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Massive galaxy clusters have been found that date to times as early as three billion years after the Big Bang, containing stars that formed at even earlier epochs1,2,3. The high-redshift progenitors of these galaxy clusters—termed ‘protoclusters’—can be identified in cosmological simulations that have the highest overdensities (greater-than-average densities) of dark matter4,5,6. Protoclusters are expected to contain extremely massive galaxies that can be observed as luminous starbursts7. However, recent detections of possible protoclusters hosting such starbursts8,9,10,11 do not support the kind of rapid cluster-core formation expected from simulations12: the structures observed contain only a handful of starbursting galaxies spread throughout a broad region, with poor evidence for eventual collapse into a protocluster. Here we report observations of carbon monoxide and ionized carbon emission from the source SPT2349-56. We find that this source consists of at least 14 gas-rich galaxies, all lying at redshifts of 4.31. We demonstrate that each of these galaxies is forming stars between 50 and 1,000 times more quickly than our own Milky Way, and that all are located within a projected region that is only around 130 kiloparsecs in diameter. This galaxy surface density is more than ten times the average blank-field value (integrated over all redshifts), and more than 1,000 times the average field volume density. The velocity dispersion (approximately 410 kilometres per second) of these galaxies and the enormous gas and star-formation densities suggest that this system represents the core of a cluster of galaxies that was already at an advanced stage of formation when the Universe was only 1.4 billion years old. A comparison with other known protoclusters at high redshifts shows that SPT2349-56 could be building one of the most massive structures in the Universe today.

Two extremely massive galaxies are seen 800 million years after the Big Bang, showing the rapid growth of early structure and marking the most massive halo known in that era. A massive galaxy in the early Universe The most massive galaxies in the early Universe were very rare objects and observing them during their growing stage is a challenge. Daniel Marrone and collaborators report observations of one of them less than 800 million years after the Universe began, which high-resolution imaging reveals to in fact be a closely interacting pair of galaxies. The larger one is forming stars at a rate of 2,900 solar masses per year, and contains 270 billion solar masses of gas. The rapid star formation was probably triggered by the interaction with its close companion, whose properties are closer to those of galaxies observed in the nearby Universe. 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 field 1 , 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 far 4 , 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 survey 6 . 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 Universe 7 . 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.

Strong gravitational magnification enables the detection of faint background sources and allows researchers to resolve their internal structures and even identify individual stars in distant galaxies. Highly magnified individual stars are useful in various applications, including studies of stellar populations in distant galaxies and constraining dark matter structures in the lensing plane. However, these applications have been hampered by the small number of individual stars observed, as typically one or a few stars are identified from each distant galaxy. Here, we report the discovery of more than 40 microlensed stars in a single galaxy behind Abell 370 at redshift of 0.725 (dubbed ‘the Dragon arc’) when the Universe was half of its current age, using James Webb Space Telescope observations with the time-domain technique. These events were found near the expected lensing critical curves, suggesting that these are magnified stars that appear as transients from intracluster stellar microlenses. Through multi-wavelength photometry, we constrained their stellar types and found that many of them are consistent with red giants or supergiants magnified by factors of hundreds. This finding reveals a high occurrence of microlensing events in the Dragon arc and demonstrates that time-domain observations by the James Webb Space Telescope could lead to the possibility of conducting statistical studies of high-redshift stars. Using JWST, more than 40 individual stars have been detected in a distant galaxy, dating back to when the Universe was only half of its current age. The stars appear to be red (super)giants that are magnified by factors of hundreds.

The cosmic evolution of obscured star formation, dust properties and production mechanisms, and the prevalence of dust-obscured AGN out to high redshifts are currently some of the hot topics in astrophysics. While much progress has been made in the early days with Spitzer and Herschel, these facilities have not reached the necessary depths to observe the mid-IR light of high-redshift (z > 3) galaxies. Recently, the James Webb Space Telescope (JWST) has filled in the blue side of the rest-frame mid-IR. The Atacama Large (Sub)Millimeter Array (ALMA), on the other hand, provides excellent sensitivity in the far-IR regime, allowing the study of dust and gas properties at high redshifts. Filling the wavelength gap between JWST and ALMA is crucial to progress our understanding of early galaxy evolution - and this will be an important goal in the next decades. The Probe far-IR Mission for Astrophysics (PRIMA), with sensitive imaging and spectroscopic capabilities at 24-240\\(\\mu\\)m and currently in Phase A study, will achieve this and provide insights into early galaxy evolution, Black Hole growth, and dust production mechanisms. Here we present PRIDES, a possible deep and wide-area survey over 1.6 square-degrees of the COSMOS field with PRIMA to study these science cases.

