Empirical constraints on the star formation and redshift dependence of the lyα 'effective' escape fraction

Abstract

We derive empirical constraints on the volume-averaged 'effective' escape fraction of Lya photons from star-forming galaxies as a function of redshift, by comparing star formation functions inferred directly from observations to observed Lya luminosity functions. Our analysis shows that the effective escape fraction increases from f effesc ~ 1-5 per cent at z = 0 to f effesc ~ 10 per cent at z = 3-4 and to f effesc = 35-50 per cent at z = 6. Our constraint at z = 6 lies above predictions by models that do not include winds, and therefore hints at the importance of winds in the Lya transfer process (even) at this redshift. We can reproduce Lya luminosity functions with an f effesc that does not depend on the galaxies star formation rates (Ψ) over up to ~2 orders of magnitude in the Lya luminosity. It is possible to reproduce the luminosity functions with an f effesc that decreases with Ψ - which appears favoured by observations of drop-out galaxies - in models which include a large scatter (σ {greater than or approximate} 1.0 dex) in f effesc and/or in which star-forming galaxies only have a non-zero f effesc for a fraction of their lifetime or a fraction of sightlines. We provide a fitting formula that summarizes our findings. © The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.