Simulations and Analytic Calculations of Bubble Growth during Hydrogen Reionization

Abstract

We present results from a large volume simulation of Hydrogen reionization.\nWe combine 3d radiative transfer calculations and an N-body simulation,\ndescribing structure formation in the intergalactic medium, to detail the\ngrowth of HII regions around high redshift galaxies. Our simulation tracks\n1024^3 dark matter particles, in a box of co-moving side length 65.6 Mpc/h.\nThis large volume allows us to accurately characterize the size distribution of\nHII regions throughout most of the reionization process. At the same time, our\nsimulation resolves many of the small galaxies likely responsible for\nreionization. It confirms a picture anticipated by analytic models: HII regions\ngrow collectively around highly-clustered sources, and have a well-defined\ncharacteristic size, which evolves from a sub-Mpc scale at the beginning of\nreionization to R > 10 Mpc towards the end. We present a detailed statistical\ndescription of our results, and compare them with a numerical scheme based on\nthe analytic model by Furlanetto, Zaldarriaga, and Hernquist. We find that the\nanalytic calculation reproduces the size distribution of HII regions and the 21\ncm power spectrum of the radiative transfer simulation remarkably well. The\nionization field from the simulation, however, has more small scale structure\nthan the analytic calculation, owing to Poisson scatter in the simulated\nabundance of galaxies on small scales. We propose and validate a simple scheme\nto incorporate this scatter into our calculations. Our results suggest that\nanalytic calculations are sufficiently accurate to aid in predicting and\ninterpreting the results of future 21 cm surveys. In particular, our fast\nnumerical scheme is useful for forecasting constraints from future 21 cm\nsurveys, and in constructing mock surveys to test data analysis procedures.\n