SPECTROSCOPIC CONFIRMATION OF A PROTOCLUSTER AT z ≈ 3.786

We present new observations of the field containing the z = 3.786 protocluster PC 217.96+ 32.3. We confirm that it is one of the largest known and most overdense high-redshift structures. Such structures are rare even in the largest cosmological simulations. We used the Mayall/MOSAIC1.1 imaging camera to image a 1 degrees.2 x 0 degrees.6 area (approximate to 150 x 75 comoving Mpc) surrounding the protocluster's core and discovered 165 candidate Ly alpha emitting galaxies (LAEs) and 788 candidate Lyman Break galaxies (LBGs). There are at least two overdense regions traced by the LAEs, the largest of which shows an areal overdensity in its core (i. e., within a radius of 2.5 comoving Mpc) of 14 +/- 7 relative to the average LAE spatial density ((rho) over bar) in the imaged field. Further, (rho) over bar is twice that derived by other field LAE surveys. Spectroscopy with Keck/DEIMOS yielded redshifts for 164 galaxies (79 LAEs and 85 LBGs); 65 lie at a redshift of 3.785 +/- 0.010. The velocity dispersion of galaxies near the core is sigma = 350 +/- 40 km s(-1), a value robust to selection effects. The overdensities are likely to collapse into systems with present-day masses of > 10(15)M(circle dot) and > 6 x 10(14)M(circle dot) The low velocity dispersion may suggest a dynamically young protocluster. We find a weak trend between narrow-band (Ly alpha) luminosity and environmental density: the Ly alpha luminosity is enhanced on average by 1.35x within the protocluster core. There is no evidence that the Ly alpha equivalent width depends on environment. These suggest that star formation and/or active galactic nucleus (AGN) activity is enhanced in the higher-density regions of the structure. PC. 217.96+ 32.3 is a Coma cluster analog, witnessed in the process of formation.