Spectroscopic Identification of a Proto-Cluster at z=2.300: Environmental Dependence of Galaxy Properties at High Redshift

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.