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The emission of singly ionized carbon is used to identify two galaxies with redshifts of nearly 7—corresponding to the Universe’s first billion years—and with velocity structures suggestive of rotation. Rotation in two high-redshift galaxies The forbidden emission line of singly ionized carbon [C ɪɪ] at a wavelength of 157.7 micrometres is one of the main lines for cooling gas in nearby star-forming galaxies, and has been expected, although not yet proved, to be bright in the early Universe. Renske Smit and collaborators report spectroscopic confirmation of the redshifts of two infrared-selected galaxies at redshifts of 6.85 and 6.81, using the [C ɪɪ] line. The galaxies are luminous, with velocity gradients across their surfaces. If those gradients represent rotation, then the galaxies have dynamical properties like those of Hα-bright galaxies two billion years later in the history of the Universe. The earliest galaxies are thought to have emerged during the first billion years of cosmic history, initiating the ionization of the neutral hydrogen that pervaded the Universe at this time. Studying this ‘epoch of reionization’ involves looking for the spectral signatures of ancient galaxies that are, owing to the expansion of the Universe, now very distant from Earth and therefore exhibit large redshifts. However, finding these spectral fingerprints is challenging. One spectral characteristic of ancient and distant galaxies is strong hydrogen-emission lines (known as Lyman-α lines), but the neutral intergalactic medium that was present early in the epoch of reionization scatters such Lyman-α photons. Another potential spectral identifier is the line at wavelength 157.4 micrometres of the singly ionized state of carbon (the [C ii ] λ  = 157.74 μm line), which signifies cooling gas and is expected to have been bright in the early Universe. However, so far Lyman-α-emitting galaxies from the epoch of reionization have demonstrated much fainter [C ii ] luminosities than would be expected from local scaling relations 1 , 2 , 3 , 4 , 5 , and searches for the [C ii ] line in sources without Lyman-α emission but with photometric redshifts greater than 6 (corresponding to the first billion years of the Universe) have been unsuccessful. Here we identify [C ii ] λ  = 157.74 μm emission from two sources that we selected as high-redshift candidates on the basis of near-infrared photometry; we confirm that these sources are two galaxies at redshifts of z  = 6.8540 ± 0.0003 and z  = 6.8076 ± 0.0002. Notably, the luminosity of the [C ii ] line from these galaxies is higher than that found previously in star-forming galaxies with redshifts greater than 6.5. The luminous and extended [C ii ] lines reveal clear velocity gradients that, if interpreted as rotation, would indicate that these galaxies have similar dynamic properties to the turbulent yet rotation-dominated disks that have been observed in Hα-emitting galaxies two billion years later, at ‘cosmic noon’.

The oldest known galaxy The galaxy described on page 186 may be, for the moment, the most distant and hence oldest galaxy known. Large samples of galaxies have been found at redshifts of z ∼6, but detections at earlier times tend to be uncertain and unreliable. But this 'new' old galaxy has a spectroscopic redshift of z =6.96, corresponding to just 750 million years after the Big Bang; and a Lyman-α emission line in its spectrum suggests that active star formation was under way when the Universe was only about 6% of its present age. This galaxy was detected during a survey using the Subaru Suprime-Cam on the summit of Mauna Kea. Looking at the galaxy population as a whole, the same survey produced a number density of galaxies at z ≈7 that is only 18-36% that at z =6.6. A separate search for galaxies at at z ∼7–8 using data from the Hubble Space Telescope yielded (conservatively) only one candidate galaxy, where 10 would be expected if there were no evolution in the galaxy population between z ∼7 and z ∼6. The simplest explanation for this is that the Universe is just too young to have built up many luminous galaxies at z ∼7–8 by hierarchical merging of small galaxies. A search for galaxies at z ≈7–8, roughly 700 million years from the Big Bang finds only one candidate galaxy at z ≈7–8, where ten would be expected if there were no evolution in the galaxy population between z ≈7 and z ≈6. The simplest explanation is that the Universe is just too young to have built up many luminous galaxies at z ≈7–8 by hierarchical merging of small galaxies. A search for galaxies at z ≈7–8, roughly 700 million years from the Big Bang finds only one candidate galaxy at z ≈7–8, where ten would be expected if there were no evolution in the galaxy population between z ≈7 and z ≈6. The simplest explanation is that the Universe is just too young to have built up many luminous galaxies at z ≈7–8 by hierarchical merging of small galaxies. The first 900 million years (Myr) to redshift z ≈ 6 (the first seven per cent of the age of the Universe) remains largely unexplored for the formation of galaxies. Large samples of galaxies have been found at z ≈ 6 (refs 1 –4 ) but detections at earlier times are uncertain and unreliable. It is not at all clear how galaxies built up from the first stars when the Universe was about 300 Myr old ( z ≈ 12–15) to z ≈ 6, just 600 Myr later. Here we report the results of a search for galaxies at z ≈ 7–8, about 700 Myr after the Big Bang, using the deepest near-infrared and optical images ever taken. Under conservative selection criteria we find only one candidate galaxy at z ≈ 7–8, where ten would be expected if there were no evolution in the galaxy population between z ≈ 7–8 and z ≈ 6. Using less conservative criteria, there are four candidates, where 17 would be expected with no evolution. This demonstrates that very luminous galaxies are quite rare 700 Myr after the Big Bang. The simplest explanation is that the Universe is just too young to have built up many luminous galaxies at z ≈ 7–8 by the hierarchical merging of small galaxies.

Recent James Webb Space Telescope (JWST) observations have revealed an unexpected abundance of massive-galaxy candidates in the early Universe, extending further in redshift and to lower luminosity than what had previously been found by submillimetre surveys 1 – 6 . These JWST candidates have been interpreted as challenging the Λ  cold dark-matter cosmology (where Λ is the cosmological constant) 7 – 9 , but, so far, these studies have mostly relied on only rest-frame ultraviolet data and have lacked spectroscopic confirmation of their redshifts 10 – 16 . Here we report a systematic study of 36 massive dust-obscured galaxies with spectroscopic redshifts between 5 and 9 from the JWST FRESCO survey. We find no tension with the Λ  cold dark-matter model in our sample. However, three ultra-massive galaxies (log M ★ / M ⊙  ≳ 11.0, where M ★ is the stellar mass and M ⊙ is the mass of the Sun) require an exceptional fraction of 50 per cent of baryons converted into stars—two to three times higher than the most efficient galaxies at later epochs. The contribution from an active galactic nucleus is unlikely because of their extended emission. Ultra-massive galaxies account for as much as 17 per cent of the total cosmic star-formation-rate density 17 at redshifts between about five and six. A study of 36 massive galaxies at redshifts between 5 and 9 from the JWST FRESCO survey finds that galaxy formation of the most massive galaxies is 2–3 times higher than the most efficient galaxies at later epochs.