Spectroscopic confirmation of an ultra-faint galaxy at the epoch of reionization

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Within one billion years of the Big Bang, intergalactic hydrogen was ionized by sources emitting ultraviolet and higher energy photons. This was the final phenomenon to globally affect all the baryons (visible matter) in the Universe. It is referred to as cosmic reionization and is an integral component of cosmology. It is broadly expected that intrinsically faint galaxies were the primary ionizing sources due to their abundance in this epoch1,2. However, at the highest redshifts (z > 7.5; lookback time 13.1 Gyr), all galaxies with spectroscopic confirmations to date are intrinsically bright and, therefore, not necessarily representative of the general population3. Here, we report the unequivocal spectroscopic detection of a low luminosity galaxy at z > 7.5. We detected the Lyman-α emission line at 10,504 Å in two separate observations with MOSFIRE4 on the Keck I Telescope and independently with the Hubble Space Telescope’s slitless grism spectrograph, implying a source redshift of z = 7.640 ± 0.001. The galaxy is gravitationally magnified by the massive galaxy cluster MACS J1423.8+2404 (z = 0.545), with an estimated intrinsic luminosity of MAB = −19.6 ± 0.2 mag and a stellar mass of M=3.00.8+1.5×108 solar masses. Both are an order of magnitude lower than the four other Lyman-α emitters currently known at z > 7.5, making it probably the most distant representative source of reionization found to date.

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Author information


  1. Department of Physics, University of California, Davis, California 95616, USA.

    • Austin Hoag
    • , Maruša Bradacˇ
    • , Kuang-Han Huang
    • , Brian C. Lemaux
    •  & Julie He
  2. School of Physics, University of Melbourne, Melbourne, Victoria 3010, Australia.

    • Michele Trenti
    •  & Stephanie R. Bernard
  3. Department of Physics and Astronomy, University of California, Los Angeles, California 90095, USA.

    • Tommaso Treu
    • , Louis E. Abramson
    • , Charlotte A. Mason
    •  & Takahiro Morishita
  4. Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany.

    • Kasper B. Schmidt
  5. Department of Physics, University of California, Santa Barbara, California 93106-9530, USA.

    • Charlotte A. Mason
  6. Astronomical Institute, Tohoku University, Aramaki, Aoba, Sendai 980-8578, Japan.

    • Takahiro Morishita
  7. Institute for International Advanced Research and Education, Tohoku University, Aramaki, Aoba, Sendai 980-8578, Japan.

    • Takahiro Morishita
  8. INAF Osservatorio Astronomico di Roma, Via Frascati 33, I-00040 Monteporzio (RM), Italy.

    • Laura Pentericci
  9. Argelander-Institut für Astronomie, Auf dem Hügel 71, D-53121 Bonn, Germany.

    • Tim Schrabback


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A.H. handled the MOSFIRE reduction and analysis, led the lens modelling analysis and was the principal author of the paper. M.B. and M.T. designed and planned the MOSFIRE observations, contributed to the GLASS survey and contributed to writing the paper. M.B. also carried out the 27 May 2015 MOSFIRE observations. T.T. designed the GLASS survey and contributed to the design of the MOSFIRE observations and to writing the paper. K.B.S. handled the GLASS reduction and analysis and contributed to writing the paper. K.H.H. performed the HST and Spitzer photometry, led the stellar population modelling and contributed to writing the paper. B.C.L. contributed to the MOSFIRE analysis and to writing the paper. J.H. contributed to the lens modelling analysis. S.R.B. carried out the 19 March 2016 MOSFIRE observations. L.E.A., C.A.M., T.M. and L.P. contributed to the GLASS survey and to writing the paper. T.S. contributed to the lens modelling analysis.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Austin Hoag.

Supplementary information

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    Supplementary Information

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