We report the first direct and robust measurement of the faint-end slope of the Ly- \alpha emitter ( LAE ) luminosity function at z = 5.7 .
Candidate LAEs from a low-spectral-resolution blind search with IMACS on Magellan-Baade were targeted at higher resolution to distinguish high redshift LAEs from foreground galaxies .
All but 2 of our 42 single-emission-line systems have flux F < 2.0 \times 10 ^ { -17 } ergs s ^ { -1 } cm ^ { -2 } , making these the faintest emission-lines observed for a z = 5.7 sample with known completeness , an essential property for determining the faint end slope of the LAE luminosity function .
We find 13 LAEs as compared to 29 foreground galaxies , in very good agreement with the modeled foreground counts predicted in Dressler et al .
( 2011a ) that had been used to estimate a faint-end slope of \alpha = -2.0 for the LAE luminosity function .
A 32 % LAE fraction , LAE/ ( LAE+foreground ) , within the flux interval F = 2 - 20 \hbox { $ \times~ { } 10 ^ { -18 } $ } ergs s ^ { -1 } cm ^ { -2 } , constrains the faint end slope of the luminosity function to -2.35 < \alpha < -1.95 ( 1 \sigma ) .
We show how this steep LF should provide , to the limit of our observations , M _ { UV } \sim -16 , more than 20 % of the flux necessary to maintain ionization at z = 5.7 , with a factor-of-ten extrapolation in flux reaching more than 50 % .
This is in addition to a comparable contribution by brighter Lyman Break Galaxies M _ { UV } \mathrel { \raise 1.505 pt \hbox { $ \scriptstyle < $ } \kern - 6.0 pt \lower 1.72 pt% \hbox { { $ \scriptstyle \sim$ } } } -18 .
We suggest that this bodes well for a sufficient supply of Lyman continuum photons by similar , low-mass star forming galaxies within the reionization epoch at z \approx 7 , only 250 Myr earlier .