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

TL;DR

This paper investigates the large-scale structure of neutral hydrogen during reionization caused by radiation, using high-resolution simulations to analyze the autocorrelation function and its evolution.

Contribution

It introduces a detailed simulation approach to study radiation-induced structures and characterizes the autocorrelation function of neutral hydrogen during reionization.

Findings

Autocorrelation function becomes initially antibiased, reaching a minimum bias at 10-20% ionization.

HI fluctuations grow exponentially faster than gravitational growth over a short redshift interval.

The autocorrelation function maintains a power-law shape with a flattened slope at late stages.

Abstract

The structures produced during the epoch of reionization by the action of radiation on neutral hydrogen are in principle different to 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 raytraces directly through 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 hydrogren 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, xi_HI(r) as a basic quantitive measure of Radiation Induced Structure (RIS.) All the models we test exhibit a characteristic behaviour, with xi_HI(r) 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% ionized. After this xi_HI(r) increases rapidly in amplitude, overtaking the matter correlation function. It keeps a power law shape, but flattens considerably, reaching an asymptotic logarithmic slope of gamma ~-0.5. The growth rate of HI fluctuations is exponentially more rapid than gravitational growth over a brief interval of redshift Deltaz ~ 2-3.