Serendipitous Discovery of an Overdensity of Lyman-Alpha Emitters at
  z~4.8 in the Cl1604 Supercluster Field
  We present results of a spectroscopic search for Lyman-alpha emitters (LAEs)
in the Cl1604 supercluster field using the extensive spectroscopic Keck/DEIMOS
database taken as part of the Observations of Redshift Evolution in Large Scale
Environments (ORELSE) survey. A total of 12 slitmasks were observed and
inspected in the Cl1604 field, spanning a survey volume of 1.365x10^4 co-moving
Mpc^3. We find a total of 17 high redshift (4.39 < z < 5.67) LAE candidates
down to a limiting flux of 1.9x10^(-18) ergs/s/cm (~0.1L* at z~5), 13 of which
we classify as high quality. The resulting LAE number density is nearly double
that of LAEs found in the Subaru deep field at z~4.9 and nearly an order of
magnitude higher than in other surveys of LAEs at similar redshifts, an excess
that is essentially independent of LAE luminosity. We also report on the
discovery of two possible LAE group structures at z~4.4 and z~4.8 and
investigate the effects of cosmic variance of LAEs on our results. Fitting a
simple truncated single Gaussian model to a composite spectrum of the 13 high
quality LAE candidates, we find a best-fit stellar velocity dispersion of 136
km/s. Additionally, we see modest evidence of a second peak in the composite
spectrum, possibly caused by galactic outflows, as well as evidence for a
non-trivial Lyman-alpha escape fraction. We find an average LAE star formation
rate density (SFRD) of ~5x10^(-3) M_solar/yr/Mpc^3 with moderate evidence for
negative evolution in the LAE SFRD from z~4.6 to z~5.7. We measure a best-fit
luminosity function generally consistent with measurements from other surveys
at similar epochs. Finally, we investigate any possible effects from weak or
strong gravitational lensing induced by the foreground supercluster, finding
that our LAE candidates are minimally affected by lensing processes.
