The Evolution of the Ultraluminous Ly-alpha Luminosity Function over
  z=5.7-6.6
  Ultraluminous Lyman alpha (Lya) emitting galaxies (ULLAEs) with log L
(Lya)>43.5 erg/s near the epoch of reionization (z>5) make up the bright end of
the LAE luminosity function (LF) and may provide insight into the process of
reionization, including the formation of ionized bubbles around these extreme
systems. We present a spectroscopic LF for ULLAEs at z=5.7. We used data from
the HEROES ~45 sq. deg Subaru Hyper Suprime-Cam survey, which is centered on
the North Ecliptic Pole and has both broadband (grizY) and narrowband (NB816
and NB921) imaging, to select candidate ULLAEs based on a NB816 excess and a
strong broadband Lyman break. We spectroscopically observed 17 ULLAE candidates
with DEIMOS on Keck II. We confirmed 12 as LAEs at z=5.7, 9 of which are
ULLAEs. The remaining sources are an AGN at z=5.7, an [OIII]5007 emitter at
z=0.63, a red star, and two spectroscopic non-detections. Using the 9 confirmed
ULLAEs, we construct a ULLAE LF at z=5.7. After applying a comprehensive
incompleteness correction, we compare our new z=5.7 LF with our recent z=6.6 LF
and with other LFs from the literature to look for evolution at the
ultraluminous end. We find the overall ratio of the z=5.7 to z=6.6 ULLAE
comoving number densities to be 1.92 (+1.12, -0.71), which corresponds to a LF
offset of 0.28 (+0.20, -0.20) dex.
