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 square degrees 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 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 Zel'dovich 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.
