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

TL;DR

This study explores how anisotropic photon emissions from early cosmic sources affect the reionisation process, revealing differences in ionised bubble growth and observable 21-cm power spectra compared to isotropic assumptions.

Contribution

It introduces the first detailed simulations of anisotropic photon emissions during reionisation, showing their impact on ionisation geometry and 21-cm signal power spectra.

Findings

Anisotropic emissions lead to smaller initial ionised bubbles.

Power spectrum suppression of 10-40% at key scales.

No measurable anisotropy in the 21-cm signal due to emission directionality.

Abstract

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