Spectroscopic Identification of a Protocluster at z = 2.300: Environmental Dependence of Galaxy Properties at High Redshift^*

We have discovered a highly significant overdensity 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 HS 1700+643. The structure has a redshift-space galaxy overdensity of δ ≃ 7 and an estimated matter overdensity in real space of δ_m ≃ 1.8, indicating that it will virialize by z ~ 0 with a mass scale of ≃1.4 × 10¹⁵ M_☉, that of a rich galaxy cluster. Detailed modeling of the spectral energy distribution—from the rest-frame far-UV to the rest-frame 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 protocluster 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 we show that this is consistent with simple theoretical expectations for the acceleration of structure formation in a region that is overdense on large scales by the observed amount. The protocluster environment contains a significant number of galaxies that already appear old, with large stellar masses (>10¹¹ M_☉), by z = 2.3.