Constraints on the Star Formation Efficiency of Galaxies During the Epoch of Reionization

TL;DR

This paper develops an empirically-calibrated model for star formation efficiency in early galaxies during reionization, matching observed luminosity functions and exploring implications for cosmic reionization and future JWST observations.

Contribution

It introduces a new model linking halo mass to star formation efficiency at high redshifts, constrained by galaxy luminosity functions and reionization data.

Findings

Star formation efficiency peaks at ~30% for halos of 10^{11}-10^{12} M_sun.

Maximal efficiency in low-mass halos can reproduce Planck's optical depth.

Models suggest higher escape fractions or increased efficiency are needed if star formation is inefficient in small halos.

Abstract

Reionization is thought to have occurred in the redshift range of $6 < z < 9$, which is now being probed by both deep galaxy surveys and CMB observations. Using halo abundance matching over the redshift range $5<z<8$ and assuming smooth, continuous gas accretion, we develop a model for the star formation efficiency $f_{\star}$ of dark matter halos at $z>6$ that matches the measured galaxy luminosity functions at these redshifts. We find that $f_{\star}$ peaks at $\sim 30\%$ at halo masses $M \sim 10^{11}$--$10^{12}$~M$_\odot$, in qualitative agreement with its behavior at lower redshifts. We then investigate the cosmic star formation histories and the corresponding models of reionization for a range of extrapolations to small halo masses. We use a variety of observations to further constrain the characteristics of the galaxy populations, including the escape fraction of UV photons. Our approach provides an empirically-calibrated, physically-motivated model for the properties of star-forming galaxies sourcing the epoch of reionization. In the case where star formation in low-mass halos is maximally efficient, an average escape fraction $\sim0.1$ can reproduce the optical depth reported by Planck, whereas inefficient star formation in these halos requires either about twice as many UV photons to escape, or an escape fraction that increases towards higher redshifts. Our models also predict how future observations with JWST can improve our understanding of these galaxy populations.