Constraining reionization with the evolution of the luminosity function of Lyα emitting galaxies

At redshifts beyond z greater than or similar to 6, as the mean fraction of neutral hydrogen < x(HI)> in the intergalactic medium (IGM) increases, the line flux of L alpha emitters can be significantly attenuated, which can result in a decrease in the observed number of emitters at a given L alpha flux. However, cosmological H II regions surrounding the Ly alpha emitting galaxies alleviate these effects. We use simple models of the Ly alpha line attenuation that incorporate the presence of H II regions to predict the overall effect of the Ly alpha absorption on the Ly alpha luminosity function (LF). We find, in agreement with other recent studies, that a fully neutral IGM is inconsistent with the Ly alpha LF remaining unchanged from z 5: 7 to z 6: 5, as suggested by recent observations. However, the presence of local H II regions prohibits placing a tight constraint on the mean neutral fraction. We find < x(HI)> less than or similar to 0.25; the presence of strong winds and/or the clustering of ionizing sources would further weaken this constraint. We conclude that the evolution of the Ly alpha LF is consistent with reionization occurring near this redshift, as suggested by other observations. Finally, we suggest that a measurement of observed Ly alpha line width as a function of the Ly alpha luminosity, in a future, larger sample of Ly alpha emitters, may serve as a robust diagnostic of the neutral fraction in the IGM.