Confirmation of a Steep Luminosity Function for Lyman-alpha Emitters at
  z = 5.7: A Major Component of Reionization
  We report the first direct and robust measurement of the faint-end slope of
the Lyman-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 = $2-20 \times 10^{-18}$ 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, 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 55%. This
is in addition to a comparable contribution from Lyman Break Galaxies M$_{UV}
\le$ -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.
