The abundance of Lyα emitters in hierarchical models

We present predictions for the abundance of Lya emitters in hierarchical structure formation models. We use the GALFORM semi-analytical model of galaxy formation to explore the impact on the predicted counts of varying assumptions about the escape fraction of Lya photons, the redshift at which the Universe reionized and the cosmological density parameter. A model with a fixed escape fraction gives a remarkably good match to the observed counts over a wide redshift interval. The counts at bright fluxes are dominated by ongoing starbursts. We present predictions for the expected counts in a typical observation with the Multi-Unit Spectroscopic Explorer (MUSE) instrument proposed for the Very Large Telescope.