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Our current knowledge of cosmic star-formation history during the first two billion years (corresponding to redshift z  > 3) is mainly based on galaxies identified in rest-frame ultraviolet light 1 . However, this population of galaxies is known to under-represent the most massive galaxies, which have rich dust content and/or old stellar populations. This raises the questions of the true abundance of massive galaxies and the star-formation-rate density in the early Universe. Although several massive galaxies that are invisible in the ultraviolet have recently been confirmed at early epochs 2 – 4 , most of them are extreme starburst galaxies with star-formation rates exceeding 1,000 solar masses per year, suggesting that they are unlikely to represent the bulk population of massive galaxies. Here we report submillimetre (wavelength 870 micrometres) detections of 39 massive star-forming galaxies at z  > 3, which are unseen in the spectral region from the deepest ultraviolet to the near-infrared. With a space density of about 2 × 10 −5 per cubic megaparsec (two orders of magnitude higher than extreme starbursts 5 ) and star-formation rates of 200 solar masses per year, these galaxies represent the bulk population of massive galaxies that has been missed from previous surveys. They contribute a total star-formation-rate density ten times larger than that of equivalently massive ultraviolet-bright galaxies at z  > 3. Residing in the most massive dark matter haloes at their redshifts, they are probably the progenitors of the largest present-day galaxies in massive groups and clusters. Such a high abundance of massive and dusty galaxies in the early Universe challenges our understanding of massive-galaxy formation. Submillimetre-wavelength observations reveal a sample of galaxies that have no detectable emission in the ultraviolet-to-near-infrared region, and are probably the progenitors of the largest present-day galaxies in clusters.

Distortions of the observed cosmic microwave background provide a direct measurement of the microwave background temperature at redshifts from 0 to 1 (refs.  1 , 2 ). Some additional background temperature estimates exist at redshifts from 1.8 to 3.3 based on molecular and atomic line-excitation temperatures in quasar absorption-line systems, but are model dependent 3 . No deviations from the expected (1 +  z ) scaling behaviour of the microwave background temperature have been seen 4 , but the measurements have not extended deeply into the matter-dominated era of the Universe at redshifts z  > 3.3. Here we report observations of submillimetre line absorption from the water molecule against the cosmic microwave background at z  = 6.34 in a massive starburst galaxy, corresponding to a lookback time of 12.8 billion years (ref.  5 ). Radiative pumping of the upper level of the ground-state ortho-H 2 O(1 10 –1 01 ) line due to starburst activity in the dusty galaxy HFLS3 results in a cooling to below the redshifted microwave background temperature, after the transition is initially excited by the microwave background. This implies a microwave background temperature of 16.4–30.2 K (1 σ range) at z  = 6.34, which is consistent with a background temperature increase with redshift as expected from the standard ΛCDM cosmology 4 . Measurement of the cosmic microwave background temperature using H 2 O absorption at a redshift of 6.34 is reported, the results of which were consistent with those from standard ΛCDM cosmology.

Magnetar giant flares are rare explosive events releasing up to 10 47  erg in gamma rays in less than 1 second from young neutron stars with magnetic fields up to 10 15−16  G (refs. 1 , 2 ). Only three such flares have been seen from magnetars in our Galaxy 3 , 4 and in the Large Magellanic Cloud 5 in roughly 50 years. This small sample can be enlarged by the discovery of extragalactic events, as for a fraction of a second giant flares reach luminosities above 10 46  erg s −1 , which makes them visible up to a few tens of megaparsecs. However, at these distances they are difficult to distinguish from short gamma-ray bursts (GRBs); much more distant and energetic (10 50−53  erg) events, originating in compact binary mergers 6 . A few short GRBs have been proposed 7 – 11 , with different amounts of confidence, as candidate giant magnetar flares in nearby galaxies. Here we report observations of GRB 231115A, positionally coincident with the starburst galaxy M82 (ref. 12 ). Its spectral properties, along with the length of the burst, the limits on its X-ray and optical counterparts obtained within a few hours, and the lack of a gravitational wave signal, unambiguously qualify this burst as a giant flare from a magnetar in M82. We report observations of GRB 231115A, positionally coincident with the starburst galaxy M82, that unambiguously qualify this burst as a giant flare from a magnetar, which is a rare explosive event releasing gamma rays.

