While limited to low spatial resolution, the next generation low-frequency radio interferometers that target 21 cm observations during the era of reionization and prior will have instantaneous fields-of-view that are many tens of square degrees on th
e sky. Predictions related to various statistical measurements of the 21 cm brightness temperature must then be pursued with numerical simulations of reionization with correspondingly large volume box sizes, of order 1000 Mpc on one side. We pursue a semi-numerical scheme to simulate the 21 cm signal during and prior to Reionization by extending a hybrid approach where simulations are performed by first laying down the linear dark matter density field, accounting for the non-linear evolution of the density field based on second-order linear perturbation theory as specified by the Zeldovich approximation, and then specifying the location and mass of collapsed dark matter halos using the excursion-set formalism. The location of ionizing sources and the time evolving distribution of ionization field is also specified using an excursion-set algorithm. We account for the brightness temperature evolution through the coupling between spin and gas temperature due to collisions, radiative coupling in the presence of Lyman-alpha photons and heating of the intergalactic medium, such as due to a background of X-ray photons. The hybrid simulation method we present is capable of producing the required large volume simulations with adequate resolution in a reasonable time so a large number of realizations can be obtained with variations in assumptions related to astrophysics and background cosmology that govern the 21 cm signal.
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 temp
erature of the 21-cm signal is derived by post-processing this numerical simulation with a semi-analytical prescription. Our study extends to high redshifts (z ~ 25) where, in addition to collisional coupling, our post-processed simulations take into account the inhomogeneities in the heating of the neutral gas by X-rays and the effect of an inhomogeneous Lya radiation field. Unlike the well-studied case where 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 Lya radiation field and X-ray intensity impact 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, non-negligible differences that can be attributed to differences in the inhomogeneous X-ray heating and Lya 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.
