Probing reionization with Lyman-alpha emission lines

Abstract

Lyman-alpha emission from high-redshift galaxies may be a powerful probe of the ionization history of the IGM at z>6: the observed Lyman-alpha emission line is sensitive to the neutral fraction of IGM hydrogen in the range 0.1-1. We present calculations of observed Lyman-alpha emission lines from z>6 galaxies, illustrating the effect of varying the many free parameters associated with the emitting galaxy, its halo, and the IGM around the galaxy. In particular, we use a dynamic model of the IGM that includes the effect of IGM infall toward the emitting galaxy. Galactic winds may play a crucial role in determining observed Lyman-alpha line fluxes. We compare our model predictions with observations of two z=6.5 galaxies and conclude that, if galactic winds are allowed for, existing observations place no constraint on the neutral fraction of the IGM at z=6.5. Future space-based observations will constrain the importance of galactic winds; if winds are unimportant for the observed z=6.5 galaxies, our models suggest that the IGM neutral fraction at z=6.5 is <~0.1.