The Formation of the massive galaxies in the SSA22 z=3.1 protocluster
Abstract
The properties of K-band selected galaxies (KAB<24) in the z = 3.09 SSA22 protocluster field are studied. 430 galaxies at 2.6 < zphot < 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 Msun at zphot 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 24μ m sources at zphot 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 (AGN) is also high compared with the field. The sky distribution of the quiescent galaxies, the 24μ m sources, and the X-ray AGNs show clustering around a density peak of z=3.1 Lyα emitters (LAEs). A significant fraction of the massive galaxies have already become quiescent, while the 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 at the center of predecessors to present-day rich galaxy clusters.