Connecting faint-end slopes of the Lyman a emitter and Lyman-break galaxy luminosity functions

Abstract

We predict Lyman α (Lyα) luminosity functions (LFs) of Lyα-selected galaxies (Lyα emitters, or LAEs) at z = 3 - 6 using the phenomenological model. This model combines observed UV-LFs of Lyman-break galaxies (LBGs, or drop-out galaxies), with constraints on their distribution of Lyα line strengths as a function of UV-luminosity and redshift. Our analysis shows that while Lyα LFs of LAEs are generally not Schechter functions, these provide a good description over the luminosity range of log10(Lα/erg s-1) = 41-44. Motivated by this result, we predict Schechter function parameters at z = 3-6. Our analysis further shows that (i) the faint-end slope of the Lyα LF is steeper than that of the UV-LF of LBGs, (with a median αLyα < -2.0 at z ≳ 4), and (ii) a turnover in the Lyα LF of LAEs at Lyα luminosities 1040 ≲ Lα ≲ 1041 erg s-1 may signal a flattening of UV-LF of LBGs at -12 > MUV > -14. We discuss the implications of these results - which can be tested directly with upcoming surveys - for the Epoch of Reionization.