Extending Semi-numeric Reionisation Models to the First Stars and Galaxies

TL;DR

This paper introduces a mass-dependent ionising efficiency parameter in semi-numeric reionisation models, enabling more accurate simulations of ionising sources across a broad range of halo masses during the epoch of reionisation.

Contribution

The paper develops a new parameterization that incorporates physical constraints from simulations, extending semi-numeric models to include a wider range of host halo masses.

Findings

Model aligns with Planck's optical depth constraints.

Bubble size distributions are broader and peak at smaller sizes.

Small halos contribute mainly at the start of EoR, ceasing after z=10.

Abstract

Semi-numeric methods have made it possible to efficiently model the epoch of reionisation (EoR). While most implementations involve a reduction to a simple three-parameter model, we introduce a new mass-dependent ionising efficiency parameter that folds in physical parameters that are constrained by the latest numerical simulations. This new parameterization enables the effective modeling of a broad range of host halo masses containing ionising sources, extending from the smallest Population III host halos with $M \sim 10^6 M_\odot$, which are often ignored, to the rarest cosmic peaks with $M \sim 10^{12} M_\odot$ during EoR. We compare the resulting ionising histories with a typical three-parameter model and also compare with the latest constraints from the Planck mission. Our model results in a optical depth due to Thomson scattering, $\tau_{\mathrm{e}}$ = 0.057, that is consistent with Planck. The largest difference in our model is shown in the resulting bubble size distributions which peak at lower characteristic sizes and are broadened. We also consider the uncertainties of the various physical parameters and comparing the resulting ionising histories broadly disfavors a small contribution from galaxies. As the smallest haloes cease a meaningful contribution to the ionising photon budget after $z = 10$, implying they play a role in determining the start of EoR and little else.