(Re)Solving Reionization with Ly{\alpha}: How Bright Ly{\alpha} Emitters account for the $z\approx2-8$ Cosmic Ionizing Background

TL;DR

This paper introduces a Lyα-based framework to understand the cosmic ionizing background, highlighting the role of bright Lyα emitters in reionization and their contribution to the ionizing photon budget from redshift 2 to 8.

Contribution

It presents a novel Lyα emitter-centric model for cosmic reionization, linking Lyα escape fractions to ionizing photon production and accounting for the observed ionizing background evolution.

Findings

LAEs and quasars together reproduce the flat ionizing emissivity at z≈2-6.

LAEs can drive rapid reionization between z≈6-9.

A small, highly ionizing galaxy population accounts for the entire ionizing budget.

Abstract

The cosmic ionizing emissivity from star-forming galaxies has long been anchored to UV luminosity functions. Here we introduce an emissivity framework based on Ly$\alpha$ emitters (LAEs), which naturally hones in on the subset of galaxies responsible for the ionizing background due to the intimate connection between the production and escape of Ly$\alpha$ and LyC photons. Using constraints on the escape fractions of bright LAEs ($L_{\rm{Ly\alpha}}>0.2 L^{*}$) at $z\approx2$ obtained from resolved Ly$\alpha$ profiles, and arguing for their redshift-invariance, we show that: (i) quasars and LAEs together reproduce the relatively flat emissivity at $z\approx2-6$, which is non-trivial given the strong evolution in both the star-formation density and quasar number density at these epochs and (ii) LAEs produce late and rapid reionization between $z\approx6-9$ under plausible assumptions. Within this framework, the $>10\times$ rise in the UV population-averaged $f_{\rm{esc}}$ between $z\approx3-7$ naturally arises due to the same phenomena that drive the growing Ly$\alpha$ emitter fraction with redshift. Generally, a LAE dominated emissivity yields a peak in the distribution of the ionizing budget with UV luminosity as reported in latest simulations. Using our adopted parameters ($f_{\rm{esc}}=50\%$, $\xi_{\rm{ion}}=10^{25.9}$ Hz erg$^{-1}$ for half the bright LAEs), a highly ionizing minority of galaxies with $M_{\rm UV}<-17$ accounts for the entire ionizing budget from star-forming galaxies. Rapid flashes of LyC from such rare galaxies produce a "disco" ionizing background. We conclude proposing tests to further develop our suggested Ly$\alpha$-anchored formalism.