Spectroscopic Confirmation of a Protocluster at z=3.786
  We present new observations of the field containing the z=3.786 protocluster,
PC217.96+32.3. We confirm that it is one of the largest and most overdense
high-redshift structures known. Such structures are rare even in the largest
cosmological simulations. We used the Mayall/MOSAIC1.1 imaging camera to image
a 1.2x0.6 deg area (~150x75 comoving Mpc) surrounding the protocluster's core
and discovered 165 candidate Lyman Alpha emitting galaxies (LAEs) and 788
candidate Lyman Break galaxies (LBGs). There are at least 2 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 in the imaged field. Further, the average LAE spatial
density in the imaged field 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 350+/-40 km/s, a value robust to selection effects.
The overdensities are likely to collapse into systems with present-day masses
of >10^{15} solar masses and >6x10^{14} solar masses. The low velocity
dispersion may suggest a dynamically young protocluster. We find a weak trend
between narrow-band (Lyman Alpha) luminosity and environmental density: the
Lyman Alpha luminosity is enhanced on average by 1.35X within the protocluster
core. There is no evidence that the Lyman Alpha equivalent width depends on
environment. These suggest that star-formation and/or AGN activity is enhanced
in the higher density regions of the structure. PC217.96+32.3 is a Coma cluster
analog, witnessed in the process of formation.
