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Normal, dust-obscured galaxies in the epoch of reionization

Abstract

Over the past decades, rest-frame ultraviolet (UV) observations have provided large samples of UV luminous galaxies at redshift (z) greater than 6 (refs. 1,2,3), during the so-called epoch of reionization. While a few of these UV-identified galaxies revealed substantial dust reservoirs4,5,6,7, very heavily dust-obscured sources at these early times have remained elusive. They are limited to a rare population of extreme starburst galaxies8,9,10,11,12 and companions of rare quasars13,14. These studies conclude that the contribution of dust-obscured galaxies to the cosmic star formation rate density at z > 6 is sub-dominant. Recent ALMA and Spitzer observations have identified a more abundant, less extreme population of obscured galaxies at z = 3−6 (refs. 15,16). However, this population has not been confirmed in the reionization epoch so far. Here, we report the discovery of two dust-obscured star-forming galaxies at z = 6.6813 ± 0.0005 and z = 7.3521 ± 0.0005. These objects are not detected in existing rest-frame UV data and were discovered only through their far-infrared [C ii] lines and dust continuum emission as companions to typical UV-luminous galaxies at the same redshift. The two galaxies exhibit lower infrared luminosities and star-formation rates than extreme starbursts, in line with typical star-forming galaxies at z ≈ 7. This population of heavily dust-obscured galaxies appears to contribute 10–25% to the z > 6 cosmic star formation rate density.

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Data availability

The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request. This paper makes use of the following ALMA data: ADS/JAO.ALMA #2019.1.01634.L.

Code availability

The codes used to reduce and analyse the ALMA data are publicly available. The code used to model the optical-to-infrared SEDs is accessible through GitHub (https://github.com/ACCarnall/bagpipes).

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Acknowledgements

The authors thank C. Williams for helpful discussions. We acknowledge support from: the Swiss National Science Foundation through the SNSF Professorship grant 190079 (Y.F., P.A.O., L.B.); NAOJ ALMA Scientific Research Grant 2020-16B (Y.F.); TOP grant TOP1.16.057 (RJB, MS); the Nederlandse Onderzoekschool voor Astronomie (S.S.); STFC Ernest Rutherford Fellowship ST/S004831/1 (R. Smit) and ST/T003596/1 (R.B.); JSPS KAKENHI JP19K23462 and JP21H01129 (HI); European Research Council’s starting grant ERC StG-717001 (P.D., A.H., G.U.); the NWO’s VIDI grant 016.vidi.189.162 and the European Commission’s and University of Groningen’s CO-FUND Rosalind Franklin program (P.D.); the Amaldi Research Center funded by the MIUR program “Dipartimento di Eccellenza” CUP:B81I18001170001 (L.G., R. Schneider); the National Science Foundation MRI-1626251 (Y.L.); FONDECYT grant 1211951, “CONICYT+PCI+INSTITUTO MAX PLANCK DE ASTRONOMIA MPG190030” and “CONICYT+PCI+REDES 190194” (M.A.); ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D) CE170100013 (E.d.C.); Australian Research Council Laureate Fellowship FL180100060 (T.N.); the ERC Advanced Grant INTERSTELLAR H2020/740120 (A.P., A.F.) and the Carl Friedrich von Siemens-Forschungspreis der Alexander von Humboldt-Stiftung Research Award (A.F.); the VIDI research program 639.042.611 (J.H.); JWST/NIRCam contract to the University of Arizona, NAS5-02015 (R.E.); ERC starting grant 851622 (IDL); the National Science Foundation under grant numbers AST-1614213, AST-1910107, and the Alexander von Humboldt Foundation through a Humboldt Research Fellowship for Experienced Researchers (D.R.). The Cosmic Dawn Center (DAWN) is funded by the Danish National Research Foundation under grant no. 140. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ.

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Authors

Contributions

Y.F. wrote the main part of the text, analysed the data and produced most of the figures. P.A.O. contributed text and led the SED fitting and data analysis. S.S. calibrated the ALMA data and produced images. M.S. performed detailed photometric measurements from the ground-based images. R.S. contributed comparison plots of different galaxy samples. All co-authors contributed to the successful execution of the ALMA program, to the scientific interpretation of the results, and helped to write up this manuscript.

Corresponding author

Correspondence to Y. Fudamoto.

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The authors declare no competing interests.

Peer review information Nature thanks Marcel Neeleman and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer review reports are available.

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Extended data figures and tables

Extended Data Fig. 1 Optical/NIR images and full SEDs of the UV-luminous targets REBELS-29 and REBELS-12.

The cutouts show images from which photometry was extracted. SED fits (bottom-right panels) are performed using the BAGPIPES33. In b and d, blue solid lines and bands represent the median posterior SEDs together with their 68% confidence contours for REBELS-29 and REBELS-12, respectively. Error bars corresponds to 1σ uncertainties, and downward arrows show 2σ upper limits. a and c show that the [C ii] 158 μm emission line redshifts (red) are in perfect agreement with the photometric redshift probability distributions (blue), that had been previously estimated from the optical/NIR photometry for both sources. This confirms their high-redshift nature.

Extended Data Fig. 2 Optical/NIR/FIR cutouts of the dusty sources REBELS-29-2 and REBELS-12-2.

$${6.5}^{{\prime\prime} }\times {6.5}^{{\prime\prime} }$$ cutouts show the existing ground- and space-based observations: Subaru Hyper Suprime Cam, VISTA VIRCAM, Spitzer IRAC, in addition to the ALMA dust continuum images and continuum subtracted [C ii] 158 μm moment-0 images. White contours show $$+2,+3,+4,+5\,\sigma$$ (solid contour) and $$-5,-4,-3,-2\,\sigma$$ (dashed contour), if present. A faint low-surface brightness foreground neighbour can be seen $$\sim {2.0}^{{\prime\prime} }$$ to the SE of REBELS-29-2. However, the photometric redshift of this foreground source is $${z}_{{\rm{ph}}}={2.46}_{-0.07}^{+0.08}$$, and the line frequency of REBELS-29-2 is not consistent with bright FIR emission lines (for example, CO lines) from this foreground redshift. No optical counterparts are found at the location of the ALMA [C ii] and dust continuum positions for both REBELS-29-2 and REBELS-12-2.

Extended Data Fig. 3 Probing a new parameter space of DSFGs.

a, The stellar mass as a function of redshift for DSFGs from the literature. IRAC-selected, H-dropout galaxies (light-grey dots with 1σ errorbars15) are generally more massive than the two serendipitously detected REBELS galaxies (red dots). Additionally, the redshifts of H-dropouts are extremely uncertain (photo-z). The extremely star-bursting SMG population only shows a small tail of rare sources at z > 4 (shown by dark dots11). The blue squares show all the previously known DSFGs at z > 5.5 with spectroscopically measured redshifts, while purple squares correspond to z ≈ 6 QSO companion galaxies13. These are more extreme sources than REBELS-12-2 and REBELS-29-2. b, The infrared luminosity/SFRIR as a function of redshift for the same galaxy samples as on the left. The infrared luminosities and hence SFRs of the newly identified galaxies are substantially lower than typical SMGs at these redshifts. For both panels, error bars correspond to 1σ uncertainties, and arrows show 2σ upper/lower limits.

Extended Data Fig. 4 Fraction of obscured star-formation as a function of stellar mass.

The fraction of obscured star-formation, $${f}_{{\rm{obs}}}={{\rm{SFR}}}_{{\rm{IR}}}/({{\rm{SFR}}}_{{\rm{IR}}}+{{\rm{SFR}}}_{{\rm{UV}}})$$, of REBELS-29-2 and REBELS-12-2 (dark coloured squares) is significantly higher than for typical LBGs at their stellar mass. The line shows the observed, constant relation between z ≈ 0 and z ≈ 2.5 (ref. 63) assuming a given set of SED templates from Bethermin and colleagues65. Blue and brown small points with error bars show stacked results of star-forming galaxies at z ≈ 4.5 and at z ≈ 5.5, respectively64. The star-formation of extreme starburst galaxies at z ≈ 5.7–6.9 is essentially 100% obscured (SMGs;12 green small points). The highly obscured star-forming galaxies found as companions of high-redshift quasars at z > 6 (refs. 13,14) (yellow diamonds) are substantially more massive than the galaxies identified here, as estimated from their dynamical masses. Squares show the obscured fraction of our UV-bright and dusty galaxies. Error bars correspond to 1σ uncertainty, and arrows show 2σ lower/upper limits. Our discovery of lower mass, obscured galaxies shows that fobs is likely to vary much more strongly at a fixed stellar mass than previously estimated even in the epoch of reionization.

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Fudamoto, Y., Oesch, P.A., Schouws, S. et al. Normal, dust-obscured galaxies in the epoch of reionization. Nature 597, 489–492 (2021). https://doi.org/10.1038/s41586-021-03846-z

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• DOI: https://doi.org/10.1038/s41586-021-03846-z

• A dusty compact object bridging galaxies and quasars at cosmic dawn

• S. Fujimoto
• G. B. Brammer
• P. A. Oesch

Nature (2022)