• A Corrigendum to this article was published on 13 March 2017

Abstract

Deep observations are revealing a growing number of young galaxies in the first billion years of cosmic time1. Compared to typical galaxies at later times, they show more extreme emission-line properties2, higher star formation rates3, lower masses4, and smaller sizes5. However, their faintness precludes studies of their chemical abundances and ionization conditions, strongly limiting our understanding of the physics driving early galaxy build-up and metal enrichment. Here we study a rare population of ultraviolet-selected, low-luminosity galaxies at redshift 2.4 < z < 3.5 that exhibit all the rest-frame properties expected from primeval galaxies. These low-mass, highly compact systems are rapidly forming galaxies able to double their stellar mass in only a few tens of millions of years. They are characterized by very blue ultraviolet spectra with weak absorption features and bright nebular emission lines, which imply hard radiation fields from young hot massive stars6,7. Their highly ionized gas phase has strongly sub-solar carbon and oxygen abundances, with metallicities more than a factor of two lower than that found in typical galaxies of similar mass and star formation rate at z≤2.58. These young galaxies reveal an early and short stage in the assembly of their galactic structures and their chemical evolution, a vigorous phase that is likely to be dominated by the effects of gas-rich mergers, accretion of metal-poor gas and strong outflows.

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Acknowledgements

This work is supported by funding from the European Research Council Advanced Grant ERC-2010-AdG-268107-EARLY and by INAF Grants PRIN 2010, PRIN 2012 and PICS 2013.This work is based on data products made available at the CESAM data center, Laboratoire d’Astrophysique de Marseille, France. This research leading to these results has received funding from the European Union Seventh Framework Programme ASTRODEEP (FP7 2007/2013) under grant agreement no. 312725. R.A. acknowledges support from the ERC Advanced Grant 695671 ‘QUENCH’. E.P.M. acknowledges support from Spanish MICINN grants AYA2010-21887-C04-01 and AYA2013-47742-C4-1-P. We thank V. Sommariva for her contribution to the initial steps of this work.

Author information

Affiliations

  1. INAF-Osservatorio Astronomico di Roma, Via Frascati 33, I-00078, Monte Porzio Catone, Italy

    • Ricardo Amorín
    • , Adriano Fontana
    • , Marco Castellano
    • , Lucia Guaita
    • , Andrea Grazian
    • , Laura Pentericci
    •  & Emiliano Merlin
  2. Cavendish Laboratory, University of Cambridge, 19 J.J. Thomson Ave., Cambridge CB30HE, UK

    • Ricardo Amorín
  3. Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB30HA, UK

    • Ricardo Amorín
  4. Instituto de Astrofísica de Andalucía, CSIC, E-18008 Granada, Spain

    • Enrique Pérez-Montero
  5. Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France

    • Olivier Le Fèvre
    • , Bruno Ribeiro
    • , Lidia A.M. Tasca
    • , Nimish Hathi
    • , Vincent Le Brun
    • , Janine Pforr
    •  & Laurence Tresse
  6. Geneva Observatory, University of Geneva, ch. des Maillettes 51, 1290 Versoix, Switzerland

    • Daniel Schaerer
    •  & Stephane de Barros
  7. Institut de Recherche en Astrophysique et Planétologie – IRAP, CNRS, Université de Toulouse, UPS-OMP, 14, Avenue E. Belin, 31400 Toulouse, France

    • Daniel Schaerer
    •  & Thierry Contini
  8. Instituto de Física y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Gran Bretaña 1111, Playa Ancha, Valparaíso, Chile

    • Romain Thomas
    •  & Paolo Cassata
  9. INAF–Osservatorio Astronomico di Bologna, via Ranzani, 1 - 40127, Bologna, Italy

    • Sandro Bardelli
    • , Stephane de Barros
    • , Eros Vanzella
    • , Daniela Vergani
    • , Giovanni Zamorani
    •  & Elena Zucca
  10. INAF–IASF, via Bassini 15, 20133 Milano, Italy

    • Letizia Cassarà
    • , Bianca Garilli
    •  & Dario Maccagni
  11. Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing, National Observatory of Athens, Penteli, 15236, Athens, Greece

    • Letizia Cassarà
  12. University of Bologna, Department of Physics and Astronomy (DIFA), V.le Berti Pichat, 6/2 – 40127 Bologna, Italy

    • Andrea Cimatti
    •  & Margherita Talia
  13. Astronomy Department, University of Massachusetts, Amherst, Massachusetts 01003, USA

    • Mauro Giavalisco
  14. Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, Maryland 21218, USA

    • Nimish Hathi
    •  & Anton Koekemoer
  15. Department of Physics, University of California, Davis, One Shields Ave., Davis, California 95616, USA

    • Brian C. Lemaux
  16. INAF–IASF Bologna, via Gobetti 101, 40129 Bologna, Italy

    • Daniela Vergani

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Contributions

R.A. discovered the described objects and analysed the spectroscopic data, contributed to designing the methodology for abundance determinations, and wrote the manuscript. A.F. contributed to the photometric analysis and assisted in writing the manuscript. E.P.M. contributed to designing the methodology and wrote the code for abundance determination. L.G. performed the stacking analysis. M.C., A.G., O.LF., B.R., D.S., L.A.M.T., R.T., S.B., L.C., A.C., T.C., S.DB., B.G., M.G., N.H., A.K., V.LB., B.C.L., D.M., L.P., J.P., M.T., L.T., E.V, D.V., G.Z. and E.Z., contributed to the data survey design, observations and data reduction and redshift measurements. B.R. also produced Fig. 2. E.M. assisted the photometric analysis. All the authors contributed to the interpretation of the data and provided comments and corrections on the original manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Ricardo Amorín.

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https://doi.org/10.1038/s41550-017-0052