Probing reionization with Lyman α emission lines

Lyman alpha emission from high-redshift galaxies may be a powerful probe of the ionization history of the intergalactic medium (IGM) at z > 6; the observed Lyman alpha emission line is sensitive to the IGM hydrogen neutral fraction over 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 infrared 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 less than or similar to0.1.