Fast Large Volume Simulations of the 21 cm Signal from the Reionization and pre-Reionization Epochs

TL;DR

This paper introduces a fast semi-numerical simulation method for large-volume 21 cm signal predictions during reionization and pre-reionization, enabling efficient analysis of upcoming low-frequency radio observations.

Contribution

It extends a hybrid approach combining Zel'dovich approximation and excursion-set formalism for large-scale, high-resolution 21 cm simulations in a computationally efficient manner.

Findings

Capable of producing large volume simulations with adequate resolution

Enables multiple realizations with varied astrophysical and cosmological assumptions

Accounts for spin-gas temperature coupling, Lyman-alpha radiative effects, and X-ray heating

Abstract

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.