Dark-ages reionization and galaxy formation simulation - III. Modelling galaxy formation and the epoch of reionization

TL;DR

This paper introduces Meraxes, a semi-analytic galaxy formation model that incorporates coupled reionization effects, successfully reproducing key observations of early galaxy growth and reionization history.

Contribution

Meraxes is the first model to include a coupled treatment of reionization and galaxy formation, providing new insights into galaxy growth during reionization.

Findings

Reproduces observed optical depth and galaxy stellar mass function evolution between z=5 and 7.

An evolving escape fraction of 0.05-0.1 is needed to match ionizing emissivity data.

Reionization has limited impact on galaxy stellar mass functions at M*>10^{7.5} M_sun.

Abstract

We introduce Meraxes, a new, purpose-built semi-analytic galaxy formation model designed for studying galaxy growth during reionization. Meraxes is the first model of its type to include a temporally and spatially coupled treatment of reionization and is built upon a custom (100 Mpc)$^3$ N-body simulation with high temporal and mass resolution, allowing us to resolve the galaxy and star formation physics relevant to early galaxy formation. Our fiducial model with supernova feedback reproduces the observed optical depth to electron scattering and evolution of the galaxy stellar mass function between $z$=5 and 7, predicting that a broad range of halo masses contribute to reionization. Using a constant escape fraction and global recombination rate, our model is unable to simultaneously match the observed ionizing emissivity at $z{\lesssim}6$. However, the use of an evolving escape fraction of 0.05-0.1 at $z{\sim}6$, increasing towards higher redshift, is able to satisfy these three constraints. We also demonstrate that photoionization suppression of low mass galaxy formation during reionization has only a small effect on the ionization history of the inter-galactic medium. This lack of `self-regulation' arises due to the already efficient quenching of star formation by supernova feedback. It is only in models with gas supply-limited star formation that reionization feedback is effective at regulating galaxy growth. We similarly find that reionization has only a small effect on the stellar mass function, with no observationally detectable imprint at $M_{\rm *}{>}10^{7.5}\,{\rm M_{{\odot}}}$. However, patchy reionization has significant effects on individual galaxy masses, with variations of factors of 2-3 at $z$=5 that correlate with environment.