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.
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This paper makes use of the following ALMA data (http://www.almaobservatory.org/en/home/): ADS/JAO.ALMA#2016.1.00236.T and ADS/JAO.ALMA#2015.1.01543.T. ALMA is a partnership of the European Southern Observatory (ESO, representing its member states), the National Science Foundation (NSF, USA) and the National Institute of Natural Sciences (NINS, Japan), together with the National Research Council (NRC, Canada) and the National Security Council (NSC) and the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA, Taiwan), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, Associated Universities Inc. (AUI)/National Radio Astronomy Observatory (NRAO) and the National Astronomical Observatory of Japan (NAOJ). This work is also 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 grant PHY-1125897, the Kavli Foundation and the Gordon and Betty Moore Foundation grant GBMF 947. This publication is based on data acquired with the Atacama Pathfinder Experiment (APEX) under programmes E-299.A-5045A-2017 and ID M-091.F-0031-2013. APEX is a collaboration between the Max-Planck-Institut für Radioastronomie, the ESO, and the Onsala Space Observatory. Supporting observations were obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the NSF (USA), the NRC (Canada), Comisión Nacional de Investigación Científica y Tecnológica (CONICYT, Chile), Ministerio de Ciencia, Tecnologa e Innovacion Productiva (Argentina), and Ministerio da Ciencia, Tecnologia e Inovacao (Brazil). The Australia Telescope Compact Array (ATCA) is part of the Australia Telescope National Facility which is funded by the Australian Government for operation as a National Facility managed by the Commonwealth Scientific and Industrial Research Organisation (CSIRO). 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. S.C.C., T.B.M. and A.B. acknowledge support from the National Sciences and Engineering Research Council (NSERC). S.C.C. and T.B.M. acknowledge the Canada Foundation for Innovation (CFI) and the Killam trust. M.A. acknowledges partial support from the Fondo Nacional de Desarrollo Científica y Tecnológico (FONDECYT, Chile) through grant 114009. The Flatiron Institute is supported by the Simons Foundation. J.D.V. acknowledges support from an A.P. Sloan Foundation Fellowship.Reviewer information
Nature thanks P. Capak and C. Papovich for their contribution to the peer review of this work.
Extended data figures and tables
Circles show the locations of the 14 sources detected in ALMA band 7 (see Methods). Nine of the 14 ALMA sources are detected in the IRAC bands with at least 3σ confidence, including the two faintest [C ii] sources from the blind line survey.
An RGB scale is used to represent sources selected at wavelengths of 500 μm (red), 350 μm (green) and 250 μm (blue), with the red SPT2349-56 extended complex clearly visible in a relative void in the foreground z ≈ 1 cosmic infrared background (blue to green coloured galaxies).
A wide-field LABOCA image (21″ beam size; white circle) of SPT2349-56. The image r.m.s. noise is 1.3 mJy at the centre of region shown to the right, increasing to 2 mJy at the edges of this region. The total flux density recovered is 110.0 ± 9.5 mJy. Subregions are drawn with black outlines, showing three different regions and their recovered flux densities. Grey contours start at 3σ and increase in steps of 3σ. SPT 1.4-mm contours are also shown (blue), revealing that even with the 1′ beam of SPT, SPT2349-56 is resolved. One additional submillimetre source is detected at > 5σ in the LABOCA image to the east (left) of the primary source, though Herschel–SPIRE photometry indicates that it is unlikely to be at z ≈4.
a, The colour map and red contours trace the CO(2–1) line integrated over the central 830 km s−1, with the contours spaced by 2σ, starting at 2σ. The grey contours show the 1.1-mm ALMA continuum detections. N, north; W, west; C, centre. Black circles show the location of the 14 [C ii]/continuum sources identified with ALMA. b, One-dimensional spectra extracted at the positions indicated with blue crosses in panel a.
a, A deep SPIRE false-colour image is shown with LABOCA contours overlaid. Locations of the 250-μm peaks used for analysis are marked with crosses (the faintest are not visible because of the contrast adopted in the image). The turquoise, blue, green and white crosses relate to the turquoise, blue, green and black lines used in panels b and c. b, c, Colour–colour (b) and colour–flux (c) diagrams for the 250-μm sources. Error bars represent 1σ standard deviation. The colour–colour diagram shows sources with SNR(250 μm) ratios of more than 3 and is dominated by the z ≈ 1 cosmic infrared background in the foreground of SPT2349-56 (sources with colours ranging from blue to green). The colour–flux diagram applies an additional cut for SNR(500 μm) ratios of more than 3. The colour–colour and colour–flux diagrams show that one of three peaks associated with SPT2349-56 is probably a lower-redshift interloper (green symbol), but also that there are five additional sources (blue symbols) in the surrounding region with colours (S500μm > S350μm > S250μm) that are suggestive of z = 4.3.
The SED of the extended SPT2349-56 source is shown, including the summed deconvolved Herschel–SPIRE flux densities, the total 870-μm LABOCA flux density, and the summed IRAC flux densities. Error bars represent 1σ measurement errors. We do not include the SPT 1.4-mm, 2.0-mm and 3.0-mm points because the source is elongated and flux measurements are difficult with the filtering used to make the map. Fitting the SED yields an infrared luminosity of (8.0 ± 1.0) × 1013Lʘ.
a, Velocity offsets of the 14 sources versus projected (physical) distance, compared with the escape velocity of a 1.16 × 1013Mʘ NFW halo (virial radius ≈ 200 kpc; concentration = 5). The grey shaded region shows our estimated halo-mass uncertainty; also shown is the escape velocity assuming a point mass halo of this same mass. Centres for the distribution of the 14 galaxies are shown at the mean of the distribution, and centred on the ‘B’ galaxy. All galaxies are bound for all but the lowest range of NFW halo masses. b, The cumulative velocity distribution of the SPT2349-56 galaxies, compared with a Gaussian distribution with our estimated dispersion (σ, 408 km s−1), is at least consistent with expectations for a relatively bound system. c, The physical distribution of the SPT2349-56 galaxies (blue squares), assuming that their redshifts are due to cosmic expansion rather than peculiar motions. This gives an extreme (but unlikely) possibility that the SPT2349-56 galaxies are stretched out along a filament compared with their 130-kpc maximum tangential extent, but this requires that none of the velocity offsets is a peculiar motion. The open symbols show analogues of SPT2349-56 found when we searched specifically for maximally extended filamentary structures in our N-body simulations. These simulated structures are not filaments; they are instead rather like collapsed structures that are slightly cigar-shaped. The SPT2349-56 galaxies could in principle be distributed like this, but it does not fundamentally change our discussion here. We also note that we are plotting two different things here: velocity offsets for SPT2349-56, and actual geometry (three-dimensional positions) for the simulation galaxies. Even though our search allowed for structures extending by about 5 Mpc along the line of sight (LOS), we found none that stretches beyond 1 Mpc. d, As for panel c, except that the full extent of our ALMA band 3 and 7 observations is shown. No structures are observed in the sidebands surrounding the 14 observed sources.