Rapidly star-forming galaxies adjacent to quasars at redshifts exceeding 6

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The existence of massive (1011 solar masses) elliptical galaxies by redshift z ≈ 4 (refs 1, 2, 3; when the Universe was 1.5 billion years old) necessitates the presence of galaxies with star-formation rates exceeding 100 solar masses per year at z > 6 (corresponding to an age of the Universe of less than 1 billion years). Surveys have discovered hundreds of galaxies at these early cosmic epochs, but their star-formation rates are more than an order of magnitude lower4. The only known galaxies with very high star-formation rates at z > 6 are, with one exception5, the host galaxies of quasars6, 7, 8, 9, but these galaxies also host accreting supermassive (more than 109 solar masses) black holes, which probably affect the properties of the galaxies. Here we report observations of an emission line of singly ionized carbon ([C ii] at a wavelength of 158 micrometres) in four galaxies at z > 6 that are companions of quasars, with velocity offsets of less than 600 kilometres per second and linear offsets of less than 100 kiloparsecs. The discovery of these four galaxies was serendipitous; they are close to their companion quasars and appear bright in the far-infrared. On the basis of the [C ii] measurements, we estimate star-formation rates in the companions of more than 100 solar masses per year. These sources are similar to the host galaxies of the quasars in [C ii] brightness, linewidth and implied dynamical mass, but do not show evidence for accreting supermassive black holes. Similar systems have previously been found at lower redshift10, 11, 12. We find such close companions in four out of the twenty-five z > 6 quasars surveyed, a fraction that needs to be accounted for in simulations13, 14. If they are representative of the bright end of the [C ii] luminosity function, then they can account for the population of massive elliptical galaxies at z ≈ 4 in terms of the density of cosmic space.

At a glance


  1. Images and spectra of the quasars and their companion galaxies discovered in this study.
    Figure 1: Images and spectra of the quasars and their companion galaxies discovered in this study.

    a, The dust continuum at 1.2 mm from ALMA is shown by red contours, which mark the ±2σ, ±4σ, ±6σ, … isophotes, with σ = (81, 86, 65, 63) μJy per beam (left to right). The images were obtained with natural weighting, yielding beams of 1.20″ × 1.06″, 0.74″ × 0.63″, 1.24″ × 0.89″ and 0.85″ × 0.65″ (left to right), shown as black ellipses. The grey scale shows the near-infrared images of the Y- + J- (left) or J-band (otherwise) flux of the fields, obtained with (left to right) the WFC3 instrument on the Hubble Space Telescope, the LUCI camera on the Large Binocular Telescope (LBT), the SofI instrument on the European Southern Observatory (ESO) New Technology Telescope or the GROND instrument on the Max Planck Gesellschaft (MPG)/ESO 2.2-m telescope. The quasars are clearly detected in their rest-frame ultraviolet emission, which is probed by these images, but their companion galaxies are not, implying that any accreting black holes, if present, are either intrinsically faint or heavily obscured. b, The continuum-subtracted ALMA [C ii] line maps are shown as black contours, which mark the ±2σ, ±4σ, ±6σ, … isophotes, with σ = (0.13, 0.11, 0.15, 0.03) Jy km s−1 per beam (left to right). The colour scale shows the image of the 1.2-mm continuum flux density. Black ellipses are as in a. The width of each image in a and b corresponds to 15″ (about 80 kpc at the redshift of the quasars). c, Spectra of the [C ii] emission and underlying continuum emission of the quasars and their companions. The channels used to create the [C ii] line maps are highlighted in yellow. The spectra are modelled as a flat continuum plus a Gaussian line (red lines). The velocity differences Δv between the quasar and the companion galaxy, derived from the line fit, are listed at the top of each column. The ALMA observations were carried out in compact array configuration between 27 January and 27 March 2016, in conditions of modest precipitable water vapour columns (1–2 mm). In each observation, 38 to 48 of the 12-m antennas were used, with on-source integration times of about 10 min. Nearby radio quasars were used for calibration. Typical system temperatures ranged between 70 K and 130 K.

  2. Velocity structure in the system PJ308−21.
    Figure 2: Velocity structure in the system PJ308−21.

    a, Continuum-subtracted [C ii] channel maps of PJ308−21 and its companion (contours). The underlying continuum is shown in colour. The velocity zero point is set by the redshift of the quasar (z = 6.2342). Each panel corresponds to 10″ × 10″, or about 50 kpc × 50 kpc. Contours mark the ±2σ, ±4σ, ±6σ, … isophotes. The black ellipse shows the synthesized beam. b, Velocity field (colour scale) of PJ308−21. The iso-velocity lines are marked in white (in units of km s−1). c, Position–velocity diagram along the white line in b. A clear velocity gradient is observed in the [C ii] emission that extends over 4.5″ (about 25 kpc) and more than 1,000 km s−1, connecting the companion source in the east with the host galaxy of the quasar and extending even further towards the west.

