Impact of anisotropic photon emission from sources during the epoch of reionization

Abstract

The reionization of the intergalactic medium (IGM) was driven by the first stars, galaxies, and accreting black holes. However, the relative importance of these sources and the efficiency by which ionizing photons escape into the IGM remain poorly understood. Most reionization modelling frameworks assume idealized, isotropic emissions. We investigate this assumption by examining a suite of simulations incorporating directed, anisotropic photon emissions. We find that such anisotropic emissions of ionizing photons yield a different reionization geometry compared to the standard, isotropic, case. During the early stages of reionization (when less than 30 per cent of the Universe is ionized), simulations with narrow photon leakage channels produce smaller ionized bubbles on average. However, these bubbles grow to similar sizes during the middle stages of reionization. This anisotropy not only produces a distinctive evolution of the size distribution of the ionized regions, but also imprints a feature onto the spherically averaged power spectra of the 21-cm signal throughout reionization. We observe a suppression in power by about 10–40 per cent at scales corresponding to wavenumbers k = 0.1-1 h Mpc-1, corresponding to the range in which current radio interferometers are most likely to measure the power spectrum. The simulation with the narrowest channel of ionization emission shows the strongest suppression. However, this anisotropic emission process does not introduce any measurable anisotropy in the 21-cm signal.