Evolution in the Escape Fraction of Ionizing Photons and the Decline in
  Strong Lya Emission from z>6 Galaxies
  The rapid decline in the number of strong Lyman Alpha (Lya) emitting galaxies
at z > 6 provides evidence for neutral hydrogen in the IGM, but is difficult to
explain with plausible models for reionization. We demonstrate that the
observed reduction in Lya flux from galaxies at z > 6 can be explained by
evolution in the escape fraction of ionizing photons, f_esc. We find that the
median observed drop in the fraction of galaxies showing strong Lya emission,
as well as the observed evolution of the Lya luminosity function both follow
from a small increase in f_esc of Delta f_esc ~ 0.1 from f_esc ~ 0.6 at z ~ 6.
This high escape fraction may be at odds with current constraints on the
ionising photon escape fraction, which favor smaller values of f_esc < 20%.
However, models that invoke a redshift evolution of f_ esc that is consistent
with these constraints can suppress the z~7 Lya flux to the observed level, if
they also include a small evolution in global neutral fraction of Delta x_HI ~
0.2. Thus, an evolving escape fraction of ionising photons can be a plausible
part of the explanation for evolution in the Lya emission of high redshift
galaxies. More generally, our analysis also shows that the drop in the Lya
fraction is quantitatively consistent with the observed evolution in the Lya
luminosity functions of Lya Emitters.
