Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

A close nuclear black-hole pair in the spiral galaxy NGC 3393


The current picture of galaxy evolution1 advocates co-evolution of galaxies and their nuclear massive black holes, through accretion and galactic merging. Pairs of quasars, each with a massive black hole at the centre of its galaxy, have separations of 6,000 to 300,000 light years (refs 2 and 3; 1 parsec = 3.26 light years) and exemplify the first stages of this gravitational interaction. The final stages of the black-hole merging process, through binary black holes and final collapse into a single black hole with gravitational wave emission, are consistent with the sub-light-year separation inferred from the optical spectra4 and light-variability5 of two such quasars. The double active nuclei of a few nearby galaxies with disrupted morphology and intense star formation (such as NGC 6240 with a separation6 of about 2,600 light years and Mrk 463 with a separation7 of about 13,000 light years between the nuclei) demonstrate the importance of major mergers of equal-mass spiral galaxies in this evolution; such mergers lead to an elliptical galaxy8, as in the case of the double-radio-nucleus elliptical galaxy 0402+379 (with a separation of about 24 light years between the nuclei)9. Minor mergers of a spiral galaxy with a smaller companion should be a more common occurrence, evolving into spiral galaxies with active massive black-hole pairs10, but have hitherto not been seen. Here we report the presence of two active massive black holes, separated by about 490 light years, in the Seyfert11 galaxy NGC 3393 (50 Mpc, about 160 million light years). The regular spiral morphology and predominantly old circum-nuclear stellar population12 of this galaxy, and the closeness of the black holes embedded in the bulge, provide a hitherto missing observational point to the study of galaxy/black hole evolution. Comparison of our observations with current theoretical models of mergers suggests that they are the result of minor merger evolution10.

Your institute does not have access to this article

Access options

Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Figure 1: Chandra ACIS-S images.
Figure 2: X-ray spectra.


  1. Colpi, M. & Dotti, M. Massive binary black holes in the cosmic landscape. In Advanced Science Letters, Special Issue on Computational Astrophysics (ed. Mayer, L. ). (2009)

    Google Scholar 

  2. Green, P. J. et al. SDSS J1254+0846: a binary quasar caught in the act of merging. Astrophys. J. 710, 1578–1588 (2010)

    ADS  Article  Google Scholar 

  3. Civano, F. et al. A runaway black hole in COSMOS: gravitational wave or slingshot recoil? Astrophys. J. 717, 209–222 (2010)

    ADS  CAS  Article  Google Scholar 

  4. Boroson, T. A. & Lauer, T. R. A candidate sub-parsec supermassive binary black hole system. Nature 458, 53–55 (2009)

    ADS  CAS  Article  Google Scholar 

  5. Valtonen, M. J. et al. A massive binary black-hole system in OJ287 and a test of general relativity. Nature 452, 851–853 (2008)

    ADS  CAS  Article  Google Scholar 

  6. Komossa, S. et al. Discovery of a binary active galactic nucleus in the ultraluminous infrared galaxy NGC 6240 using Chandra. Astrophys. J. 582, L15–L19 (2003)

    ADS  CAS  Article  Google Scholar 

  7. Hutchings, J. B. & Neff, S. G. The double-nucleus galactic merger MKN 463. Astron. J. 97, 1306–1311 (1989)

    ADS  CAS  Article  Google Scholar 

  8. Mayer, L. et al. Rapid formation of supermassive black hole binaries in galaxy mergers with gas. Science 316, 1874–1877 (2007)

    ADS  CAS  Article  Google Scholar 

  9. Rodriguez, C. et al. A compact supermassive binary black hole system. Astrophys. J. 646, 49–60 (2006)

    ADS  CAS  Article  Google Scholar 

  10. Callegari, S. et al. Growing massive black hole pairs in minor mergers of disk galaxies. Astrophys. J. 729, 85. (2011)

    ADS  Article  Google Scholar 

  11. Diaz, A. I., Prieto, M. A. & Wamsteker, W. The optical and UV spectrum of the Seyfert type 2 galaxy NGC 3393. Astron. Astrophys. 195, 53–59 (1988)

    ADS  CAS  Google Scholar 

  12. Cid Fernandes, R. et al. The star formation history of Seyfert 2 nuclei. Mon. Not. R. Astron. Soc. 355, 273–296 (2004)

    ADS  Article  Google Scholar 

  13. Levenson, N. A., Heckman, T. M., Krolik, J. H., Weaver, K. A. & Z˙ycki, P. T. Penetrating the deep cover of Compton-thick active galactic nuclei. Astrophys. J. 648, 111–127 (2006)

    ADS  CAS  Article  Google Scholar 

  14. Kondratko, P. T., Greenhill, L. J. & Moran, J. M. The parsec-scale accretion disk in NGC 3393. Astrophys. J. 678, 87–95 (2008)

