2007ApJ...670..919K

We present a new theoretical model for the luminosity functions (LFs) of Lyα-emitting galaxies in the framework of hierarchical galaxy formation. We extend a semianalytical model of galaxy formation that reproduces a number of observations for local and high-z galaxies, without changing the original model parameters but with the introduction of physically motivated modeling to describe the escape fraction of Lyα photons from host galaxies (f^Lyα_esc). Although previous studies using hierarchical clustering models simply assumed a constant and universal value of f^Lyα_esc, we incorporate two new effects on f^Lyα_esc: extinction by interstellar dust and galaxy-scale outflows induced as star formation feedback. It is found that the new model nicely reproduces all the observed Lyα LFs of Lyα emitters (LAEs) with redshifts in the range z∼3-6. In particular, the surprisingly small evolution of the observed LAE Lyα LFs as compared with the dark halo mass function is naturally reproduced. Our model predicts that galaxies with strong outflows and f^Lyα_esc∼1 should dominate the observed LFs. This is also consistent with available observations, whereas the simple universal f^Lyα_esc model requires f^Lyα_esc≪1 in order not to overproduce the brightest LAEs. On the other hand, we find that our model significantly overpredicts LAEs at z≳6, and that absorption of Lyα photons by neutral hydrogen in the intergalactic medium (IGM) provides a reasonable interpretation of this discrepancy. This indicates that the IGM neutral fraction x_HI rapidly evolves from x_HI≪1 at z≲6 to a value of order unity at z∼6-7, which is broadly consistent with other observational constraints on the history of reionization.