Lyman Alpha Emitting Galaxies as a Probe of Reionization

TL;DR

This paper reviews how Lyman Alpha emitting galaxies can be used to study the Epoch of Reionization, discussing radiative processes, IGM effects, and observational strategies to understand the universe's reionization history.

Contribution

It provides a comprehensive review of Lyman Alpha radiative transfer, optical depth calculations, and the potential of galaxy surveys to probe the reionization epoch.

Findings

Observed Lya flux suppression at z > 6 indicates rapid reionization.

Inhomogeneous models suggest a swift evolution of neutral hydrogen fraction.

Future surveys can improve constraints on reionization history.

Abstract

The Epoch of Reionization (EoR) represents a milestone in the evolution of our Universe. Star-forming galaxies that existed during the EoR likely emitted a significant fraction (~5-40%) of their bolometric luminosity as Lyman Alpha (Lya) line emission. However, neutral intergalactic gas that existed during the EoR was opaque to Lya emission that escaped from galaxies during this epoch, which makes it difficult to observe. The neutral intergalactic medium (IGM) may thus reveal itself by suppressing the Lya flux from background galaxies. Interestingly, a `sudden' reduction in the observed Lya flux has now been observed in galaxies at z >6. This review contains a detailed summary of Lya radiative processes: I describe (i) the main Lya emission processes, including collisional-excitation & recombination (and derive the famous factor `0.68'), and (ii) basic radiative transfer concepts, including e.g. partially coherent scattering, frequency diffusion, resonant versus wing scattering, optically thick versus 'extremely' optically thick (static/outflowing/collapsing) media, and multiphase media. Following this review, I derive expressions for the Gunn-Peterson optical depth of the IGM during (inhomogeneous) reionization and post-reionization. I then describe why current observations appear to require a very rapid evolution of volume-averaged neutral fraction of hydrogen in the context of realistic inhomogeneous reionization models, and discuss uncertainties in this interpretation. Finally, I describe how existing & futures surveys and instruments can help reduce these uncertainties, and allow us to fully exploit Lya emitting galaxies as a probe of the EoR.