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.
This is a preview of subscription content
Subscribe to Nature+
Get immediate online access to the entire Nature family of 50+ journals
Subscribe to Journal
Get full journal access for 1 year
only $8.25 per issue
All prices are NET prices.
VAT will be added later in the checkout.
Tax calculation will be finalised during checkout.
Get time limited or full article access on ReadCube.
All prices are NET prices.
Bouwens, R. J. et al. UV luminosity functions at redshifts z~4 to z~10: 10,000 galaxies from HST legacy fields. Astrophys. J. 803, 34–49 (2015).
Smit, R. et al. Evidence for ubiquitous high-equivalent-width nebular emission in z~7 galaxies: toward a clean measurement of the specific star-formation rate using a sample of bright, magnified galaxies. Astrophys. J. 784, 58 (2014).
Tasca, L. A. M. et al. The evolving star formation rate: M⋆ relation and sSFR since z ≃ 5 from the VUDS spectroscopic survey. Astron. Astrophys. 581A, 54 (2015).
Grazian, A. et al. The galaxy stellar mass function at 3.5 ≤ z ≤ 7.5 in the CANDELS/UDS, GOODS-South, and HUDF fields. Astron. Astrophys. 575, A96 (2015).
Shibuya, T. et al. Morphologies of ∼190,000 galaxies at z = 0–10 revealed with HST legacy data. I. Size evolution. Astrophys. J. Suppl. 219, 15 (2015).
Erb, D. et al. Physical conditions in a young, unreddened, low-metallicity galaxy at high redshift. Astrophys. J. 719, 1168–1190 (2010).
Stark, D. et al. Ultraviolet emission lines in young low-mass galaxies at z ≃ 2: physical properties and implications for studies at z > 7. Mon. Not. R. Astron. Soc. 445, 3200 (2014).
Mannucci, F. et al. A fundamental relation between mass, star formation rate and metallicity in local and high-redshift galaxies. Mon. Not. R. Astron. Soc. 408, 2115–2127 (2010).
Shapley, A. et al. Rest-frame ultraviolet spectra of z~3 Lyman break galaxies. Astrophys. J. 588, 65–89 (2003).
Vanzella, E. et al. High-resolution spectroscopy of a young, low-metallicity optically thin L = 0.02L* star-forming galaxy at z = 3.12. Astrophys. J. Lett. 821, L27 (2016).
de Barros, S. et al. An extreme [O iii] emitter at z = 3.2: a low metallicity Lyman continuum source. Astron. Astrophys. 585, A51 (2016).
Vanzella, E. et al. Hubble imaging of the ionizing radiation from a star-forming galaxy at z=3.2 with fesc>50%. Astrophys. J. 825, 41 (2016).
Le Fèvre, O. et al. The VIMOS Ultra-Deep Survey: ~10 000 galaxies with spectroscopic redshifts to study galaxy assembly at early epochs 2 < z ≲ 6. Astron. Astrophys. 576, A79 (2015).
Steidel, C. C. et al. Strong nebular line ratios in the spectra of z ~ 2–3 star forming galaxies: first results from KBSS-MOSFIRE. Astrophys. J. 795, 165 (2014).
Onodera, M. et al. ISM excitation and metallicity of star-forming galaxies at z ≃ 3.3 from near-IR spectroscopy. Astrophys. J. 822, 42 (2016).
Perez-Montero, E. Deriving model-based Te-consistent chemical abundances in ionized gaseous nebulae. Mon. Not. R. Astron. Soc. 441, 2663–2675 (2014).
Pérez-Montero, E. & Amorín, R. Using photo-ionisation models to derive carbon and oxygen abundances in the rest UV. Mon. Not. R. Astron. Soc. accepted (2017).
Asplund, M. et al. The chemical composition of the sun. Ann. Rev. Astron. Astrophys. 47, 481–522 (2009).
Mattsson, L. et al. The origin of carbon: low-mass stars and an evolving, initially top-heavy IMF? Astron. Astrophys. 515, A68 (2010).
