  We present a semi-analytical model of star formation which explains
simultaneously the observed UV luminosity function of high redshift Lyman break
galaxies (LBGs) and luminosity functions 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 halos. The Lyman-alpha
luminosity functions at z < 4 are well reproduced with only <10% 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 luminosity
function 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 inter stellar
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 luminosity function 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, upto z = 6.5, we do not see the
effect of evolving inter galactic medium (IGM) opacity on the Lyman-alpha
escape from these galaxies.