A commonly employed method to detect protoclusters in the young universe is the search for overdensities of massive star forming galaxies, such as submillimeter galaxies (SMGs), around high-mass halos, including those hosting quasars. In this work, we study the Megaparsec environment surrounding nine physically associated quasar pairs between \\(z=2.45\\) and \\(z=3.82\\) with JCMT/SCUBA-2 observations at 450 \\(\\)m and 850 \\(\\)m covering a field of view of roughly 13.7 arcmin in diameter (or 32 Mpc\\(^2\\) at the median redshift) for each system. We identify a total of 170 SMG candidates and 26 non-SMG and interloper candidates. A comparison of the underlying 850 \\(\\)m source models recovered with Monte Carlo simulations to the blank field model reveals galaxy overdensities in all fields, with a weighted average overdensity factor of \\(_ cumul = 3.4 0.3\\). From this excess emission at 850 \\(\\)m, we calculate a star formation rate density of \\(1700 100\\) M\\(_\\) yr\\(^-1\\) Mpc\\(^-3\\), consistent with predictions from protocluster simulations and observations. Compared to fields around single quasars, those surrounding quasar pairs have higher excess counts and more centrally peaked star formation, further highlighting the co-evolution of SMGs and quasars. We do not find preferential alignment of the SMGs with the quasar pair direction or their associated Ly\\(\\) nebulae, indicating that cosmic web filaments on different scales might be traced by the different directions. Overall, this work substantiates the reliability of quasar pairs to detect overdensities of massive galaxies and likely sites of protocluster formation. Future spectroscopic follow-up observations are needed to confirm membership of the SMG candidates with the physically associated quasar pairs and definitively identify the targeted fields as protoclusters.

We present an analysis of the JWST NIRCam and MIRI morphological properties of 80 massive (\\(\\log_{10}(M_\\ast[M_{\\odot}])\\)=11.2\\(\\pm\\)0.1) dusty star-forming galaxies at $z$$=\\(2.7\\)^{+1.2}_{-0.7}\\(, identified as sub-millimetre galaxies (SMGs) by ALMA, that have been observed as part of the JWST PRIMER project. To compare the structure of these massive, active galaxies to more typical less actively star-forming galaxies, we define two comparison samples. The first of 850 field galaxies matched in specific star-formation rate and redshift and the second of 80 field galaxies matched in stellar mass. We identify 20\\)\\pm\\(5% of the SMGs as candidate late-stage major mergers, a further 40\\)\\pm\\(10% as potential minor mergers and 40\\)\\pm\\(10% which have comparatively undisturbed disk-like morphologies, with no obvious massive neighbours. These rates are comparable to those for the field samples and indicate that the majority of the sub-millimetre-detected galaxies are not late-stage major mergers, but have interaction rates similar to the less-active population at \\)z$$\\sim\\(2-3. We establish that SMGs have comparable near-infrared sizes to the less active populations, but exhibit lower Sérsic indices, consistent with bulge-less disks and have more structured morphologies at 2\\)\\mu\\(m relative to 4\\)\\mu\\(m. We find evidence for dust reddening as the origin of the morphological differences between the populations, identifying a strong correlation between the F200W\\)-\\(F444W pixel colour and the 870\\)\\mu$m surface brightness. We conclude that SMGs and less active galaxies at the same epochs share a common disk-like structure, but the weaker bulge components of the SMGs results in a lower dynamical stability. Consequently, instabilities triggered either secularly or by minor external perturbations result in higher levels of activity (and dust content) in SMGs compared to typical star-forming galaxies. [Abridged]

Observing galaxies in the radio and submillimeter continuum has the advantage of being unaffected by dust extinction, which is a major drawback of studying galaxy evolution using optical data. Submillimeter single-dish surveys have made tremendous progress in understanding the high-redshift dusty population, but the low angular resolution of single-dish telescopes has also hampered these studies. Our recent JCMT and SMA imaging of high-redshift submillimeter sources revealed z > 4 objects that are radio and optically faint. Such objects cannot be easily identified with the combination of submillimeter single-dish and radio imaging. We also found a large fraction of multiple objects that are blended in single-dish images. Such objects may be early-stage mergers, or dusty starbursts in group environments. Since our work, larger surveys with PdBI and ALMA have been carried out to further address these issues. Additional to submillimeter imaging, future ultradeep EVLA imaging at 20 cm can also detect large samples of ultraluminous star forming galaxies at z ≳ 2. Sensitivities in radio and submillimeter observations have different redshift and dust temperature dependencies. Radio observations are also less affected by confusion. It will be necessary to combine deep surveys in both wavebands in order to achieve a more complete picture of the evolution of high-redshift star forming galaxies.