    ADS  CAS  Article  Google Scholar 

  15. Guainazzi, M., Fabian, A. C., Iwasawa, K., Matt, G. & Fiore, F. On the transmission-dominated to reprocessing-dominated spectral state transitions in Seyfert 2 galaxies. Mon. Not. R. Astron. Soc. 356, 295–308 (2005)

    ADS  CAS  Article  Google Scholar 

  16. Risaliti, G. et al. The XMM-Newton long look of NGC 1365: uncovering of the obscured X-ray source. Mon. Not. R. Astron. Soc. 393, L1–L5 (2009)

    ADS  Article  Google Scholar 

  17. Cooke, A. J. et al. The narrow-line region in the Seyfert 2 galaxy NGC 3393. Astrophys. J. Suppl. Ser. 129, 517–545 (2000)

    ADS  CAS  Article  Google Scholar 

  18. Strohmayer, T. E. & Mushotzky, R. F. Discovery of X-ray quasi-periodic oscillations from an ultraluminous x-ray source in M82: evidence against beaming. Astrophys. J. 586, L61–L64 (2003)

    ADS  Article  Google Scholar 

  19. George, I. M. & Fabian, A. C. X-ray reflection from cold matter in active galactic nuclei and X-ray binaries. Mon. Not. R. Astron. Soc. 249, 352–367 (1991)

    ADS  CAS  Article  Google Scholar 

  20. Fabbiano, G. et al. The time-variable ultraluminous X-ray sources of “The Antennae”. Astrophys. J. 584, L5–L8 (2003)

    ADS  Article  Google Scholar 

  21. Maiolino, R. et al. Heavy obscuration in X-ray weak AGNs. Astron. Astrophys. 338, 781–794 (1998)

    ADS  Google Scholar 

  22. Elvis, M. et al. Atlas of quasar energy distributions. Astrophys. J. Suppl. Ser. 95, 1–68 (1994)

    ADS  Article  Google Scholar 

  23. Magorrian, J. et al. The demography of massive dark objects in galaxy centers. Astron. J. 115, 2285–2305 (1998)

    ADS  Article  Google Scholar 

  24. Gandhi, P. et al. Resolving the mid-infrared cores of local Seyferts. Astron. Astrophys. 502, 457–472 (2009)

    ADS  Article  Google Scholar 

  25. Schmitt, H. R., Ulvestad, J. S., Antonucci, R. R. J. & Kinney, A. L. Jet directions in Seyfert galaxies: radio continuum imaging data. Astrophys. J. Suppl. Ser. 132, 199–209 (2001)

    ADS  Article  Google Scholar 

  26. Miyazawa, G. T. Haba, Y. & Kunieda, H. Broad-band temporal and spectral variation of 36 active galactic nuclei observed with Suzaku. Publ. Astron. Soc. Jpn 61, 1331–1354 (2009)

    ADS  Article  Google Scholar 

  27. Fukazawa, Y. et al. Fe-K line probing of material around the active galactic nucleus central engine with Suzaku. Astrophys. J. 727, 19–31 (2011)

    ADS  Article  Google Scholar 

  28. Cusumano, G. et al. The Palermo Swift-BAT hard X-ray catalogue. III. Results after 54 months of sky survey. Astron. Astrophys. 524, A64–A101 (2010)

    Article  Google Scholar 

  29. Palmeri, P., Mendoza, C., Kallman, T. R., Bautista, M. A. & Meléndez, M. Modeling of iron K lines: radiative and Auger decay data for Fe II-Fe IX. Astron. Astrophys. 410, 359–364 (2003)

    ADS  CAS  Article  Google Scholar 

  30. Magdziarz, P. & Zdziarski, A. A. Angle-dependent Compton reflection of X-rays and gamma-rays. Mon. Not. R. Astron. Soc. 273, 837–848 (1995)

    ADS  Article  Google Scholar 

Download references


This work was supported by a NASA grant (with Principal Investigator J.W.). We thank P. Gandhi for discussions on the 13 μm observations. We used the NASA ADS and NED services. This work includes archival Chandra and Hubble Space Telescope data. We acknowledge discussions with T. Di Matteo and L. Mayer at the Aspen Center for Physics 2011 Summer Program.

Author information

Authors and Affiliations



G.F. suggested the possibility of double active galactic nuclei, designed the data analysis approach, directed the interpretation of the results and wrote the paper. J.W. is the Principal Investigator of the Chandra proposal, performed the data analysis and participated in the interpretation of the results. M.E. and G.R. contributed to the interpretation of the results and revisions of the manuscript.

Corresponding author

Correspondence to G. Fabbiano.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

Supplementary information

Supplementary Information

This file contains Supplementary Test and Data and Supplementary Figure 1 and a legend. (PDF 113 kb)

PowerPoint slides

Rights and permissions

Reprints and Permissions

About this article

Cite this article

Fabbiano, G., Wang, J., Elvis, M. et al. A close nuclear black-hole pair in the spiral galaxy NGC 3393. Nature 477, 431–434 (2011).

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI:

Further reading


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing