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 delta(z)(g)similar or equal to 7 and an estimated matter overdensity in real space of delta(m) similar or equal to 1.8, indicating that it will virialize by z similar to 0 with a mass scale of similar or equal to 1.4; 10(15) M-circle dot, 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 similar to 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 (> 10(11) M-circle dot), by z = 2.3.