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© R. P. Deane et al.
role in the evolution of their hosts. How these giant black holes were formed
in the first place and how they grew with time is not clearly understood,
but galaxy mergers -that may have eventually led to the merging of their
central black holes- appear to be a natural explanation. If this is true,
then one expects to see a number of dual or even multiple supermassive
black hole systems in some galaxies. However, we only know of very few
examples so far, in particular at low separations. Very Long Baseline
Interferometry (VLBI) is a powerful tool to find and confirm close pairs
of supermassive black hole systems due to its great resolution.
Dr. Roger Deane et al. announced the discovery of a triple black hole system
in which the tight inner pair of black holes were resolved by the
European VLBI Network (EVN). The Westerbork Synthesis Radio Telescope
was part of the observations, and the data were correlated at the
Joint Institute for VLBI in Europe in Dwingeloo. These systems are important
for various reasons. In terms of galaxy evolution it is known that black holes
-especially in active galactic nuclei (AGN)- influence how galaxies evolve,
and understanding how often black holes themselves merge is key to this work.
Furthermore, closely orbiting systems such as this are sources of gravitational
waves in the Universe, if General Relativity is correct. Future radio telescopes
such as the SKA will be able to measure the gravitational waves from such
systems as their orbits decrease.
While the VLBI technique was essential to discover the inner two black holes
(which are in fact the second closest pair of supermassive black holes known),
Deane and co-authors have also shown that the binary black hole presence can be
revealed by much larger scale features. The orbital motion of the black hole is
imprinted onto its large jets, twisting them into a helical or corkscrew-like
shape. So even though black holes may be so close together that our telescopes
can’t tell them apart, their twisted jets may provide easy-to-find pointers to
them, much like using a flare to mark your location at sea. This may provide
sensitive future telescopes an additional way to find binary black holes with
much greater efficiency.
The image shows a sketch of the triple system with helical jets from one of the
supermassive black holes. The third black hole is part of the system, but farther
away and is likely to emit relatively straight jets. The inset shows the
EVN 1.7 GHz image (contours) overlaid on the EVN 5 GHz image (colour scale).
The results are published in Nature:
http://dx.doi.org/10.1038/nature13454
See also the JIVE press release here:
http://www.jive.nl/trio-supermassive-black-holes-shake-space-time