Berg, D. A. et al. Carbon and oxygen abundances in low metallicity dwarf galaxies. Astrophys. J. 827, 126 (2016).
Malhotra, S. & Rhoads, J. E. Large equivalent width Lyα line emission at z=4.5: young galaxies in a young universe? Astrophys. J. Lett. 565, 71–74 (2002).
Steidel, C. et al. Reconciling the stellar and nebular spectra of high-redshift galaxies. Astrophys. J. 826, 159 (2016).
Cassata, P. et al. He ii emitters in the VIMOS VLT Deep Survey: population III star formation or peculiar stellar populations in galaxies. Astron. Astrophys. 556, A68 (2013).
Kehrig, C. et al. The extended He ii λ4686-emitting region in IZw 18 unveiled: clues for peculiar ionizing sources. Astrophys. J. Lett. 801, 28–34 (2015).
Andrews, B.H. & Martini, P. The mass-metallicity relation with the direct method on stacked spectra of SDSS galaxies. Astrophys. J. 765, 140 (2013).
Maiolino, R., et al. AMAZE. I. The evolution of the mass-metallicity relation at z > 3. Astron. Astrophys. 488, 463 (2008).
Troncoso, P. et al. Metallicity evolution, metallicity gradients, and gas fractions at z~3.4. Astron. Astrophys. 563, A58 (2014).
Sánchez Almeida, J. et al. Localized starbursts in dwarf galaxies produced by the impact of low-metallicity cosmic gas clouds. Astrophys. J. Lett. 810, L15 (2016).
Ceverino, D. et al. Gas inflow and metallicity drops in star-forming galaxies. Mon. Not. R. Astron. Soc. 457, 2605–2612 (2016).
Bekki, K. Formation of blue compact dwarf galaxies from merging and interacting gas-rich dwarfs. Mon. Not. R. Astron. Soc. Lett. 388, L10–14 (2008).
Bournaud, F. et al. The long lives of giant clumps and the birth of outflows in gas-rich galaxies at high redshift. Astrophys. J. 780, 57 (2014).
Zanella, A. et al. An extremely young massive clump forming by gravitational collapse in a primordial Galaxy. Nature 521, 54–56 (2015).
Erb, D. Feedback in low-mass galaxies in the early Universe. Nature 523, 169–176 (2014).
Tasca, L. A. M. The VIMOS Ultra Deep Survey first data release: spectra and spectroscopic redshifts of 698 objects up to z~6 in CANDELS. Preprint at https://arxiv.org/abs/1602.01842 (2016).
Karman, W. et al. MUSE integral-field spectroscopy towards the Frontier Fields cluster Abell S1063. II. Properties of low luminosity Lyman alpha emitters at z>3. Preprint at https://arxiv.org/abs/1606.01471 (2016).
Christensen, L. et al. Gravitationally lensed galaxies at 2 < z < 3.5: direct abundance measurements of Lyα emitters. Mon. Not. R. Astron. Soc. 427, 1973–1982 (2012).
James, B. L., et al. Testing metallicity indicators at z~1.4 with the gravitationally lensed galaxy CASSOWARY 20. Mon. Not. R. Astron. Soc. 440, 1794–1809 (2014).
Bayliss, M. B., et al. The physical conditions, metallicity and metal abundance ratios in a highly magnified galaxy at z = 3.6252. Astrophys. J. 790, 144 (2014).
Garnett, D. et al. High carbon in I Zwicky 18: new results from Hubble Space Telescope spectroscopy. Astrophys. J. 481, 174–178 (1997).
Garnett, D. et al. Carbon in spiral galaxies from Hubble Space Telescope spectroscopy. Astrophys. J. 513, 168–179 (1999).
Pettini, M. et al. C, N, O abundances in the most metal-poor damped Lyman alpha systems. Mon. Not. R. Astron. Soc. 385, 2011–2024 (2008).
Jaskot, A. E. & Ravindranath, S. Photoionization models for the semi-forbidden C iii] 1909 emission in star-forming galaxies. Astrophys. J. 833, 136 (2016).
