The impact of varying inhomogeneous reionization histories on metrics of Lyα opacity

Abstract

The epoch of hydrogen reionization is complete by z = 5, but its progression at higher redshifts is uncertain. Measurements of Lyα forest opacity show large scatter at z < 6, suggestive of spatial fluctuations in neutral fraction, temperature, or ionizing background, either individually or in combination. However, there are degeneracies in the impact of such fluctuations, necessitating careful modeling. We develop a framework for modeling the reionization history and associated temperature fluctuations, with the intention of incorporating ionizing background fluctuations at a later time. We generate several reionization histories using semi-numerical code AMBER, and implement them in the Nyx cosmological hydrodynamics code to examine the impact on the evolution of gas within the simulation and the associated metrics of the Lyα forest opacity. We find that the pressure smoothing scale within the IGM is strongly correlated with the adiabatic index of the temperature-density relation. We find that while models with 20,000 K photoheating at reionization are better able to reproduce the shape of the observed z = 5 1D flux power spectrum than colder ones, they fail to match the highest wavenumbers. The simulated autocorrelation function and optical depth distributions are systematically low and narrow, respectively, compared to the observed values, but are in better agreement when the reionization history is longer in duration, more symmetric in its distribution of reionization redshifts, or if there are remaining neutral regions at z < 6.

intergalactic medium, dark ages, reionization, first stars, quasars: absorption lines

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