The formation of the massive galaxies in the SSA22 z = 3.1 protocluster

We study the properties of K-band-selected galaxies (K _AB < 24) in the z = 3.09 SSA22 protocluster field. 430 galaxies at 2.6 < z _phot < 3.6 are selected as potential protocluster members in a 112 arcmin² area based on their photometric redshifts. We find that ≈20% of the massive galaxies with stellar masses >10¹¹ M _⊙ at z _phot ∼ 3.1 have colors consistent with those of quiescent galaxies with ages >0.5 Gyr. This fraction increases to ≈50% after correcting for unrelated foreground/background objects. We also find that 30% of the massive galaxies are heavily reddened, dusty, star-forming galaxies. Few such quiescent galaxies at similar redshifts are seen in typical survey fields. An excess surface density of 24 μm sources at z _phot ∼ 3.1 is also observed, implying the presence of dusty star-formation activity in the protocluster. Cross-correlation with the X-ray data indicates that the fraction of K-band-selected protocluster galaxies hosting active galactic nuclei (AGNs) is also high compared with the field. The sky distribution of the quiescent galaxies, the 24 μm sources, and the X-ray AGNs show clustering around a density peak of z = 3.1 Lyα emitters. A significant fraction of the massive galaxies have already become quiescent, while dusty star-formation is still active in the SSA22 protocluster. These findings indicate that we are witnessing the formation epoch of massive early-type galaxies in the centers of the predecessors to present-day rich galaxy clusters.