The extreme astrophysical processes and conditions that characterize the early Universe are expected to result in young galaxies that are dynamically different from those observed today 1 – 5 . This is because the strong effects associated with galaxy mergers and supernova explosions would lead to most young star-forming galaxies being dynamically hot, chaotic and strongly unstable 1 , 2 . Here we report the presence of a dynamically cold, but highly star-forming, rotating disk in a galaxy at redshift 6 z  = 4.2, when the Universe was just 1.4 billion years old. Galaxy SPT–S J041839–4751.9 is strongly gravitationally lensed by a foreground galaxy at z  = 0.263, and it is a typical dusty starburst, with global star-forming 7 and dust properties 8 that are in agreement with current numerical simulations 9 and observations 10 . Interferometric imaging at a spatial resolution of about 60 parsecs reveals a ratio of rotational to random motions of 9.7 ± 0.4, which is at least four times larger than that expected from any galaxy evolution model at this epoch 1 – 5 but similar to the ratios of spiral galaxies in the local Universe 11 . We derive a rotation curve with the typical shape of nearby massive spiral galaxies, which demonstrates that at least some young galaxies are dynamically akin to those observed in the local Universe, and only weakly affected by extreme physical processes. A strongly lensed galaxy at redshift 4.2 appears to be a dynamically cold disk galaxy, similar to spiral galaxies in the local neighbourhood and weakly affected by extreme physical processes.

Local and low-redshift ( z  < 3) galaxies are known to broadly follow a bimodal distribution: actively star-forming galaxies with relatively stable star-formation rates and passive systems. These two populations are connected by galaxies in relatively slow transition. By contrast, theory predicts that star formation was stochastic at early cosmic times and in low-mass systems 1 – 4 . These galaxies transitioned rapidly between starburst episodes and phases of suppressed star formation, potentially even causing temporary quiescence—so-called mini-quenching events 5 , 6 . However, the regime of star-formation burstiness is observationally highly unconstrained. Directly observing mini-quenched galaxies in the primordial Universe is therefore of utmost importance to constrain models of galaxy formation and transformation 7 , 8 . Early quenched galaxies have been identified out to redshift z  < 5 (refs.  9 – 12 ) and these are all found to be massive ( M ⋆  > 10 10   M ⊙ ) and relatively old. Here we report a (mini-)quenched galaxy at z  = 7.3, when the Universe was only 700 Myr old. The JWST/NIRSpec spectrum is very blue ( U – V  = 0.16 ± 0.03 mag) but exhibits a Balmer break and no nebular emission lines. The galaxy experienced a short starburst followed by rapid quenching; its stellar mass (4–6 × 10 8   M ⊙ ) falls in a range that is sensitive to various feedback mechanisms, which can result in perhaps only temporary quenching. Analysis of the JWST/NIRSpec spectrum of the recently observed Lyman-break galaxy JADES-GS+53.15508-27.80178 revealed a redshift of z  = 7.3, a Balmer break and a complete absence of nebular emission lines, indicating that quenching occurred only 700 million years after the Big Bang.

Post-starburst (or “E+A”) galaxies trace the fastest and most dramatic processes in galaxy evolution. Recent work studying the evolution of galaxies through this phase has revealed insights on how galaxies undergo structural and stellar population changes as well as the role of various feedback mechanisms. In this review, I summarize recent work on identifying post-starburst galaxies; tracing the role of this phase through cosmic time; measuring stellar populations, on-going star formation, morphologies, kinematics, interstellar medium properties, and active galactic nucleus activity; mechanisms to cause the recent starburst and its end; and the future evolution to quiescence (or not). The review concludes with a list of open questions and exciting possibilities for future facilities.

Most stars in today’s Universe reside within spheroids, which are bulges of spiral galaxies and elliptical galaxies 1 , 2 . Their formation is still an unsolved problem 3 , 4 – 5 . Infrared/submillimetre-bright galaxies at high redshifts 6 have long been suspected to be related to spheroid formation 7 , 8 , 9 , 10 , 11 – 12 . Proving this connection has been hampered so far by heavy dust obscuration when focusing on their stellar emission 13 , 14 – 15 or by methodologies and limited signal-to-noise ratios when looking at submillimetre wavelengths 16 , 17 . Here we show that spheroids are directly generated by star formation within the cores of highly luminous starburst galaxies in the distant Universe. This follows from the ALMA submillimetre surface brightness profiles, which deviate substantially from those of exponential disks, and from the skewed-high axis-ratio distribution. Most of these galaxies are fully triaxial rather than flat disks: the ratio of the shortest to the longest of their three axes is half, on average, and increases with spatial compactness. These observations, supported by simulations, reveal a cosmologically relevant pathway for in situ spheroid formation through starbursts that is probably preferentially triggered by interactions (and mergers) acting on galaxies fed by non-coplanar gas accretion streams. Deep ALMA archival observations of submillimetre-bright galaxies at high redshifts show that spheroidal bulges formed much earlier than expected and are directly generated by star formation within the cores of highly luminous starburst galaxies.