Cassata, P. et al. The VIMOS Ultra-Deep Survey (VUDS): fast increase in the fraction of strong Lyman-α emitters from z = 2 to z = 6. Astrophys. J. 573, A24 (2015).
Calzetti, D. et al. The dust content and opacity of actively star-forming galaxies. Astrophys. J. 533, 682–695 (2000).
Grogin, N. A. et al. CANDELS: the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey. Astrophys. J. Suppl. 197, 35 (2011).
Koekemoer, A. M. et al. The COSMOS survey: Hubble Space Telescope Advanced Camera for Surveys observations and data processing. Astrophys. J. Suppl. 172, 196–202 (2007).
Koekemoer, A. M. et al. CANDELS: the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey — the Hubble Space Telescope observations, imaging data products, and mosaics. Astrophys. J. Suppl. 197, 36 (2011).
Civano, F. et al. The Chandra COSMOS Legacy survey: overview and point source catalog. Astrophys. J. 819, 62 (2016).
Marchesi, S., et al. The Chandra COSMOS Legacy survey: optical/IR identifications. Astrophys. J. 817, 34 (2016).
Feltre, A., Charlot, S. & Gutkin, J. Nuclear activity versus star formation: emission-line diagnostics at ultraviolet and optical wavelengths. Mon. Not. R. Astron. Soc. 456, 3354–3374 (2016).
Gutkin, J., Charlot, S. & Bruzual, G. Modelling the nebular emission from primeval to present-day star-forming galaxies. Mon. Not. R. Astron. Soc. 462, 1757–1774 (2016).
Hainline, K. N. et al. The rest-frame ultraviolet spectra of UV-selected active galactic nuclei at z ~ 2–3. Astrophys. J. 733, 31 (2011).
Keenan, F. P., Feibelman, W. A. & Berrington, K. A. Improved calculations for the C iii 1907, 1909 and Si iii 1883, 1892 electron density sensitive emission-line ratios, and a comparison with IUE observations. Astrophys. J. 389, 443–446 (1992).
Steidel, C. et al. The structure and kinematics of the circumgalactic medium from far-ultraviolet spectra of z ~= 2–3 Galaxies. Astrophys. J. 717, 289–322 (2010).
Hashimoto, T. et al. Gas motion study of Lyα emitters at z ~ 2 using FUV and optical spectral lines, Astrophys. J. 765, 70 (2013).
Shibuya, T. et al. What is the physical origin of strong Lyα emission? II. Gas kinematics and distribution of Lyα emitters. Astrophys. J. 788, 48 (2014).
Erb, D. et al. The Lyα properties of faint galaxies at z ~ 2–3 with systemic redshifts and velocity dispersions from Keck-MOSFIRE. Astrophys. J. 795, 33 (2014).
Trainor, R. et al. The spectroscopic properties of Lyα-emitters at z ∼2.7: escaping gas and photons from faint galaxies. Astrophys. J. 809, 89 (2015).
Bradač, M. ALMA, [Cii] detection of a redshift 7 lensed galaxy behind RXJ1347.1-1145T. Astrophys. J. Lett. 836, L2 (2017).
Heckman, T. M. et al. Extreme feedback and the epoch of reionization: clues in the local universe. Astrophys. J. 730, 5 (2011).
Jaskot, A. E. & Oey, M. S. Linking Lyα and low-ionization transitions at low optical depth. Astrophys. J. Lett. 791, L19 (2014).
Henry, A. et al. Lyα emission from green peas: the role of circumgalactic gas density, covering, and kinematics. Astrophys. J. 809, 19 (2015).
Verhamme, A. et al. Using Lyman-α to detect galaxies that leak Lyman continuum. Astron. Astrophys. 578, A7 (2015).
Dijkstra, M., Gronke, M. & Venkatesan, A. The Lyα-LyC connection: evidence for an enhanced contribution of UV-faint galaxies to cosmic reionization. Astrophys. J. 828, 71 (2016).
