Photoevaporation of cosmological minihaloes during reionization

Energy released by a small fraction of the baryons in the Universe, which condensed out while the intergalactic medium (IGM) was cold, dark and neutral, reheated and reionized it by redshift 6, exposing other baryons already condensed into dwarf-galaxy minihaloes to the glare of ionizing radiation. We present the first gas dynamical simulations of the photoevaporation of cosmological minihaloes overtaken by the ionization fronts which swept through the IGM during the reionization epoch in the currently favoured Λ cold dark matter (ΛCDM) universe, including the effects of radiative transfer. These simulations demonstrate the phenomenon of I-front trapping inside minihaloes, in which the weak, R-type fronts which travelled supersonically across the IGM decelerated when they encountered the dense, neutral gas inside minihaloes, and were thereby transformed into D-type I-fronts, preceded by shock waves. For a minihalo with virial temperature below 10⁴ K, the I-front gradually burned its way through the minihalo which trapped it, removing all of its baryonic gas by causing a supersonic, evaporative wind to blow backwards into the IGM, away from the exposed layers of minihalo gas just behind the advancing I-front. We describe this process in detail, along with some of its observable consequences, for the illustrative case of a minihalo of total mass 10⁷ M_solar, exposed to a distant source of ionizing radiation with either a stellar or quasar-like spectrum, after it was overtaken at redshift z= 9 by the weak, R-type I-front which ionized the IGM surrounding the source. For a source at z= 9 which emits 10⁵⁶ ionizing photons per second at 1 Mpc (or, equivalently, 10⁵² ionizing photons per second at 10 kpc), the photoevaporation of this minihalo takes about 100-150 Myr, depending on the source spectrum, ending at about z= 7.5.

Such hitherto neglected feedback effects were widespread during the reionization epoch. N-body simulations and analytical estimates of halo formation in the ΛCDM model suggest that sub-kpc minihaloes such as these, with virial temperatures below 10⁴ K, were so common as to cover the sky around larger-mass source haloes and possibly dominate the absorption of ionizing photons during reionization. This means that previous estimates of the number of ionizing photons per hydrogen atom required to complete reionization which neglected this effect may be too low. Regardless of their effect on the progress of reionization, however, the minihaloes were so abundant that random lines of sight through the high-z Universe should encounter many of them, which suggests that it may be possible to observe the processes described here in the absorption spectra of distant sources.