Fast large volume simulations of the 21-cm signal from the reionization and pre-reionization epochs

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 deg 2 on the 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 the order of 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 Zel'dovich approximation, and then specifying the location and mass of collapsed dark matter haloes 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 α 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.