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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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.
The authors declare no competing financial interests.
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.
Extended data figures and tables
The VIS and NIR arms are plotted in blue and red respectively. The best-fitting step function is plotted as a dashed line. The break in the spectrum is clearly detected with the NIR arm only. Gaps in the cumulative spectrum are due to removal of regions affected by strong sky absorption.
The slit consistently covered both the emission from the high redshift galaxy and the galaxy located 2″ below it. This spectrum uses approximately half of the total exposure time. The upper panel shows the two-dimensional rectified spectrum, the lower panel the one-dimensional spectrum of the companion. Error bars are 68% confidence. The spectrum of the companion galaxy is recovered through the entire spectral range, including that covered by the transition from the VIS to the NIR data, and shows no indication of the sharp break seen in A1689-zD1.
Extended Data Figure 3 Probability distribution as a function of redshift for galaxy template fits to the Hubble and Spitzer IRAC photometry data.
The probability distribution is based on fitting galaxies using the New-HyperZ code33.
Extended Data Figure 4 The tapered ALMA flux image at 226 GHz, centred on A1689-zD1; the image is primary-beam-corrected.
The depth of the map at the location of A1689-zD1is 0.12 mJy per beam (42% of the deepest part of the mosaic). The sensitivity decreases towards the edge of the mosaic owing to the overlap of multiple pointings and primary beam correction. The structure north of A1689-zD1 is a probable detection of a different source in the field and will be presented in a forthcoming paper (K.K. et al., manuscript in preparation).
Tracks show how the parameters change with temperature, with different tracks for different opacity wavelengths, λ0. Varying βIR is shown for λ0 = 200 µm (black and grey lines). Intrinsic (CMB-corrected) and measured temperatures are indicated for λ0 = 300 µm (orange line) on the concave and convex sides respectively. A diamond marks our fiducial model: uncorrected T = 35 K, λ0 = 200 µm, βIR = 1.92. Solid-colour regions show <90%, <95%, and <99% confidence intervals due to the βUV–IRX relation (including measurement uncertainties, βIR = 1.72–2.12, and λ0 < 300 µm), with solid, dashed, and dot-dashed lines indicating these intervals for the tracks. Dotted lines mark >99%.
Stellar synthesis models from ref. 43 (BC03) are fitted to the photometric data (squares). Error bars are 68% confidence. The best-fitting model is shown in green with the resultant fluxes in the different bands shown as circles. The Very Large Telescope (VLT)/X-shooter spectrum is also plotted (solid histogram) for comparison.
Full, self-consistent ultraviolet-to-far-infrared models are fitted to the data using the GRASIL (dashed line) and MAGPHYS (dash-dotted line) codes. The values derived from these models fitted to the photometric data (squares) are largely consistent with those derived from the modified blackbody (solid line) and ultraviolet–optical-only fit, though with an additional contribution from the restframe mid-infrared flux. A CMB correction has not been applied here. Error bars are 68% confidence. Upper limits are 68% confidence for all points except the 8.0-µm band, for which the upper limit is 95% confidence.
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Watson, D., Christensen, L., Knudsen, K. et al. A dusty, normal galaxy in the epoch of reionization. Nature 519, 327–330 (2015). https://doi.org/10.1038/nature14164
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