A close-pair binary in a distant triple supermassive black-hole system

Jun 24, 2014
21 pages
Published in:
  • Nature 511 (2014) 57
e-Print:

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Abstract: (arXiv)
Galaxies are believed to evolve through merging, which should lead to multiple supermassive black holes in some. There are four known triple black hole systems, with the closest pair being 2.4 kiloparsecs apart (the third component is more distant at 3 kiloparsecs), which is far from the gravitational sphere of influence of a black hole with mass \sim109^9 M_\odot (about 100 parsecs). Previous searches for compact black hole systems concluded that they were rare, with the tightest binary system having a separation of 7 parsecs. Here we report observations of a triple black hole system at redshift z=0.39, with the closest pair separated by \sim140 parsecs. The presence of the tight pair is imprinted onto the properties of the large-scale radio jets, as a rotationally-symmetric helical modulation, 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 experiments.
Note:
  • 21 pages, 6 figures. Published online by Nature on 25 June 2014. Extremely minor differences with published version exist
  • black hole
  • binary
  • galaxy
  • gravitational radiation
  • gravitation
  • observatory
  • radio wave
  • [1]
    Both components have flat spectra, which is uncharacteristic of hotspots which have typical spectral indices of αhotspots < -0.542. Spectral indices are even steeper for CSOs with a projected linear size (hotspot separation) greater than ∼100 pc42
    • [2]
      The population of CSOs have typical luminosities of 1027 which is 4 orders of magnitude larger than J1502SE/W41,42. While this may be a selection effect, the lowest luminosity CSO that we have found in the literature has a 1.4 GHz radio luminosity that is 1 order of magnitude larger than that of J1502SE/W42
      • W. Hz-1
      • [3]
        Similar to the double-jet/core-jet scenario discussed in the previous section, higher frequency observations probe nearer the base of the jet since it becomes optically thin closer to the central black hole39,40. In the core-hotspot scenario where the distance is as large as 138 pc, this would result in a observable frequency dependent core-shift (i.e. decreasing separation between the two components) which we do not see: the separation of J1502SE and J1502SW appears independent of observing frequency
        • [4]
          In the double-hotspot scenario, there is no core emission detected (to L1.7GHz ∼ 1022
          • W. Hz-1
          • [5]
            The straight ∼2 kpc long inner jet of J1502S has a 45 degree misalignment with the vector between J1502SE/W. This would imply a dramatic, almost instantaneous change in angular momentum
            • [6]
              Assuming that one or both J1502SE/W are young hotspots
              • [6]
                this would imply that we happened to observe J1502+1115 within a very short time period since this dramatic change in angular momentum. If we assume that the jet axis is in the plane of the sky and a very conservative jet speed of 0.01-0.1c, the shift in momentum must have occurred within the last 2 - 20 × 103 years. For a nominal radio source lifetime of ∼107 years, this represents a 0.02 - 0.2 percent probability. Indeed, hotspot velocities from young radio sources are typically 0.3-0.5c, and almost never below 0.1c, corroborating our claim that this is a very conservative estimate42. So in summary, in order for a core-hotspot or double-hotspot alternative to be true, the hotspot(s) would not only need to have highly uncharacteristic flat spectral indices (and potentially an order of magnitude larger core size compared to what is observed in the highest luminosity radio sources)
                • [6]

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