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

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 alpha 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 alpha LFs at z less than or similar to 4 are well reproduced with only less than or similar to 10 per cent of the LBGs emitting Lyman alpha 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 alpha 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 alpha 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 alpha photons. At z > 5, the observed Lyman alpha 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 alpha escape from these galaxies.