Discovery of a protocluster at z ∼ 6

Jun Toshikawa, Nobunari Kashikawa, Kazuaki Ota, Tomoki Morokuma, Takatoshi Shibuya, Masao Hayashi, Tohru Nagao, Linhua Jiang, Matthew A. Malkan, Eiichi Egami, Kazuhiro Shimasaku, Kentaro Motohara, Yoshifumi Ishizaki

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63 Scopus citations


We report the discovery of a protocluster at z∼6 containing at least eight cluster member galaxies with spectroscopic confirmations in the wide-field image of the Subaru Deep Field (SDF). The overdensity of the protocluster is significant at the 6σ level, based on the surface number density of i′-dropout galaxies. The overdense region covers 6′ × 6′ (14 Mpc × 14 Mpc in comoving units at z = 6) and includes 30 i′-dropout galaxies. Follow-up spectroscopy revealed that 15 of these are real z 6 galaxies (5.7 < z < 6.3). Of these 15, 8 are clustering in a narrow redshift range (Δz < 0.05 centered at z = 6.01), corresponding to a seven-fold increase in number density over the average in redshift space. We found no significant difference in the observed properties, such as Lyα luminosities and UV continuum magnitudes, between the eight protocluster members and the seven non-members. The velocity dispersion of the eight protocluster members is 647 ± 124 km s-1, which is about three times higher than that predicted by the standard cold dark matter model. This discrepancy could be attributed to the distinguishing three-dimensional distribution of the eight protocluster members. We discuss two possible explanations for this discrepancy: either the protocluster is already mature, with old galaxies at the center, or it is still immature and composed of three subgroups merging to become a larger cluster. In either case, this concentration of z = 6.01 galaxies in the SDF may be one of the first sites of formation of a galaxy cluster in the universe.

Original languageEnglish (US)
Article number137
JournalAstrophysical Journal
Issue number2
StatePublished - May 10 2012


  • early Universe
  • galaxies: clusters: general
  • galaxies: high-redshift
  • large-scale structure of Universe

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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