Modelling reionization in a bursty universe

Abstract

We present semi-analytic models of the epoch of reionization focusing on the differences between continuous and bursty star formation (SF). Our model utilizes physically motivated analytic fits to 1D radiative transfer simulations of HII regions around dark matter haloes in a representative cosmic volume. Constraining our simulations with observed and extrapolated UV luminosity functions of high-redshift galaxies, we find that for a fixed halo mass, stellar populations forming in bursty models produce larger HII regions which leave behind longlived relic HII regions which are able to maintain partial ionization in the intergalactic medium (IGM) in a manner similar to an early X-ray background. The overall effect is a significant increase in the optical depth of the IGM, τe, and amilder increase of the redshift of reionization. To produce τe = 0.066 observed by Planck and complete reionization by redshift zre ~ 6, models with bursty SF require an escape fraction fesc ~ 2-10 per cent that is 2-10 times lower than fesc ~ 17 per cent found assuming continuous SF and is consistent with upper limits on fesc from observations at z = 0 and z ~ 1.3-6. The ionizing photon budget needed to reproduce the observed τe and zre depends on the period and duty cycle of the bursts of SF and the temperature of the neutral IGM. These results suggest that any remaining tension between observed and predicted ionizing photon budget for reionization can be alleviated if reionization is driven by short bursts of SF, perhaps relating to the formation of Population III stars and compact star clusters such as protoglobular clusters.