  We report on a search for ultraluminous Lyman alpha emitting galaxies (LAEs)
at z=6.6 using the NB921 filter on Hyper Suprime-Cam on the Subaru telescope.
We searched a 30 degree squared area around the North Ecliptic Pole, which we
observed in broadband g', r', i', z', and y' and narrowband NB816 and NB921,
for sources with NB921 < 23.5 and z' - NB921 > 1.3. This corresponds to a
selection of log L(Ly-alpha) > 43.5 erg/s. We followed up seven candidate LAEs
(out of thirteen) with the Keck DEIMOS spectrograph and confirmed five z=6.6
LAEs, one z=6.6 AGN with a broad Ly-alpha line and a strong red continuum, and
one low-redshift ([OIII]5007) galaxy. The five ultraluminous LAEs have wider
line profiles than lower luminosity LAEs, and one source, NEPLA4, has a complex
line profile similar to that of COLA1. In combination with previous results, we
show that the line profiles of the z=6.6 ultraluminous LAEs are systematically
different than those of lower luminosity LAEs at this redshift. This result
suggests that ultraluminous LAEs generate highly ionized regions of the
intergalactic medium in their vicinity that allow the full Lyman alpha profile
of the galaxy---including any blue wings---to be visible. If this
interpretation is correct, then ultraluminous LAEs offer a unique opportunity
to determine the properties of the ionized zones around them, which will help
in understanding the ionization of the z ~ 7 intergalactic medium. A simple
calculation gives a very rough estimate of 0.015 for the escape fraction of
ionizing photons, but more sophisticated calculations are needed to fully
characterize the uncertainties.