Understanding how super-massive black holes form and grow in the early Universe has become a major challenge 1 , 2 since it was discovered that luminous quasars existed only 700 million years after the Big Bang 3 , 4 . Simulations indicate an evolutionary sequence of dust-reddened quasars emerging from heavily dust-obscured starbursts that then transition to unobscured luminous quasars by expelling gas and dust 5 . Although the last phase has been identified out to a redshift of 7.6 (ref. 6 ), a transitioning quasar has not been found at similar redshifts owing to their faintness at optical and near-infrared wavelengths. Here we report observations of an ultraviolet compact object, GNz7q, associated with a dust-enshrouded starburst at a redshift of 7.1899 ± 0.0005. The host galaxy is more luminous in dust emission than any other known object at this epoch, forming 1,600 solar masses of stars per year within a central radius of 480 parsec. A red point source in the far-ultraviolet is identified in deep, high-resolution imaging and slitless spectroscopy. GNz7q is extremely faint in X-rays, which indicates the emergence of a uniquely ultraviolet compact star-forming region or a Compton-thick super-Eddington black-hole accretion disk at the dusty starburst core. In the latter case, the observed properties are consistent with predictions from cosmological simulations 7 and suggest that GNz7q is an antecedent to unobscured luminous quasars at later epochs. An unusual ultraviolet compact object associated with a dusty starburst has been observed at a redshift of about 7.2, with a luminosity that falls between that of quasars and galaxies, possibly in transition between the two. 

Far-ultraviolet observations of the nearby low-mass star-forming galaxy J0925+1403 show that the galaxy is leaking ionizing radiation with an escape fraction of about 8 per cent, which is sufficient to ionize intergalactic medium material that is about 40 times as massive as the stellar mass of the galaxy. Observation of a 'reionizing' galaxy The early Universe went through a period known as the cosmic 'Dark Ages', when matter was largely transparent to radiation and transformed to neutral gas. Later, some 800 million years after the Big Bang, it was ionized again. Which sources were responsible for this re-ionization? Low-mass, star-forming galaxies are prime candidates, but are hard to observe. Here Yuri Izotov et al . present far-ultraviolet observations of a nearby low-mass star-forming galaxy that can be considered a proxy for the reionizing galaxy population. The galaxy, J0925+1403, is leaking ionizing radiation with an escape fraction of ∼8 per cent. The total number of photons emitted during the starburst phase is sufficient to ionize intergalactic medium material that is about 40 times as massive as the stellar mass of the galaxy. One of the key questions in observational cosmology is the identification of the sources responsible for ionization of the Universe after the cosmic ‘Dark Ages’, when the baryonic matter was neutral. The currently identified distant galaxies are insufficient to fully reionize the Universe by redshift z  ≈ 6 (refs 1 , 2 , 3 ), but low-mass, star-forming galaxies are thought to be responsible for the bulk of the ionizing radiation 4 , 5 , 6 . As direct observations at high redshift are difficult for a variety of reasons, one solution is to identify local proxies of this galaxy population. Starburst galaxies at low redshifts, however, generally are opaque to Lyman continuum photons 7 , 8 , 9 . Small escape fractions of about 1 to 3 per cent, insufficient to ionize much surrounding gas, have been detected only in three low-redshift galaxies 10 , 11 . Here we report far-ultraviolet observations of the nearby low-mass star-forming galaxy J0925+1403. The galaxy is leaking ionizing radiation with an escape fraction of about 8 per cent. The total number of photons emitted during the starburst phase is sufficient to ionize intergalactic medium material that is about 40 times as massive as the stellar mass of the galaxy.

Ninety per cent of baryons are located outside galaxies, either in the circumgalactic or intergalactic medium 1 , 2 . Theory points to galactic winds as the primary source of the enriched and massive circumgalactic medium 3 – 6 . Winds from compact starbursts have been observed to flow to distances somewhat greater than ten kiloparsecs 7 – 10 , but the circumgalactic medium typically extends beyond a hundred kiloparsecs 3 , 4 . Here we report optical integral field observations of the massive but compact galaxy SDSS J211824.06+001729.4. The oxygen [O  ii ] lines at wavelengths of 3726 and 3729 angstroms reveal an ionized outflow spanning 80 by 100 square kiloparsecs, depositing metal-enriched gas at 10,000 kelvin through an hourglass-shaped nebula that resembles an evacuated and limb-brightened bipolar bubble. We also observe neutral gas phases at temperatures of less than 10,000 kelvin reaching distances of 20 kiloparsecs and velocities of around 1,500 kilometres per second. This multi-phase outflow is probably driven by bursts of star formation, consistent with theory 11 , 12 . Theory predicts that winds expel baryons from galaxies into intergalactic space; now optical observations of the massive, but compact, galaxy SDSS J211824.06+001729.4 show that it is ejecting an enormous ionized outflow of gas.