Images from the National Science Foundation’s Very Large Array (VLA)
radio telescope have uncovered compelling evidence that supermassive
black holes at the hearts of large galaxies collide when their host
galaxies merge.

“What we have found is the smoking gun for black hole collision in
merging galaxies,” said David Merritt, an astrophysicist at Rutgers
University in New Brunswick, New Jersey. Merritt and his colleague
Ronald D. Ekers of CSIRO’s Australia Telescope National Facility in
Sydney, Australia, present their findings in the August 1 edition of
Science Express.

When large galaxies merge, current models predict that their central
black holes would sink toward the center of the combined galaxy and form
a binary pair. “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 Merritt. “What we
have found in the VLA data is the first direct evidence that the black
holes actually do coalesce.”

The evidence for these mergers comes from the “jets” of radio emitting
particles that shoot from the cores of large galaxies. These jets are
oriented parallel to the spin axis of the supermassive spinning black
hole and are generated by a disk of material being pulled into it.

Images taken with the VLA reveal that about 7 percent of radio emitting
galaxies appear to have their jets flipped, forming what is known as an
“X-type” radio source, so named because of the “X” shape of the radio
lobes. “Flipped jets suggest that the black hole has suddenly been
realigned,” said Ekers.

These features are formed, the astronomers believe, when black holes
collide in a cosmic version of a demolition derby. “Black holes are so
large and so massive,” said Merritt, “that the only thing we can imagine
that would have enough force to realign them is another black hole.” The
astronomers’ calculations show for the first time that even a small
black hole can significantly impact the orientation of another one up to
five times more massive.

The mechanism of how these objects collide, however, has not been well
understood. Theory predicts that as the black holes draw near, they
kick out the surrounding stars in their neighborhood, which initially
provided the breaking power to bring them together. Merritt believes
that when the distance between the black holes shrinks to about the size
of the solar system, they start to radiate away energy as gravity waves.
This then brings the black holes closer and closer, causing them to
spin faster and faster, until they eventually collide in an enormous
burst of gravitational radiation.

Considering the large population of radio galaxies that appear to have
flipped jets, and the estimated 100 million-year lifetime of the jets,
the astronomers believe that these collisions happen somewhere in the
Universe at the rate of about one a year. Proposed space-based
gravitational wave detectors, therefore, should be able to detect these
dramatic events.

Tht8Hœtronomers used existing images from the VLA to produce their
results. “The VLA has dominated the whole field of radio galaxy
studies. In this current research, the quality of the images has been of
crucial importance. In each case, the evidence for the black hole having
suddenly been realigned has been the detailed morphology of the radio
galaxy. We’ve used the results of many observing programs using the
VLA, involving many different groups from around the world,” Ekers said.

The National Radio Astronomy Observatory is a facility of the National
Science Foundation, operated under cooperative agreement by Associated
Universities, Inc.

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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

Credit: National Radio Astronomy Observatory / AUI, observers Murgia et al.
STScI (inset)

Radio image
Credit: National Radio Astronomy Observatory / AUI, observers Murgia et al.

Credit: STScI