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

TL;DR

This paper presents a semi-analytical model explaining the evolution of Lyman-alpha emitter luminosity functions across redshifts, highlighting a transition around z=4-5 related to galaxy properties and IGM effects.

Contribution

It introduces a model that simultaneously reproduces UV and Lyman-alpha luminosity functions using different halo mass functions, revealing a key epoch change at z=4-5.

Findings

Lyman-alpha luminosity functions at z<4 are explained with <10% LBGs emitting Lyman-alpha.

At z>5, nearly all LBGs are Lyman-alpha emitters, indicating a shift in galaxy properties.

No significant impact of IGM opacity evolution on Lyman-alpha escape up to z=6.5.

Abstract

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.