The Radiative Feedback from the First Stars and Quasars and the End of the Dark Age

Abstract.

In hierarchical models of structure formation, an early cosmic UV background (UVB) is produced by the small ( $T_{vir}\lesssim 10^{4}$K) halos that collapse before cosmological reionization. If the first collapsed halos harbor stars, then their UV flux below 13.6 eV photo-dissociates the molecular hydrogen H₂ in subsequently collapsing halos within a large sphere of influence. This causes a pause in the cosmic star-formation history: the buildup of the UVB and the epoch of reionization are delayed until larger halos ( $T_{vir}\gtrsim 10^{4}$K) collapse. In contrast, if the small halos harbor mini-quasars with hard spectra extending to $\sim$1keV, then their X-rays balance the effects of the UVB, the negative feedback does not occur, and reionization can be caused early on by the small halos.