Understanding the redshift evolution of the luminosity functions of Lyman-α emitters

Abstract

We present a semi-analytical model of star formation which explains simultaneously the observed ultraviolet (UV) luminosity function (LF) of high-redshift Lyman break galaxies (LBGs) and LFs of Lyman emitters. We consider both models that use the Press-Schechter (PS) and Sheth-Tormen (ST) halo mass functions to calculate the abundances of dark matter haloes. The Lyman α LFs at z ≾ 4 are well reproduced with only ≾ 10 per cent of the LBGs emitting Lyman lines with rest equivalent width greater than the limiting equivalent width of the narrow band surveys. However, the observed LF at z > 5 can be reproduced only when we assume that nearly all LBGs are Lyman emitters. Thus, it appears that 4 < z < 5 marks the epoch when a clear change occurs in the physical properties of the high-redshift galaxies. As Lyman α escape depends on dust and gas kinematics of the interstellar medium (ISM), this could mean that on an average the ISM at z > 5 could be less dusty, more clumpy and having more complex velocity field. All of these will enable easier escape of the Lyman photons. At z > 5, the observed Lyman α LF are well reproduced with the evolution in the halo mass function along with very minor evolution in the physical properties of high-redshift galaxies. In particular, up to z= 6.5, we do not see the effect of evolving intergalactic medium opacity on the Lyman α escape from these galaxies.