The Formation of a Massive Galaxy Cluster Core at Z = 4.3

The Formation of a Massive Galaxy Cluster Core at z = 4.3 T. B. Miller1;2;∗, S. C. Chapman1;3, M. Aravena4, M. L. N. Ashby5, C. C. Hayward5;6, J. D. Vieira7, A. Weiß8, A. Babul9, M. Bethermin´ 10, C. M. Bradford11;12, M. Brodwin13, J. E. Carlstrom14;15;16;17, Chian-Chou Chen18, D. J. M. Cunningham1;19, C. De Breuck 18, A. H. Gonzalez20, T. R. Greve21, Y. Hezaveh22, K. Lacaille1;23, K. C. Litke24, J. Ma20, M. Malkan25, D. P. Marrone24, W. Morningstar22, E. J. Murphy26, D. Narayanan20, E. Pass1;27, R. Perry1, K. A. Phadke7, K. M. Rotermund1, J. Simpson28;29, J. S. Spilker24, J. Sreevani7, A. A. Stark5, M. L. Strandet8;30 & A. L. Strom31 1Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Canada 2Department of Astronomy, Yale University, 52 Hillhouse Avenue, New Haven, CT 06511, USA 3Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK 4Nucleo´ de Astronom´ıa, Facultad de Ingenier´ıa y Ciencias, Universidad Diego Portales, Av. Ejercito 441, Santiago, Chile 5Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA 6Center for Computational Astrophysics, Flatiron Institute, 162 Fifth Avenue, New York, NY 10010, USA 7Department of Astronomy, University of Illinois, 1002 West Green Street, Urbana, IL 61801, USA 8Max-Planck-Institut fur Radioastronomie, Auf dem Hugel¨ 69 D-53121 Bonn, Germany 9Department of Physics and Astronomy, University of Victoria, Victoria, BC V8P 1A1, Canada 10Aix-Marseille Universite,´ CNRS, LAM, Laboratoire d’Astrophysique de Marseille, Marseille, France 11California Institute of Technology, 1200 E. California Boulevard, Pasadena, CA 91125, USA 12Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, USA 13Department of Physics and Astronomy, University of Missouri, 5110 Rockhill Road, Kansas City, MO 64110, USA 14 Kavli Institute for Cosmological Physics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA 15Department of Physics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA 16Enrico Fermi Institute, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA 17Department of Astronomy and Astrophysics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637, USA 18European Southern Observatory, Karl Schwarzschild Straße 2, 85748 Garching, Germany 19Department of Astronomy and Physics, Saint Mary’s University, Halifax, Nova Scotia, Canada 20Department of Astronomy, University of Florida, Bryant Space Sciences Center, Gainesville, FL 32611, USA 21Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK 22Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Stanford, CA 94305, USA 23Department of Physics and Astronomy, McMaster University, Hamilton, ON L8S4M1, Canada 24Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA 25Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA 26National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA 27Department of Physics and Astronomy, University of Waterloo, 200 University Avenue West, Waterloo, ON N2L 3G1, Canada 28Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK 29Centre for Extragalactic Astronomy, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK 30International Max Planck Research School (IMPRS) for Astronomy and Astrophysics, Universities of Bonn and Cologne, Germany 31Observatories of The Carnegie Institution for Science, 813 Santa Barbara Street, Pasadena, CA 91101, USA Massive galaxy clusters are now found as early as ∼ 3 billion years after the Big Bang, containing stars that formed at even earlier epochs.1–3 The high- redshift progenitors of these galaxy clusters, termed ‘protoclusters’, are identi- fied in cosmological simulations with the highest dark matter overdensities.4–6 1 While their observational signatures are less well defined compared to virial- ized clusters with a substantial hot intra-cluster medium (ICM), protoclusters are expected to contain extremely massive galaxies that can be observed as luminous starbursts.7 Recent claimed detections of protoclusters hosting such starbursts8–11 do not support the kind of rapid cluster core formation expected in simulations12 because these structures contain only a handful of starburst- ing galaxies spread throughout a broad structure, with poor evidence for even- tual collapse into a protocluster. Here we report that the source SPT2349-56 consists of at least 14 gas-rich galaxies all lying at z = 4.31 based on sensitive observations of carbon monoxide and ionized carbon. We demonstrate that each of these galaxies is forming stars between 50 and 1000 times faster than our own Milky Way, and all are located within a projected region only ∼ 130 kiloparsecs in diameter. This galaxy surface density is more than 10 times the average blank field value (integrated over all redshifts) and >1000 times the average field volume density. The velocity dispersion (∼ 410 km s−1) of these galaxies and enormous gas and star formation densities suggest that this system represents a galaxy cluster core 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 is a uniquely massive and dense system that could be building one of the most massive structures in the Universe today. 