We report physical properties of the brightest (\\(S_{870\\,\\mu \\rm m}=12.4\\)-\\(19.2\\,\\)mJy) and not strongly lensed 18 870\\(\\,\\mu\\)m selected dusty star-forming galaxies (DSFGs), also known as submillimeter galaxies (SMGs), in the COSMOS field. This sample is part of an ALMA band\\(\\,\\)3 spectroscopic survey (AS2COSPEC), and spectroscopic redshifts are measured in 17 of them at \\(z=2\\)-\\(5\\). We perform spectral energy distribution analyses and deduce a median total infrared luminosity of \\(L_{\\rm IR}=(1.3\\pm0.1)\\times10^{13}\\,L_{\\odot}\\), infrared-based star-formation rate of \\({\\rm SFR}_{\\rm IR}=1390\\pm150~M_{\\odot}\\,\\rm yr^{-1}\\), stellar mass of \\(M_\\ast=(1.4\\pm0.6)\\times10^{11}\\,M_\\odot\\), dust mass of \\(M_{\\rm dust}=(3.7\\pm0.5)\\times10^9\\,M_\\odot\\), and molecular gas mass of \\(M_{\\rm gas}= (\\alpha_{\\rm CO}/0.8)(1.2\\pm0.1)\\times10^{11}\\,M_\\odot\\), suggesting that they are one of the most massive, ISM-enriched, and actively star-forming systems at \\(z=2\\)-\\(5\\). In addition, compared to less massive and less active galaxies at similar epochs, SMGs have comparable gas fractions; however, they have much shorter depletion time, possibly caused by more active dynamical interactions. We determine a median dust emissivity index of \\(\\beta=2.1\\pm0.1\\) for our sample, and by combining our results with those from other DSFG samples, we find no correlation of \\(\\beta\\) with redshift or infrared luminosity, indicating similar dust grain compositions across cosmic time for infrared luminous galaxies. We also find that AS2COSPEC SMGs have one of the highest dust-to-stellar mass ratios, with a median of \\(0.02\\pm0.01\\), significantly higher than model predictions, possibly due to too strong of a AGN feedback implemented in the model. Finally, our complete and uniform survey enables us to put constraints on the most massive end of the dust and molecular gas mass functions.

Bright galaxies at sub-millimetre wavelengths from Herschel are now well known to be predominantly strongly gravitationally lensed. The same models that successfully predicted this strongly lensed population also predict about one percent of faint \\(450\\)m-selected galaxies from deep James Clerk Maxwell Telescope (JCMT) surveys will also be strongly lensed. Follow-up ALMA campaigns have so far found one potential lens candidate, but without clear compelling evidence e.g. from lensing arcs. Here we report the discovery of a compelling gravitational lens system confirming the lensing population predictions, with a \\(z_s = 3.4 0.4\\) submm source lensed by a \\(z_spec = 0.360\\) foreground galaxy within the COSMOS field, identified through public JWST imaging of a \\(450\\)m source in the SCUBA-2 Ultra Deep Imaging EAO Survey (STUDIES) catalogue. These systems will typically be well within the detectable range of future wide-field surveys such as Euclid and Roman, and since sub-millimetre galaxies are predominantly very red at optical/near-infrared wavelengths, they will tend to appear in near-infrared channels only. Extrapolating to the Euclid-Wide survey, we predict tens of thousands of strongly lensed near-infrared galaxies. This will be transformative for the study of dusty star-forming galaxies at cosmic noon, but will be a contaminant population in searches for strongly lensed ultra-high-redshift galaxies in Euclid and Roman.

Optically dark dusty star-forming galaxies (DSFGs) play an essential role in massive galaxy formation at early cosmic time, however their nature remains elusive. Here we present a detailed case study of all the baryonic components of a \\(z=4.821\\) DSFG, XS55. Selected from the ultra-deep COSMOS-XS 3GHz map with a red SCUBA-2 450\\(\\mu\\)m/850\\(\\mu\\)m colour, XS55 was followed up with ALMA 3mm line scans and spectroscopically confirmed to be at \\(z=4.821\\) via detections of the CO(5-4) and [CI](1-0) lines. JWST/NIRCam imaging reveals that XS55 is a F150W-dropout with red F277W/F444W colour, and a complex morphology: a compact central component embedded in an extended structure with a likely companion. XS55 is tentatively detected in X-rays with both Chandra and XMM-Newton, suggesting an active galactic nucleus (AGN) nature. By fitting a panchromatic SED spanning NIR to radio wavelengths, we revealed that XS55 is a massive main-sequence galaxy with a stellar mass of \\(M_\\ast=(5\\pm1)\\times10^{10}\\,{\\rm M_\\odot}\\) and a star formation rate of \\({\\rm SFR}=540\\pm177~{\\rm M_\\odot\\,yr^{-1}}\\). The dust of XS55 is optically thick in the far infrared (FIR) with a surprisingly cold dust temperature of \\(T_{\\rm dust}=33\\pm2\\,{\\rm K}\\), making XS55 one of the coldest DSFGs at \\(z>4\\) known to date. This work unveils the nature of a radio-selected F150W-dropout, suggesting the existence of a population of DSFGs hosting active black holes embedded in optically thick dust.