Cosmic Reionization and the 21 cm Signal: Comparison between an Analytical Model and a Simulation

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Abstract

We measure several properties of the reionization process and the corresponding low-frequency 21 cm signal associated with the neutral hydrogen distribution, using a large volume, high-resolution simulation of cosmic reionization. The brightness temperature of the 21 cm signal is derived by postprocessing this numerical simulation with a semianalytical prescription. Our study extends to high redshifts (z ~ 25) where, in addition to collisional coupling, our postprocessed simulations take into account the inhomogeneities in the heating of the neutral gas by X-rays and the effect of an inhomogeneous Ly α radiation field. Unlike the well-studied case in which spin temperature is assumed to be significantly greater than the temperature of the cosmic microwave background due to uniform heating of the gas by X-rays, spatial fluctuations in both the Ly α radiation field and X-ray intensity affect predictions related to the brightness temperature at z > 10, during the early stages of reionization and gas heating. The statistics of the 21 cm signal from our simulation are then compared to existing analytical models in the literature, and we find that these analytical models provide a reasonably accurate description of the 21 cm power spectrum at z < 10. Such an agreement is useful, since analytical models are better suited to quickly explore the full astrophysical and cosmological parameter space relevant for future 21 cm surveys. We find, nevertheless, nonnegligible differences that can be attributed to differences in the inhomogeneous X-ray heating and Lyα coupling at z > 10, and, with upcoming interferometric data, these differences in return can provide a way to better understand the astrophysical processes during reionization.
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