2 1 In a multi-band survey over 2500 deg of sky, the South Pole Telescope (SPT) discovered a −2 2 population of rare (n ∼ 0:04 deg ), extremely bright (S1:4 mm > 20 mJy) millimeter-selected 13, 14 3 sources. The Atacama Large Millimeter Array (ALMA) 870-µm imaging showed that 4 more than 90% of these SPT-selected sources are single high-redshift submillimeter galaxies 15 5 (SMGs) with intrinsic flux densities of S870 µm = 5 − 10 mJy, but gravitationally lensed by 16 17 6 factors of 5 − 20, with a median redshift z ∼ 4. However, ∼ 10% of these sources show no 7 evidence for lensing and may instead be intrinsically very luminous galaxies or even groups of 2 8 multiple rapidly star-forming galaxies. The brightest such source in the SPT 2500 deg survey, 9 SPT2349-56 (S1:4 mm=23.3 mJy), is revealed by LABOCA (a low resolution bolometer cam- 10 era on the APEX telescope) observations at 870 µm to consist of two elongated sources with a 11 combined flux density S870 µm ∼ 110 mJy (Fig. 1), with the brighter southern source comprising 17 12 ∼ 77 mJy of this flux density. An ALMA redshift survey further resolved SPT2349-56 into a 13 pair of bright 3-mm sources associated with the southern LABOCA source, with both lying at 14 z = 4:3. 15 To better understand the nature of this structure, deep ALMA spectral imaging of the 16 brighter southern peak of the extended LABOCA source was undertaken. A 358-GHz map 17 containing the redshifted [CII]1900:5 GHz line was used to search for line-emitting galaxies. A 18 blind spectral line survey (described in the Methods) was performed on the data cube, revealing 19 14 z ∼ 4:31 line emitters at high significance (SNR >7). Twelve of these emitters are indi- 20 vidually detected in the 1.1-mm continuum map at > 5σ, with 1.1-mm flux densities ranging 2 21 from 0.2-5 mJy (Fig. 1). The remaining two line emitters (M,N) are both detected at lower sig- 22 nificance in the 1.1-mm continuum map but have robust IRAC infrared counterparts (Extended 23 Data Table 1, Extended Data Fig. 4). Nine of these sources are also detected (> 5σ) in the 24 CO(4-3) line. The ALMA spectra are shown in Fig. 1. 25 The measurements of both the continuum and spectral lines of the 14 galaxies allow us 26 to estimate their star formation rates (SFRs), gas masses, and dynamical masses (Tables 1 & 27 Extended Data Table 1). The physical properties of these sources indicate that this protocluster 28 already harbors massive galaxies that are rapidly forming stars from an abundant gas supply. 29 The two brightest sources, A & B, have SFRs in excess of 1000 solar masses per year within 30 their resolved ∼ 3-kpc radii (Extended Data Table ). The total SFR of the 14 sources is 6000 ± −1 31 600 M yr . Multi-colour imaging with Herschel-SPIRE (250, 350, 500 µm), in addition to 32 the 870-µm LABOCA map, shows that the northern LABOCA structure is also consistent with 33 lying at z = 4:3 (see Methods). The sources detected in the ALMA 870-µm imaging therefore 34 comprise just 50% of the total flux density of the southern LABOCA source and 36% of the total 35 LABOCA flux density, suggesting that the inner ∼ 500 kpc of this protocluster contains 16,500 −1 36 M yr of star formation. Modelling the spectral energy distribution based on this combined 13 37 submillimeter photometry yields an IR luminosity (from 8-1100 µm) of (8:0 ± 1:0) × 10 L . 38 The gas masses of the 14 protocluster galaxies, estimated from CO(4-3), or [CII] if unde- 10 11 39 tected in CO(4-3) (see Methods), range from 1 × 10 to 1 × 10 M , with a total gas mass 11 40 of ∼ 6 × 10 (XCO=0:8) M . A follow-up survey of colder molecular gas in CO(2-1) with 41 the ATCA radio telescope detects the bulk of this large gas repository, especially in the central 42 region near sources B, C, & G, and confirms that the assumptions about gas excitation used to 18 43 convert CO(4-3) to H2 gas masses of the galaxies are reasonable. Based on simulations and 44 measurements of lower-redshift systems that have a similar gravitational potential well depth, 45 we expect and calculate explicitly in the Methods that the cold gas may comprise only a small 46 fraction of the baryon budget.

Load more