We present results of a spectroscopic search for Ly \alpha emitters ( LAEs ) in the Cl1604 supercluster field using the extensive spectroscopic Keck/DEep Imaging Multi-Object Spectrograph 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 1.365 \times 10 ^ { 4 } co-moving Mpc ^ { 3 } .
We find a total of 17 high redshift ( 4.39 \leq z \leq 5.67 ) LAE candidates down to a limiting flux of 1.9 \times 10 ^ { -18 } ergs s ^ { -1 } cm ^ { -2 } ( L ( Ly \alpha ) = 4.6 \times 10 ^ { 41 } ergs s ^ { -1 } or \sim 0.1 L _ { \ast } at z \sim 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 \sim 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 \sim 4.4 and z \sim 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 ^ { -1 } .
Additionally , we see modest evidence of a second peak in the composite spectrum , possibly caused by galactic outflows , offset from the main velocity centroid of the LAE population by \sim 440 km s ^ { -1 } as well as evidence for a non-trivial Ly \alpha escape fraction .
We find an average star formation rate density ( SFRD ) of \sim 5 \times 10 ^ { -3 } M _ { \odot } yr ^ { -1 } Mpc ^ { -3 } with moderate evidence for negative evolution in the SFRD from z \sim 4.6 to z \sim 5.7 .
By simulating the statistical flux-loss due to our observational setup we measure a best-fit luminosity function characterized by \Phi _ { \ast } L _ { \ast } = 2.2 ^ { +3.9 } _ { -1.3 } \times 10 ^ { 39 } ergs s ^ { -1 } Mpc ^ { -3 } for \alpha =-1.6 , 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 .