On the redshift-evolution of the Lyman-alpha escape fraction and the
  dust content of galaxies
  The Ly-alpha emission line has been proven a powerful tool by which to study
evolving galaxies at the highest redshifts. However, in order to use Lya as a
physical probe of galaxies, it becomes vital to know the Lya escape fraction
(fescLya). Unfortunately, due to the resonant nature of Lya, fescLya may vary
unpredictably and requires empirical measurement. Here we compile Lya
luminosity functions between redshift z=0 and 8 and, combined with H-alpha and
ultraviolet data, assess how fescLya evolves with redshift. We find a strong
upwards evolution in fescLya over the range z=0.3-6, which is well-fit by the
power-law fescLya \propto (1+z)^\xi, with \xi =(2.57_-0.12^+0.19). This
predicts that fescLya should reach unity at z=11.1. By comparing fescLya and
E(B-V) in individual galaxies we derive an empirical relationship between
fescLya and E(B-V), which includes resonance scattering and can explain the
redshift evolution of fescLya between z=0 and 6 purely as a function of the
evolution in the dust content of galaxies. Beyond z~6.5, fescLya drops more
substantially; an effect attributed to either ionizing photon leakage, or an
increase in the neutral gas fraction of the intergalactic medium. While
distinguishing between those two scenarios may be extremely challenging, by
framing the problem this way we remove the uncertainty of the halo mass from
Lya-based tests of reionization. We finally derive a new method by which to
estimate the dust content of galaxies based purely upon the observed Lya and UV
LFs. These data are characterized by an exponential with an e-folding redshift
of ~3.5.
