Galaxies are believed to evolve through merging1, which should lead to some hosting multiple supermassive black holes2,3,4. There are four known triple black hole systems5,6,7,8, with the closest black hole pair being 2.4 kiloparsecs apart (the third component in this system is at 3 kiloparsecs)7, which is far from the gravitational sphere of influence (about 100 parsecs for a black hole with mass one billion times that of the Sun). Previous searches for compact black hole systems concluded that they were rare9, with the tightest binary system having a separation of 7 parsecs (ref. 10). Here we report observations of a triple black hole system at redshift z = 0.39, with the closest pair separated by about 140 parsecs and significantly more distant from Earth than any other known binary of comparable orbital separation. The effect of the tight pair is to introduce a rotationally symmetric helical modulation on the structure of the large-scale radio jets, which provides a useful way to search for other tight pairs without needing extremely high resolution observations. As we found this tight pair after searching only six galaxies, we conclude that tight pairs are more common than hitherto believed, which is an important observational constraint for low-frequency gravitational wave experiments11,12.
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We thank J. Magorrian, A. Karastergiou, S. Ransom and B. Fanaroff for discussions. The European VLBI Network is a joint facility of European, Chinese, South African and other radio astronomy institutes funded by their national research councils. e-VLBI research infrastructure in Europe was supported by the European Union’s Seventh Framework Programme (FP7/2007-2013) under grant agreement number RI-261525 NEXPReS. The financial assistance of the South African SKA Project (SKA SA) towards this research is acknowledged. Opinions expressed and conclusions arrived at are those of the authors and are not necessarily to be attributed to the SKA SA. Z.P. and S.F. acknowledge funding from the Hungarian Scientific Research Fund (OTKA 104539). Z.P. is grateful for funding support from the International Space Science Institute.
The authors declare no competing financial interests.
Extended data figures and tables
Both J1502S (open red squares) and J1502P (grey squares) are steep-spectrum radio sources between 1.4 and 8 GHz, as measured by JVLA observations. The two flat spectrum cores (J1502SE/SW, filled circles) are the likely cause of the flattened spectrum of J1502S at higher frequency, as measured by the AMI 15.7 GHz detection labelled as J1502+1115 (AMI). Error bars, ± 1s.d. on the flux measurement. J1502+1115 (GMRT) indicates the 610 MHz detection using the GMRT (Methods). Note that J1502SE and J1502SW are shown marginally offset in frequency purely for clearer illustration.
Extended Data Figure 2 Larger field-of-view JVLA 5 GHz map of J1502+1115 to demonstrate map fidelity.
The colour scale shows the same JVLA 5 GHz residuals shown in Fig. 1c but with the full 128 × 128 pixel median map generated from the Monte Carlo realizations. The small filled red square indicates the map boundary of the VLBI map shown in Fig. 1a. The red cross denotes the centroid of the point source subtracted J1502P component. Contour levels are the same as in Fig. 1c. The grey ellipse (lower left) represents the FWHM of the Briggs-weighted (robust = 1) PSF, while the white dot shows the VLBI 5 GHz PSF.
The colour scale shows the same JVLA 5 GHz residuals shown in Fig. 1c but imaged with Briggs uv-weighting (robust = 0) to highlight the position angle of the inner northeast J1502S jet. This is misaligned with the vector between J1502SE and J1502SW (red dots) by ∼45°. The black JVLA 5 GHz contours start at 60 μJy per beam (∼2σ) and increase in steps of 1σ. The grey ellipse (lower left) represents the FWHM of the Briggs-weighted (robust = 0) PSF, while the white ellipse shows the VLBI 5 GHz PSF. The red square indicates the map boundary of the VLBI map shown in Fig. 1a.
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Deane, R., Paragi, Z., Jarvis, M. et al. A close-pair binary in a distant triple supermassive black hole system. Nature 511, 57–60 (2014). https://doi.org/10.1038/nature13454
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