Major galaxy mergers are thought to play an important part in fuelling the growth of supermassive black holes1. However, observational support for this hypothesis is mixed, with some studies showing a correlation between merging galaxies and luminous quasars2,3 and others showing no such association4,5. Recent observations have shown that a black hole is likely to become heavily obscured behind merger-driven gas and dust, even in the early stages of the merger, when the galaxies are well separated6,7,9 (5 to 40 kiloparsecs). Merger simulations further suggest that such obscuration and black-hole accretion peaks in the final merger stage, when the two galactic nuclei are closely separated10 (less than 3 kiloparsecs). Resolving this final stage requires a combination of high-spatial-resolution infrared imaging and high-sensitivity hard-X-ray observations to detect highly obscured sources. However, large numbers of obscured luminous accreting supermassive black holes have been recently detected nearby (distances below 250 megaparsecs) in X-ray observations11. Here we report high-resolution infrared observations of hard-X-ray-selected black holes and the discovery of obscured nuclear mergers, the parent populations of supermassive-black-hole mergers. We find that obscured luminous black holes (bolometric luminosity higher than 2 × 1044 ergs per second) show a significant (P < 0.001) excess of late-stage nuclear mergers (17.6 per cent) compared to a sample of inactive galaxies with matching stellar masses and star formation rates (1.1 per cent), in agreement with theoretical predictions. Using hydrodynamic simulations, we confirm that the excess of nuclear mergers is indeed strongest for gas-rich major-merger hosts of obscured luminous black holes in this final stage.

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Data availability

The reduced imaging datasets from the HST are available from the Hubble Legacy Archive. The raw imaging datasets from the near-infrared adaptive optics programmes are available from the Keck Observatory Archive. Other reduced datasets generated or analysed in this study are available from the corresponding author on reasonable request.

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Change history

  • 21 March 2019

    In this Letter, the spelling of author Benny Trakhtenbrot was corrected; the affiliation for author Sylvain Veilleux was amended; and a new ref. 9 was added to the Abstract with subsequent references renumbered; these errors have been corrected online.


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This work is dedicated to the memory of our friend and collaborator N. Gehrels. We appreciate the help of R. M. McGurk with the initial night of Keck telescope observations and initial data reductions. M.J.K. acknowledges support from the Swiss National Science Foundation (SNSF) through the Ambizione fellowship grant PZ00P2 154799/1 and from NASA through ADAP award NNH16CT03C. K.S., L.F.S. and A.W. acknowledge support from SNSF grants PP00P2 138979 and PP00P2 166159.  L.B. acknowledges support from NSF award number 1715413. We acknowledge the work of the Swift/BAT team to make this study possible. This paper is part of the Swift/BAT AGN Spectroscopic Survey (BASS).

Reviewer information

Nature thanks D. Kocevski and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Author information


  1. Eureka Scientific Inc, Oakland, CA, USA

    • Michael J. Koss
  2. Institute for Particle Physics and Astrophysics, ETH Zürich, Zürich, Switzerland

    • Michael J. Koss
    • , Phillip Bernhard
    • , Anna Weigel
    • , Lia F. Sartori
    •  & Kevin Schawinski
  3. Department of Physics, University of Florida, Gainesville, FL, USA

    • Laura Blecha
  4. Department of Physics, Manhattan College, New York, NY, USA

    • Chao-Ling Hung
  5. Department of Astronomy, University of California, Berkeley, CA, USA

    • Jessica R. Lu
  6. Department of Physics, ETH Zürich, Zürich, Switzerland

    • Benny Trakhtenbrot
  7. School of Physics and Astronomy, Tel Aviv University, Tel Aviv, Israel

    • Benny Trakhtenbrot
  8. Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Santiago, Chile

    • Ezequiel Treister
  9. Department of Astronomy and Joint Space-Science Institute, University of Maryland, College Park, MD, USA

    • Richard Mushotzky
    •  & Sylvain Veilleux
  10. Núcleo de Astronomía de la Facultad de Ingeniería, Universidad Diego Portales, Santiago, Chile

    • Claudio Ricci
  11. Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing, China

    • Claudio Ricci
  12. Chinese Academy of Sciences South America Center for Astronomy, Santiago, Chile

    • Claudio Ricci
  13. Institute for Astronomy, University of Hawaii at Manoa, Honolulu, HI, USA

    • David B. Sanders


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M.J.K. drafted the manuscript, performed the observations and carried out much of the analysis. L.B. performed and interpreted the hydrodynamic simulations. P.B. carried out much of the initial data reduction. C.-L.H. ran the artificial redshifting code. J.R.L. provided the initial data reduction code and helped with the analysis. K.S. aided in the scientific interpretations and the reduction of the raw data. E.T. assisted in the initial observing runs. R.M., S.V. and D.B.S. aided in the initial proposal and scientific interpretations. B.T., L.F.S., A.W. and C.R. assisted in the scientific interpretations.

Competing interests

The authors declare no competing interests.

Corresponding author

Correspondence to Michael J. Koss.

