Lyman Alpha Emitters in the Hierarchically Clustering Galaxy Formation

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 semi-analytic model of galaxy formation that reproduces a number of observations for local galaxies, without changing the original model parameters but introducing a physically-motivated modelling to describe the escape fraction of Lyα photons from host galaxies ( f Lyα esc ). Though a previous study using a hierarchical clustering model 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 outflow induced as a star formation feedback. It is found that the new model nicely reproduces all the observed Lyα LFs of the Lyα emitters (LAEs) at different redshifts in z ∼3–6. Our model predicts that galaxies with strong outflows and f Lyα esc ∼ 1 are dominant in the observed LFs, which is consistent with available observations while the simple universal f Lyα esc model requires f Lyα esc ≪ 1 not to overproduce the brightest LAEs. On the other hand, we found that our model significantly overpredicts LAEs at z & 6, and absorption of Lyα photons by neutral hydrogen in intergalactic medium (IGM) is a reasonable interpretation for the discrepancy. This indicates that the IGM neutral fraction xHI rapidly evolves from xHI ≪ 1 at z . 6 to a value of order unity at z ∼ 6–7, which is consistent with other observational constraints on the reionization history. Subject headings: galaxies: evolution — galaxies: formation — galaxies: high-redshift — methods: numerical