DISCOVERY OF A PROTOCLUSTER AT z {approx} 6
- Department of Astronomy, School of Science, Graduate University for Advanced Studies, Mitaka, Tokyo 181-8588 (Japan)
- Department of Astronomy, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502 (Japan)
- Institute of Astronomy, University of Tokyo, Mitaka, Tokyo 181-0015 (Japan)
- Optical and Infrared Astronomy Division, National Astronomical Observatory, Mitaka, Tokyo 181-8588 (Japan)
- Steward Observatory, University of Arizona, 933 North Chery Avenue, Tucson, AZ 85721 (United States)
- Department of Physics and Astronomy, University of California, Los Angeles, CA 90095-1547 (United States)
- Department of Astronomy, University of Tokyo, Hongo, Tokyo 113-0033 (Japan)
We report the discovery of a protocluster at z {approx} 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 6{sigma} level, based on the surface number density of i'-dropout galaxies. The overdense region covers {approx}6' Multiplication-Sign 6' (14 Mpc Multiplication-Sign 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 {approx} 6 galaxies (5.7 < z < 6.3). Of these 15, 8 are clustering in a narrow redshift range ({Delta}z < 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 Ly{alpha} 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{sup -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.
- OSTI ID:
- 22034512
- Journal Information:
- Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 2 Vol. 750; ISSN ASJOAB; ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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