Radiation-induced large-scale structure during the reionization epoch: the autocorrelation function

Abstract

The structures produced during the epoch of reionization by the action of radiation on neutral hydrogen are in principle different from those that arise through gravitational growth of initially small perturbations. We explore the difference between the two mechanisms using high-resolution cosmological radiative transfer. Our computations use a Monte Carlo code which ray-traces directly through smoothed particle hydrodynamics (SPH) kernels without a grid, preserving the high spatial resolution of the underlying hydrodynamic simulation. Because the properties of the first sources of radiation are uncertain, we simulate a range of models with different source properties and recombination physics. We examine the morphology of the neutral hydrogen distribution and the reionization history in these models. We find that at fixed mean neutral fraction, structures are visually most affected by the existence of a lower limit in source luminosity, then by galaxy mass-to-light ratio, and are minimally affected by changes in the recombination rate and amplitude of mass fluctuations. We concentrate on the autocorrelation function of the neutral hydrogen, ξ Hl (r), as a basic quantitative measure of radiation-induced structure. All the models we test exhibit a characteristic behaviour, with ξ Hl becoming initially linearly antibiased with respect to the matter correlation function, reaching a minimum bias factor b ∼ 0.5 when the universe is ∼ 10-20 per cent ionized. After this ξ Hl increases rapidly in amplitude, overtaking the matter correlation function. It keeps a power-law shape, but flattens considerably, reaching an asymptotic logarithmic slope of γ Hl ≃ -0.5. The growth rate of H i fluctuations is exponentially more rapid than gravitational growth over a brief interval of redshift Δ z ∼ 2-3.