Lyman Alpha Emitters in the Hierarchically Clustering Galaxy Formation
Abstract
We present a new theoretical model for the luminosity functions (LFs) of Lyman alpha (Lya) 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 and high-z galaxies, without changing the original model parameters but introducing a physically-motivated modelling to describe the escape fraction of Lya photons from host galaxies (fesc). Though a previous study using a hierarchical clustering model simply assumed a constant and universal value of fesc, we incorporate two new effects on fesc: 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 Lya LFs of the Lya emitters (LAEs) at different redshifts in z ~ 3-6. Especially, the rather surprisingly small evolution of the observed LAE Lya LFs compared with the dark halo mass function is naturally reproduced. Our model predicts that galaxies with strong outflows and fesc ~ 1 are dominant in the observed LFs. This is also consistent with available observations, while the simple universal fesc model requires fesc << 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 Lya 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 broadly consistent with other observational constraints on the reionization history.