Lyα Emitters in Hierarchical Galaxy Formation

Abstract

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). Although previous studies using hierarchical clustering models simply assumed a constant and universal value of f, we incorporate two new effects on f: 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 ~ 1 should dominate the observed LFs. This is also consistent with available observations, whereas the simple universal f model requires f 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 xH I rapidly evolves from xH I 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.