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 Ly \alpha as a physical probe of galaxies , it becomes vital to know the Ly \alpha escape fraction ( f _ { \mathrm { esc } } ^ { \mathrm { Ly } \alpha } ) .
Unfortunately , due to the resonant nature of Ly \alpha , f _ { \mathrm { esc } } ^ { \mathrm { Ly } \alpha } may vary unpredictably and requires empirical measurement .
Here we compile Ly \alpha luminosity functions between redshift z=0 and 8 and , combined with H \alpha and ultraviolet data , assess how f _ { \mathrm { esc } } ^ { \mathrm { Ly } \alpha } evolves with redshift .
We find a strong upwards evolution in f _ { \mathrm { esc } } ^ { \mathrm { Ly } \alpha } over the range z = 0.3 - 6 , which is well-fit by the power-law f _ { \mathrm { esc } } ^ { \mathrm { Ly } \alpha } \propto ( 1 + z ) ^ { \xi } with \xi = ( 2.57 _ { -0.12 } ^ { +0.19 } ) .
This predicts that f _ { \mathrm { esc } } ^ { \mathrm { Ly } \alpha } should reach unity at z = 11.1 .
By comparing f _ { \mathrm { esc } } ^ { \mathrm { Ly } \alpha } and E _ { B - V } in individual galaxies we derive an empirical relationship between f _ { \mathrm { esc } } ^ { \mathrm { Ly } \alpha } and E _ { B - V } , which includes resonance scattering and can explain the redshift evolution of f _ { \mathrm { esc } } ^ { \mathrm { Ly } \alpha } between z = 0 and 6 purely as a function of the evolution in the dust content of galaxies .
Beyond z \approx 6.5 , f _ { \mathrm { esc } } ^ { \mathrm { Ly } \alpha } 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 Ly \alpha -based tests of reionization .
We finally derive a new method by which to estimate the dust content of galaxies based purely upon the observed Ly \alpha and UV LFs .
These data are characterized by an exponential with an e-folding redshift of \approx 3.5 .