The cool side of Lyman alpha emitters

We extend a previous study of Lyman alpha emitters (LAEs) based on hydrodynamical cosmological simulations, by including two physical processes important for LAEs: (i) Ly alpha and continuum luminosities produced by cooling of collisionally excited H i in the galaxy and (ii) dust formation and evolution; we follow these processes on a galaxy-by-galaxy basis. H i cooling on average contributes 16-18 per cent of the Ly alpha radiation produced by stars, but this value can be much higher in low-mass LAEs and further increased if the H i is clumpy. The continuum luminosity is instead almost completely dominated by stellar sources. The dust content of galaxies scales with their stellar mass, M-dust proportional to M0.7(*), and stellar metallicity, Z(*), such that M-dust proportional to Z1.7(*). As a result, the massive galaxies have Ly alpha escape fraction as low as f(alpha) = 0.1, with a LAE-averaged value decreasing with redshift: << f(alpha)>> = (0.33, 0.23) at z = (5.7, 6.6). The ultraviolet (UV) continuum escape fraction shows the opposite trend with z, possibly resulting from clumpiness evolution. The model successfully reproduces the observed Ly alpha and UV luminosity functions at different redshifts and the Ly alpha equivalent width scatter to a large degree, although the observed distribution appears to be more more extended than the predicted one. We discuss possible reasons for such tension.