  3. Intensely star-forming galaxies in the earliest galactic overdensities.
    Figure 3: Intensely star-forming galaxies in the earliest galactic overdensities.

    a, The [C ii]-to-far-infrared luminosity ratio (L[C ii]/LFIR), a key diagnostic of the contribution of the [C ii] line to cooling in the star-forming interstellar medium, as a function of the far-infrared luminosity (LFIR, in units of the luminosity of the Sun L). Sources from the literature (refs 5, 9, 12, 23, 24, 25 and references therein) are shown with small symbols: blue triangles for local (z < 1) galaxies; orange triangles for high-redshift (z > 1) sources; and red diamonds for very high-redshift (z > 6) quasars. The large yellow and red filled circles highlight sources at z > 6 from this work, with 1σ error bars; arrows mark the 3σ limits. The quasars examined here appear towards the far-infrared-bright end of the plot, consistent with other quasars observed at these redshifts. Two of the companion sources (of J2100−1715 and PJ231−20) fall in the same regime as the quasars; however, two companions (of J0842+1218 and PJ308−21) populate a different area of the plot, where less-extreme star-forming galaxies are found. b, The cumulative number of [C ii]-bright companion sources identified in our survey (yellow filled circles, with Poissonian 1σ uncertainties) compared with the constraints from the luminosity function set by blind-field searches of [C ii] at high redshift (orange25 and grey26 dashed lines) as a function of the sky-projected distance from the quasars. We adopt a cylindrical volume centred on the quasar and with depth corresponding to a difference of ±1,000 km s−1 in redshift space. The ALMA field-of-view is also shown for reference (black dotted line). There is an excess by many orders of magnitudes compared with the general field expectations; however, the observed counts can be explained if the limiting case of quasar–Lyman-break-galaxy clustering measured at z ≈ 4 is assumed. In this case, the excess in the galaxy number density at radius r due to large-scale clustering, ξ(r), is modelled as ξ(r) = (r0/r)γ, with a scale length of co-moving Mpc (h = 0.7 in the adopted cosmology) fitted for quasar–galaxy pairs at z ≈ 4 at a fixed slope γ = 2.0 (ref. 27; orange shaded area).


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Author information


  1. Max Planck Institut für Astronomie, Königstuhl 17, Heidelberg 69117, Germany

    • R. Decarli,
    • F. Walter,
    • B. P. Venemans,
    • E. P. Farina,
    • C. Mazzucchelli &
    • H.-W. Rix
  2. National Radio Astronomy Observatory, Pete V. Domenici Array Science Center, PO Box O, Socorro, New Mexico 87801, USA

    • F. Walter &
    • C. Carilli
  3. Astronomy Department, California Institute of Technology, MC105-24, Pasadena, California 91125, USA

    • F. Walter
  4. The Observatories of the Carnegie Institute of Washington, 813 Santa Barbara Street, Pasadena, California 91101, USA

    • E. Bañados
  5. Argelander Institute for Astronomy, University of Bonn, Auf dem Hügel 71, Bonn 53121, Germany

    • F. Bertoldi
  6. Battcock Centre for Experimental Astrophysics, Cavendish Laboratory, Cambridge CB3 0HE, UK

    • C. Carilli
  7. Steward Observatory, The University of Arizona, 933 North Cherry Avenue, Tucson, Arizona 85721-0065, USA

    • X. Fan
  8. Cornell University, 220 Space Sciences Building, Ithaca, New York 14853, USA

    • D. Riechers
  9. Department of Astrophysical Sciences, Princeton University, Princeton, New Jersey 08533, USA

    • M. A. Strauss
  10. Kavli Institute of Astronomy and Astrophysics at Peking University, 5 Yiheyuan Road, Haidian District, Beijing 100871, China

    • R. Wang
  11. Korea Astronomy and Space Science Institute, Daedeokdae-ro 776, Yuseong-gu Daejeon 34055, South Korea

    • Y. Yang


R.D. led the writing and analysis. F.W. was principle investigator of the ALMA programme that led to this discovery. F.W. and B.P.V. played a central part in the project design and implementation. E.P.F. provided the clustering analysis. E.B., B.P.V., E.P.F., C.M., F.W. and H.W.R. contributed to the identification of Pan-STARRS1 quasars. X.F. provided the Hubble observations of J0842+1218. All authors contributed to writing the proposal, and reviewed, discussed and commented on the manuscript.

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