback does not occur, and reionization could be caused by the small halos.