Evaluating the Accuracy of Reionization Prescriptions in Semi-analytic Models of the First Stars and Galaxies

TL;DR

This paper assesses the accuracy of semi-analytic reionization models in predicting ionized bubble distributions during the first galaxies' formation, revealing limitations of existing models and proposing a new, more accurate prescription.

Contribution

The study introduces a new semi-analytic reionization model that accounts for cosmic web density structures, improving agreement with detailed simulations.

Findings

Existing models overestimate reionization feedback effects.

New model accurately predicts halo ionization fractions.

Reionization feedback impacts low-mass galaxies significantly.

Abstract

Semi-analytic models are a valuable tool to study the first stars and galaxies. Their numerical efficiency makes it possible to survey broad regions of astrophysical parameter space across large volumes and redshift ranges. Following reionization in these models is necessary since star formation is suppressed in ionized regions due to photoheating of the gas. Here we evaluate the accuracy of three semi-analytic reionization prescriptions (two previously developed and one new model) by comparing their three-dimensional distribution of ionized bubbles to the Renaissance hydrodynamical cosmological radiative transfer simulations. We find that the previously existing models accurately determine the distribution of the larger bubbles within our ${\sim}6$ comoving Mpc simulation box, but that these models fail to take into account self-shielded neutral gas in dense filaments. Thus, these prescriptions overestimate the fraction of halos in HII regions impacted by reionization feedback by up to an order of magnitude (depending on halo mass and redshift). This leads to an unrealistically large effect of reionization feedback on Pop III stars and low-mass metal-enriched galaxies. Our newly developed model takes into account the density structure of the cosmic web, leading to good agreement with Renaissance in the fraction of halos found in ionized regions.