Extended data figures and tables

  1. Extended Data Fig. 1 Other close mergers.

    ad, Tricolour optical images in the gri band from the Sloan Digital Sky Survey or the Kitt Peak survey with about 1″ angular resolution. The galaxies shown are NGC 6240 (a), 2MASX J00253292+6821442 (b), ESO 509-G027 (c) and Mrk 975 (d) from the AGN sample. The images are 60 kpc × 60 kpc in size. Red squares indicate the size of the zoomed-in AO image on the right. eh, High-spatial-resolution images of the nuclear mergers shown in ad, 4 kpc × 4 kpc in size.

  2. Extended Data Fig. 2 Other close mergers.

    ac, Tricolour optical images in the gri band from the Sloan Digital Sky Survey or the Kitt Peak survey with about 1″ angular resolution. The galaxies shown are 2MASX J16311554+2352577 (a) and 2MASX J08434495+3549421 (b) from the AGN sample and 2MASX J08370182-4954302 (c) from the inactive-galaxy sample. d, Lower-quality red Digitized Sky Survey image of UGC02369 NED01, for which no higher-quality imaging exists. The images in ad are 60 kpc × 60 kpc in size. Red squares indicate the size of the zoomed-in AO image on the right. eh, High-spatial-resolution near-infrared images of the nuclear mergers shown in ad, 4 kpc × 4 kpc in size.

  3. Extended Data Fig. 3 Inactive-galaxy control sample.

    ad, Tricolour optical images in the gri band from Pan-STARRS imaging with about1″ angular resolution. The images show inactive galaxies in the control sample that were matched in stellar mass and SFR to the AGN: NGC 214 (a), NGC 151 (b), NGC 2998 (c) and NGC 6504 (d). The images are 60 kpc × 60 kpc in size. Red squares indicate the size of the zoomed-in AO image on the right. eh, High-spatial-resolution near-infrared images of the nuclear mergers shown in ad, 4 kpc × 4 kpc in size. Some white lines are present in NICMOS and Pan-STARRS imaging owing to bad pixels with very low or zero response or with very high or erratic dark current.

  4. Extended Data Fig. 4 Stellar mass, star formation rate and resolution of AGN and inactive galaxies.

    a, H-band luminosity of the different AGN and inactive galaxies. Inactive galaxies with considerably lower stellarmasses than the AGN samples were excluded (〈log(LH/Lʘ)〉 < 9.7). b, 60-μm luminosity of the different AGN and inactive galaxies. Inactive galaxies with lower SFR were also excluded from the comparison (〈log(νLν)60μm〉 = 43.6). For observations in which a galaxy was not detected, we show a 3σ upper limit of the SFR, indicated by a downward arrow. c, Comparison of the maximum spatial resolution (in parsecs) of the different observations. The inactive-galaxy sample typically has higher physical spatial resolutions than the AGN samples. Many galaxies observed fall along a line because of the constant physical resolution of the HST.

  5. Extended Data Fig. 5 Summary of programme types included in the HST control sample.

    The majority of archival control sample observations are of high-SFR luminous infrared galaxies (‘LIRG’) or from studies of volume-limited samples of nearby galaxies (‘Nearby Galaxy’). The remaining samples originate from observations of spiral galaxies (‘Spiral’), galaxies in the merger sequence or late-stage mergers (‘Merger’), galaxies with large or small black holes (‘Black Hole’) and elliptical galaxies (‘Elliptical’). Finally, some nearby galaxies were observed serendipitously in observations of other sources or survey fields (‘Serendip’).

  6. Extended Data Fig. 6 SFR and stellar mass.

    Measurements of SFR and stellar mass for the BAT AGN sample (purple circles) and the HST-matched archival control sample of inactive galaxies (green diamonds). The full distribution of inactive galaxies from the Sloan Digital Sky Survey (SDSS) is shown with grey shading and the full distribution of the HST archive with blue contours. The HST archival sample has an excess of high-stellar-mass, high-SFR inactive galaxies because of the large number of observations of luminous infrared galaxies.

  7. Extended Data Fig. 7 Simulated HST images of nuclear mergers at high redshift.

    Simulated images of three nuclear mergers (2MASXJ 01392400+2924067, CGCG 341-006, MCG+02-21-013) observed at z = 1 with the HST F160W filter as part of the CANDELS survey (60 mas pixel−1) using optical imaging and FERENGI software. The HST would be unable to detect these final stage mergers. All simulated images are displayed in the arcsinh scale in coupled-channel-device counts, as if observed in the HST F160W filter as part of the CANDELS survey.

  8. Extended Data Table 1 Galaxies with companions within 10 kpc

Supplementary information

  1. Supplementary Data

    This file contains a machine readable table associated with the study. The first table, All High Resolution Observations, contains a list of all the galaxies in the study and the details of their high resolution observations, H-band emission as a proxy for star formation and 60 um emission as a proxy for their star formation.

  2. Supplementary Data

    This file also contains a machine readable table associated with the study. The second table, HST archival programs, contains a list of all the HST archival programs used in the study.

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