Constraining Reionization with the Evolution of the Luminosity Function of Lyman Α Emitting Galaxies

At redshifts beyond z ∼> 6, as the mean fraction of neutral hydrogen 〈xHI〉 in the intergalactic medium (IGM) increases, the line flux of Lyα emitters can be significantly suppressed, which can result in a decrease in the observed number of emitters at a given Lyα flux. However, cosmological HII regions surrounding the Lyα emitting galaxies alleviate these effects. We use simple models of the Lyα line suppression that incorporate the presence of HII regions to predict the overall effect of the Lyα absorption on the Lyα luminosity function. We find, in agreement with other recent studies, that unless ionizing sources are unusually strongly clustered, a fully neutral IGM may be inconsistent with the large abundance of confirmed z = 6.5 Lyα emitters. However, the presence of local HII regions prohibits placing a tight constraint on the mean neutral fraction. We find 〈xHI〉 ∼< 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 Lyα LF is consistent with reionization occurring near this redshift. Finally, we suggest that a measurement of observed Lyα line width as a function of the Lyα luminosity, in a future, larger sample of Lyα emitters, may serve as a robust diagnostic of the neutral fraction in the IGM. Subject headings: cosmology: theory – cosmology: early universe – galaxies: formation – galaxies: highredshift – galaxies: quasars: general – galaxies: quasars: absorption lines