Cosmic Reionization and the 21cm Signal: Simulations and Analytical Models

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 simulation involving 24 billion dark matter particles utilizes a hybrid code with a particle-mesh N-body algorithm for dark matter and prescriptions for baryons and star formation. The radiative transfer of ionization radiation is calculated simultaneously using an adaptive algorithm as the dark matter distribution evolves in time with calculations performed on particles rather than on a coarse grid of the density distribution. The brightness temperature of the 21-cm signal is derived by post-processing this numerical simulation and compared to results based on analytical models of the reionization process. Our study extends to high redshifts where the contribution from spin temperature is non-negligible and we take into account the heating of the gas by X-rays and the effect of Lyα and inhomogeneous collisional coupling when calculating the 21-cm radio signal. In general to the extent we can compare with a single simulation, we find that analytical models provide a reasonably accurate description (within a factor of two) of the 21-cm power spectrum, which is useful in order to quickly explore the full parameter space relevant for future 21cm surveys. We find, nevertheless, non-negligible differences that can be attributed to a combination of the non-Gaussian nature of the 21cm signal and aspherical ionized patches in the simulation which are not part of the analytical calculation. Subject headings: cosmology: theory — large scale structure of Universe — diffuse radiation