SERENDIPITOUS DISCOVERY OF AN OVERDENSITY OF Lyα EMITTERS ATz∼ 4.8 IN THE CL1604 SUPERCLUSTER FIELD

Abstract

We present results of a spectroscopic search for Lyα 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 survey. A total of 12 slitmasks were observed and inspected in the Cl1604 field, spanning a survey volume of 1.365 × 10^4 comoving Mpc^3. We find a total of 17 high-redshift (4.39 ≤ z ≤ 5.67) LAE candidates down to a limiting flux of 1.9 × 10^(–18) erg s^(–1) cm^(–2) (L(Lyα) = 4.6 × 10^(41) erg s^(–1) or ~0.1 L _* 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^(–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 ~440 km s^(–1) as well as evidence for a nontrivial Lyα escape fraction. We find an average star formation rate density (SFRD) of ~5 × 10^(–3) M_☉ yr^(–1) Mpc^(–3) with moderate evidence for negative evolution in the SFRD from z ~ 4.6 to z ~ 5.7. By simulating the statistical flux loss due to our observational setup, we measure a best-fit luminosity function characterized by Φ_* L_* = 2.2^(+3.9)_(–1.3) × 10^(39) erg s^(–1) Mpc^(–3) for α = –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.