Letter | Published:

A dusty, normal galaxy in the epoch of reionization

Nature volume 519, pages 327330 (19 March 2015) | Download Citation

Abstract

Candidates for the modest galaxies that formed most of the stars in the early Universe, at redshifts z > 7, have been found in large numbers with extremely deep restframe-ultraviolet imaging1. But it has proved difficult for existing spectrographs to characterize them using their ultraviolet light2,3,4. The detailed properties of these galaxies could be measured from dust and cool gas emission at far-infrared wavelengths if the galaxies have become sufficiently enriched in dust and metals. So far, however, the most distant galaxy discovered via its ultraviolet emission and subsequently detected in dust emission is only at z = 3.2 (ref. 5), and recent results have cast doubt on whether dust and molecules can be found in typical galaxies at z ≥ 76,7,8. Here we report thermal dust emission from an archetypal early Universe star-forming galaxy, A1689-zD1. We detect its stellar continuum in spectroscopy and determine its redshift to be z = 7.5 ± 0.2 from a spectroscopic detection of the Lyman-α break. A1689-zD1 is representative of the star-forming population during the epoch of reionization9, with a total star-formation rate of about 12 solar masses per year. The galaxy is highly evolved: it has a large stellar mass and is heavily enriched in dust, with a dust-to-gas ratio close to that of the Milky Way. Dusty, evolved galaxies are thus present among the fainter star-forming population at z > 7.

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Acknowledgements

The Dark Cosmology Centre is funded by the Danish National Research Foundation. L.C. is supported by the EU under a Marie Curie Intra-European Fellowship, contract number PIEF-GA-2010-274117. K.K. acknowledges support from the Swedish Research Council and the Knut and Alice Wallenberg Foundation. J.R. acknowledges support from a European Research Council starting grant, CALENDS, and the Career Integration Grant 294074. A.G. acknowledges support from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement number 267251 (“AstroFIt”). M.J.M. acknowledges the support of the Science and Technology Facilities Council. ALMA is a partnership of the European Southern Observatory (ESO, representing its member states), the National Science Foundation (USA) and National Institutes of Natural Sciences (Japan), together with the National Research Council (Canada) and the National Science Council and the Academia Sinica Institute for Astronomy and Astrophysics (Taiwan), in cooperation with Chile. The Joint ALMA Observatory is operated by the ESO, Associated Universities Inc./National Radio Astronomy Observatory and the National Astronomical Observatory of Japan. We thank L. Lindroos, J. Hjorth, J. Fynbo, A. C. Andersen, and R. Bouwens for discussions, M. Limousin for providing a lensing map of the cluster, and the Nordic ALMA Regional Center Node for assistance.

Author information

Affiliations

  1. Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 32, København Ø, 2100, Denmark

    • Darach Watson
    • , Lise Christensen
    •  & Anna Gallazzi
  2. Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, SE-439 92 Onsala, Sweden

    • Kirsten Kraiberg Knudsen
  3. Centre de Recherche Astrophysique de Lyon, Observatoire de Lyon, Université Lyon 1, 9 Avenue Charles André, 69561 Saint Genis Laval Cedex, France

    • Johan Richard
  4. Istituto Nazionale di Astrofisica-Osservatorio Astrosico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy

    • Anna Gallazzi
  5. The Scottish Universities Physics Alliance, Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh, EH9 3HJ, UK

    • Michał Jerzy Michałowski

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Contributions

D.W. conceived the study, was Principal Investigator of the X-shooter programme, produced Fig. 1 and Extended Data Figs 1 and 4–7 and wrote the main text. L.C. reduced and analysed the X-shooter spectrum, did the HyperZ analysis and produced Fig. 2 and Extended Data Fig. 2. K.K. reduced and analysed the ALMA data and produced Fig. 3 and Extended Data Fig. 3. J.R. was Principal Investigator of the ALMA programmes and reduced and analysed the Hubble data. A.G. modelled the ultraviolet SED and determined the galaxy stellar age. M.J.M. modelled the full ultraviolet–far-infrared SED and produced Table 1. All authors contributed to the Methods and all authors discussed the results and commented on the manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Darach Watson.

This paper makes use of the following ALMA data: ADS/JAO.ALMA 2011.0.00319.S and 2012.1.00261.S available from the ALMA archive at https://almascience.eso.org/alma-data/archive.

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DOI

https://doi.org/10.1038/nature14164

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