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Durham University

Department of Physics

Staff profile

Publication details for Prof David Alexander

Kubo, M., Uchimoto, Y.K., Yamada, T., Kajisawa, M., Ichikawa, T., Matsuda, Y., Akiyama, M., Hayashino, T., Konishi, M., Nishimura, T., Omata, K., Suzuki, R., Tanaka, I., Yoshikawa, T., Alexander, D.M., Fazio, G. G., Huang, J.-S. & Lehmer, B.D. (2013). The Formation of the Massive Galaxies in the SSA22 z = 3.1 Protocluster. The Astrophysical Journal 778(2): 170.

Author(s) from Durham


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.