According to the current understanding of cosmic structure formation, the precursors of the most massive structures in the Universe began to form shortly after the Big Bang, in regions corresponding to the largest fluctuations in the cosmic density field1,2,3. Observing these structures during their period of active growth and assembly—the first few hundred million years of the Universe—is challenging because it requires surveys that are sensitive enough to detect the distant galaxies that act as signposts for these structures and wide enough to capture the rarest objects. As a result, very few such objects have been detected so far4,5. Here we report observations of a far-infrared-luminous object at redshift 6.900 (less than 800 million years after the Big Bang) that was discovered in a wide-field survey6. High-resolution imaging shows it to be a pair of extremely massive star-forming galaxies. The larger is forming stars at a rate of 2,900 solar masses per year, contains 270 billion solar masses of gas and 2.5 billion solar masses of dust, and is more massive than any other known object at a redshift of more than 6. Its rapid star formation is probably triggered by its companion galaxy at a projected separation of 8 kiloparsecs. This merging companion hosts 35 billion solar masses of stars and has a star-formation rate of 540 solar masses per year, but has an order of magnitude less gas and dust than its neighbour and physical conditions akin to those observed in lower-metallicity galaxies in the nearby Universe7. These objects suggest the presence of a dark-matter halo with a mass of more than 100 billion solar masses, making it among the rarest dark-matter haloes that should exist in the Universe at this epoch.
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ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada) and NSC and ASIAA (Taiwan), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. This work incorporates observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute (STScI) operated by AURA. This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. The SPT is supported by the NSF through grant PLR-1248097, with partial support through PHY-1125897, the Kavli Foundation and the Gordon and Betty Moore Foundation grant GBMF 947. Supporting observations were obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, under a cooperative agreement with the NSF on behalf of the Gemini partnership of NSF (USA), NRC (Canada), CONICYT (Chile), Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina) and Ministério da Ciência, Tecnologia e Inovação (Brazil). D.P.M., J.S.S., J.D.V., K.C.L. and J.S. acknowledge support from the US NSF under grant AST-1312950. D.P.M. was partially supported by NASA through grant HST-GO-14740 from the Space Telescope Science Institute. K.C.L. was partially supported by SOSPA4-007 from the National Radio Astronomy Observatory. The Flatiron Institute is supported by the Simons Foundation. J.D.V. acknowledges support from an A. P. Sloan Foundation Fellowship. Y.D.H. is a Hubble fellow.
The authors declare no competing financial interests.
Reviewer Information Nature thanks R. Davé and the other anonymous reviewer(s) for their contribution to the peer review of this work.
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Extended data figures and tables
a–d, Continuum images in ALMA bands 3 (a), 6 (b), 7 (c) and 8 (d), corresponding to rest-frame wavelengths of 380 μm, 160 μm, 110 μm and 90 μm, respectively. Note that the resolution in a is a factor of roughly ten worse than in b–d, and the displayed field of view is also larger by a factor of four. Contours at 10%, 30% and 90% of the image peak in band 6 are shown in a for scale. The ALMA synthesized beam (full-width at half-maximum) is represented as a hatched ellipse in the corner of each image.
8″ × 8″ thumbnails of SPT0311−58 in the observed optical and infrared filters are shown. ALMA band 6 continuum contours at 30% and 4% of the image peak are shown in blue; the ALMA synthesized beam is depicted as a blue ellipse in the corner of each image.
The field around SPT0311−58 is shown, as seen with ALMA and HST at 1.3 mm (ALMA band 6; red), 1,300 nm (combined HST/WFC3 F125W and F160W filters; green) and 700 nm (combined HST/ACS F606W and F775W filters; blue). For emission from z = 6.9, no emission should be visible in the ACS filters owing to the opacity of the neutral intergalactic medium, whereas the other filters correspond to rest-frame 160 nm and 160 μm.
Left to right, sky image, model and residual images. Top to bottom, HST/WFC3 F125W, Spitzer/IRAC 3.6 μm and Spitzer/IRAC 4.5 μm data. The ALMA band 6 contours are shown in the left and right columns; the red circles in the right column show the photometric extraction regions for the Spitzer/IRAC images.
For each continuum wavelength for which we have suitable data, we reconstruct the source-plane emission as described in Methods section ‘Gravitational lens modelling’. For each wavelength, from left to right, we show the ‘dirty’ (not de-convolved) image of the data, the dirty image of the model, the model residuals and the source-plane reconstruction. Because the images of the data are not de-convolved, the structure far from the object is due to side lobes in the synthesized beam, and should be reproduced by the models. The image-plane region modelled is evident in the residuals, and results in the ‘noise’ in the source-plane reconstructions. Contours in the residual panels are drawn in steps of ±2σ. The lensing caustics are shown in each source-plane panel (ellipse and diamond). The lens parameters are determined independently at 90 μm and 160 μm; at 110 μm we adopt the parameters of the 160-μm model.
For each channel (40 km s−1 wide), we reconstruct the source-plane emission using the lens parameters determined from fitting to the rest-frame 160-μm (ALMA band 6) continuum data (Methods section ‘Gravitational lens modelling’). The four images for each channel are as in Extended Data Fig. 5.
Extended Data Figure 7 Optical to submillimetre-wavelength SED modelling for SPT0311−58 E, SPT0311−58 W and the lens galaxy.
The photometric data in Extended Data Tables 2 and 3 for the three components at the position of SPT0311−58 are compared to the models determined using the CIGALE SED modelling code. The lens is modelled assuming a redshift of zphot = 1.43, as estimated with the photometric redshift code EAZY. Upper limits are shown at the 1σ threshold and error bars represent 1σ uncertainties.
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Marrone, D., Spilker, J., Hayward, C. et al. Galaxy growth in a massive halo in the first billion years of cosmic history. Nature 553, 51–54 (2018). https://doi.org/10.1038/nature24629
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