Borthakur, S. et al. A local clue to the reionization of the universe. Science 346, 216–219 (2014).
Verhamme, A. et al. Lyman-alpha spectral properties of five newly discovered Lyman continuum emitters. Astron. Astrophys. 597, A13 (2017).
Bruzual, G. & Charlot, S. Stellar population synthesis at the resolution of 2003. Mon. Not. R. Astron. Soc. 344, 1000–1028 (2003).
Fontana, A. et al. A European Southern Observatory Very Large Telescope survey of near-infrared (Z <= 25) selected galaxies at redshifts 4.5 < z < 6: constraining the cosmic star formation rate near the reionization epoch. Astrophys. J. 587, 544–550 (2003).
Castellano, M. et al. Constraints on the star-formation rate of z ~ 3 LBGs with measured metallicity in the CANDELS GOODS-South field. Astron. Astrophys. 566, A19 (2014).
Santini, P. et al. Stellar masses from the CANDELS survey: the GOODS-South and UDS fields. Astrophys. J. 801, 97 (2015).
Laigle, C. et al. The COSMOS2015 catalog: exploring the 1<z<6 universe with half a million galaxies. Astrophys. J. Suppl. S. 224, 24 (2016).
Chabrier, G. Galactic stellar and substellar initial mass function. Public Astron. Soc. Pac. 115, 763–795 (2003).
Schaerer, D. & de Barros, S. The impact of nebular emission on the ages of z≈ 6 galaxies. Astron. Astrophys. 502, 423–426 (2009).
Schaerer, D. & Vacca, W. D. New models for Wolf-Rayet and O star populations in young starbursts. Astrophys. J. 497, 618–644 (1998).
Anders, P. & Fritze-v. Alvensleben, U. Spectral and photometric evolution of young stellar populations: the impact of gaseous emission at various metallicities. Astron. Astrophys. 401, 1063–1070 (2003).
Ilbert O. et al. Cosmos photometric redshifts with 30-bands for 2-deg2. Astrophys. J. 690, 1236–1249 (2009).
Thomas, R. et al. The VIMOS Ultra-Deep Survey (VUDS): IGM transmission towards galaxies with 2.5<z<5.5 and the colour selection of high redshift galaxies. Astron. Astrophys. 597, A88 (2017).
Hathi, N. et al. The VIMOS Ultra Deep Survey: Lyα emission and stellar populations of star-forming galaxies at 2<z<2.5. Astron. Astrophys. 588, A26 (2016).
Talia, M. et al. The star formation rate cookbook at 1 < z < 3: Extinction-corrected relations for UV and [O ii]λ3727 luminosities. Astron. Astrophys. 582, A80 (2015).
Kennicutt, R. C. Jr. Star formation in galaxies along the Hubble sequence. Ann. Rev. Astron. Astrophys. 36, 189 (1998).
Ferland, G. J. et al. The 2013 release of Cloudy. Rev. Mex. Astron. Astrof. 49, 137–163 (2013).
Luridiana, V., Morrisett, C. & Shaw, R. A. in Planetary Nebulae: An Eye to the Future, Proc. International Astronomical Union Symp. 283, 422–423 (2012).
Garnett, D. et al. The evolution of C/O in dwarf galaxies from Hubble Space Telescope FOS observations. Astrophys. J. 443, 64–76 (1995).
Villar-Martín, M., Cerviño, M. & González-Delgado, R. Nebular and stellar properties of a metal-poor H ii galaxy at z= 3.36. Mon. Not. R. Astron. Soc. 355, 1132–1142 (2004).
Ribeiro, B. et al. Size evolution of star-forming galaxies with 2 < z < 4.5 in the VIMOS Ultra-Deep Survey. Astron. Astrophys. 593, A22 (2016).
Peng, C. Y. et al. Detailed structural decomposition of galaxy images. Astron J. 124, 266–293 (2002).
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.
The authors declare no competing financial interests.
About this article
Cite this article
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
The Astronomy and Astrophysics Review (2019)
Nature Astronomy (2017)