Abstract
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 are fainter than F =
2.0 $10^-17$ ergs/s/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 $10^-18$ ergs/s/cm$^2$ constrains the faint end slope of the
luminosity function to 1.95 < $\alpha$ < -2.35 (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%. We suggest that this bodes well
for a comparable contribution by similar, low-mass star forming galaxies at
higher-redshift -- within the reionization epoch at z > 7, only 250 Myr earlier
-- and that such systems provide a substantial, if not dominant, contribution
to the late-stage reionization of the IGM.
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