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# Analogues of primeval galaxies two billion years after the Big Bang

## 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

Authors

### 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.

### Corresponding author

Correspondence to Ricardo Amorín.

## Ethics declarations

### Competing interests

The authors declare no competing financial interests.

## Supplementary information

### Supplementary Information

Supplementary Tables 1–2, Supplementary Figures 1–4 and Supplementary References. (PDF 3704 kb)

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Amorín, R., Fontana, A., Pérez-Montero, E. et al. Analogues of primeval galaxies two billion years after the Big Bang. Nat Astron 1, 0052 (2017). https://doi.org/10.1038/s41550-017-0052

• Accepted:

• Published:

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

• ### De re metallica: the cosmic chemical evolution of galaxies

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• F. Mannucci

The Astronomy and Astrophysics Review (2019)

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• Alice Shapley

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