1. National Astronomical Observatory, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
2. Department of Astronomy, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
3. The Graduate University for Advanced Studies, Mitaka, Tokyo 181-8588, Japan
4. Subaru Telescope, 650 North A'ohoku Place, Hilo, Hawaii 96720, USA
5. Department of Astronomy, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
6. The Institute of Astronomy, University of Tokyo, Mitaka, Tokyo 181-0015, Japan
7. Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, Maryland 21218, USA

Correspondence to: Masanori Iye^1,2,3 Correspondence and requests for materials should be addressed to M.I. (Email: iye@optik.mtk.nao.ac.jp).

Abstract

When galaxy formation started in the history of the Universe remains unclear. Studies of the cosmic microwave background indicate that the Universe, after initial cooling (following the Big Bang), was reheated and reionized by hot stars in newborn galaxies at a redshift in the range 6 < z < 14 (ref. 1). Though several candidate galaxies at redshift z > 7 have been identified photometrically^{2, 3}, galaxies with spectroscopically confirmed redshifts have been confined to z < 6.6 (refs 4–8). Here we report a spectroscopic redshift of z = 6.96 (corresponding to just 750 Myr after the Big Bang) for a galaxy whose spectrum clearly shows Lyman- emission at 9,682 Å, indicating active star formation at a rate of 10M yr^-1, where M is the mass of the Sun. This demonstrates that galaxy formation was under way when the Universe was only 6 per cent of its present age. The number density of galaxies at z 7 seems to be only 18–36 per cent of the density at z = 6.6.

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