Supermassive black holes at the hearts of large galaxies merge when their
host galaxies do, say Professor David R. Merritt of Rutgers University, New
Jersey, and Professor Ronald D. Ekers of CSIRO’s Australia Telescope
National Facility in Sydney, Australia.

The astronomers make their claim in a paper published online in Science
Express [URL below] on August 1.

Merritt and Ekers’ model is the strongest evidence to date that the black
holes’ mutual attraction ends in an embrace rather than an endless waltz.

Simulations show that when large galaxies merge, interactions between the
black holes and stars will make the black holes sink towards the center of
the combined galaxy. But as they approach the center the black holes will
kick out nearby stars, switching off the mechanism that was drawing them
together.

“Most astronomers assume that nature finds a way to bring the black holes
together, since we don’t see strong evidence of binary black holes at the
centers of galaxies,” says David Merritt. “We now have solid evidence that
the black hole mergers actually take place.”

That ‘smoking gun’ evidence comes from powerful galaxies that shoot ‘jets’
of radio-emitting particles from their cores.

Jets signal the presence of a supermassive black hole, millions or billions
of times more massive than the Sun. The jets emerge, not from the black hole
itself, but from a disk of gas and dismembered stars whirling around it.

About 7% of these galaxies look like their jets have suddenly changed
direction. Jets are aligned with the spin axis of the black hole,
astronomers think. “Flipped jets suggest that the black hole has suddenly
been realigned,” says Ron Ekers.

Which is just what would happen if two black holes fused, the astronomers
say. They show for the first time that even a small black hole can pack
quite a punch, knocking another hole up to five times more massive through a
large angle.

The idea of black-hole mergers is not new: British cosmologist Martin Rees
raised it in a 1978 paper in the journal Nature, for example. But at that
time it was too speculative to be taken seriously, Ron Ekers says.

Theorists still can’t explain what draws the black holes together once they
have thrown the stars out of their neighbourhood. But when the gulf between
the merging holes has shrunk to the size of the solar system, the holes
start to radiate away energy as gravity waves, says David Merritt. Then the
holes slide inexorably towards fusion, spiralling together faster and
faster. Their final clinch releases an enormous burst of gravitational
radiation.

The number of radio galaxies with flipped jets, plus the estimated
100-million-year lifetime of the ‘radio lobes’ they produce, suggests that
these cosmic takeovers happen at the rate of one a year, say Merritt and
Ekers. That’s good news for those planning gravity-wave instruments to
detect them.

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SCIENCE EXPRESS
http://www.sciencemag.org/sciencexpress/recent.shtml

IMAGES OF RADIO GALAXY NGC 326

Jets from the core of the radio galaxy NGC 326 seem to have changed
direction suddenly, perhaps as a result of two black holes merging.

The jets initially pointed to the 10 o’clock and 4 o’clock directions. They
now point to 8 o’clock and 2 o’clock.

Radio image with jets inset
http://www.atnf.csiro.au/people/hsim/tmp/black_hole_mergers/NGC_326_composite.gif
Credit: National Radio Astronomy Observatory / AUI, observers Murgia et al.;
STScI (inset)

Radio image of NGC 326
http://www.atnf.csiro.au/people/hsim/tmp/black_hole_mergers/NGC_326_new.gif
Credit: National Radio Astronomy Observatory / AUI, observers Murgia et al.

Jets of NGC 326
http://www.atnf.csiro.au/people/hsim/tmp/black_hole_mergers/NGC_326_jets.gif
Credit: NASA, STScI/AURA (optical image, in red) and National Radio
Astronomy Observatory / AUI, observers Murgia et al. (radio image of jets,
in green)

Additional images of NGC 326
http://www.ira.bo.cnr.it/~murgia/Radioastronomy/NGC326/IMAGES/index.html
Credit: National Radio Astronomy Observatory / AUI, observers Murgia et al.