21cmfast: a fast, seminumerical simulation of the high-redshift 21-cm signal

We introduce a powerful seminumeric modelling tool, 21cmfast, designed to efficiently simulate the cosmological 21-cm signal. Our code generates 3D realizations of evolved density, ionization, peculiar velocity and spin temperature fields, which it then combines to compute the 21-cm brightness temperature. Although the physical processes are treated with approximate methods, we compare our results to a state-of-the-art large-scale hydrodynamic simulation, and find good agreement on scales pertinent to the upcoming observations (greater than or similar to 1 Mpc). The power spectra from 21cmfast agree with those generated from the numerical simulation to within 10s of per cent, down to the Nyquist frequency. We show results from a 1-Gpc simulation which tracks the cosmic 21-cm signal down from z = 250, highlighting the various interesting epochs. Depending on the desired resolution, 21cmfast can compute a redshift realization on a single processor in just a few minutes. Our code is fast, efficient, customizable and publicly available, making it a useful tool for 21-cm parameter studies.