On the redshift-evolution of the Lyman-alpha escape fraction and the dust content of galaxies

TL;DR

This study analyzes the evolution of the Lyman-alpha escape fraction in galaxies from redshift 0 to 8, revealing its increase with redshift and its relation to dust content, with implications for understanding galaxy evolution and reionization.

Contribution

It provides the first empirical measurement of the Lyman-alpha escape fraction evolution and links it to dust content, offering new methods to estimate dust in high-redshift galaxies.

Findings

fescLya increases with redshift, reaching unity at z=11.1

Empirical relation between fescLya and dust E(B-V) explains evolution up to z=6

fescLya drops beyond z~6.5 due to IGM effects

Abstract

The Ly-alpha emission line has been proven a powerful tool by which to study evolving galaxies at the highest redshifts. However, in order to use Lya as a physical probe of galaxies, it becomes vital to know the Lya escape fraction (fescLya). Unfortunately, due to the resonant nature of Lya, fescLya may vary unpredictably and requires empirical measurement. Here we compile Lya luminosity functions between redshift z=0 and 8 and, combined with H-alpha and ultraviolet data, assess how fescLya evolves with redshift. We find a strong upwards evolution in fescLya over the range z=0.3-6, which is well-fit by the power-law fescLya \propto (1+z)^\xi, with \xi =(2.57_-0.12^+0.19). This predicts that fescLya should reach unity at z=11.1. By comparing fescLya and E(B-V) in individual galaxies we derive an empirical relationship between fescLya and E(B-V), which includes resonance scattering and can explain the redshift evolution of fescLya between z=0 and 6 purely as a function of the evolution in the dust content of galaxies. Beyond z~6.5, fescLya drops more substantially; an effect attributed to either ionizing photon leakage, or an increase in the neutral gas fraction of the intergalactic medium. While distinguishing between those two scenarios may be extremely challenging, by framing the problem this way we remove the uncertainty of the halo mass from Lya-based tests of reionization. We finally derive a new method by which to estimate the dust content of galaxies based purely upon the observed Lya and UV LFs. These data are characterized by an exponential with an e-folding redshift of ~3.5.