Photon Consumption in Minihalos during Cosmological Reionization

At the earliest epochs of structure formation in cold dark matter (CDM) cosmologies, the smallest nonlinear objects are the numerous small halos that condense with virial temperatures below ~10⁴ K. Such ``minihalos'' are not yet resolved in large-scale three-dimensional cosmological simulations. Here we employ a semianalytic method, combined with three-dimensional simulations of individual minihalos, to examine their importance during cosmological reionization. We show that, depending on when reionization takes place, they potentially play an important role as sinks of ionizing radiation. If reionization occurs at sufficiently high redshifts (z_r>~20), the intergalactic medium is heated to ~10⁴ K and most minihalos never form. On the other hand, if z_r<~20, a significant fraction (>~10%) of all baryons have already collapsed into minihalos, and are subsequently removed from the halos by photoevaporation as the ionizing background flux builds up. We show that this process can require a significant budget of ionizing photons, exceeding the production by straightforward extrapolations back in time of known quasar and galaxy populations by a factor of up to ~10 and ~3, respectively.