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Abstract: The structures produced during the epoch of reionization by the action ofradiation on neutral hydrogen are in principle different to those that arisethrough gravitational growth of initially small perturbations. We explore thedifference between the two mechanisms using high resolution cosmologicalradiative transfer. Our computations use a Monte Carlo code which raytracesdirectly through SPH kernels without a grid, preserving the high spatialresolution of the underlying hydrodynamic simulation. Because the properties ofthe first sources of radiation are uncertain, we simulate a range of modelswith different source properties and recombination physics. We examine themorphology of the neutral hydrogren distribution and the reionization historyin these models. We find that at fixed mean neutral fraction, structures arevisually 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 inthe recombination rate and amplitude of mass fluctuations. We concentrate onthe autocorrelation function of the neutral hydrogen, xi HIr as a basicquantitive measure of Radiation Induced Structure RIS. All the models we testexhibit a characteristic behaviour, with xi HIr becoming initially linearlyantibiased with respect to the matter correlation function, reaching a minimumbias factor b~0.5 when the universe is ~10-20% ionized. After this xi HIrincreases rapidly in amplitude, overtaking the matter correlation function. Itkeeps a power law shape, but flattens considerably, reaching an asymptoticlogarithmic slope of gamma ~-0.5. The growth rate of HI fluctuations isexponentially more rapid than gravitational growth over a brief interval ofredshift Deltaz ~ 2-3.

Author: Rupert A.C. Croft CMU, Gabriel Altay CMU

Source: https://arxiv.org/

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