  We have discovered a highly significant over-density of galaxies at
z=2.300+/-0.015 in the course of a redshift survey designed to select
star-forming galaxies in the redshift range z=2.3+/-0.4 in the field of the
bright z=2.72 QSO HS1700+643. The structure has a redshift-space galaxy
over-density of delta_g,z ~= 7 and an estimated matter over-density in real
space of delta_m ~= 1.8, indicating that it will virialize by z~0 with a mass
scale of ~= 1.4x10^15 M_sun, that of a rich galaxy cluster. Detailed modeling
of the spectral energy distribution -- from the rest-far-UV to the rest-near-IR
-- of the 72 spectroscopically confirmed galaxies in this field for which we
have obtained K_s and Spitzer/IRAC photometry, allows for a first direct
comparison of galaxy properties as a function of large-scale environment at
high redshift. We find that galaxies in the proto-cluster environment have mean
stellar masses and inferred ages that are ~2 times larger (at z=2.30) than
identically UV-selected galaxies outside of the structure, and show that this
is consistent with simple theoretical expectations for the acceleration of
structure formation in a region that is over-dense on large scales by the
observed amount. The proto-cluster environment contains a significant number of
galaxies that already appear old, with large stellar masses (>10^11 M_sun), by
z=2.3.

Astrophysics