One of the more spectacular
phenomena in the cosmos might just be the collision of
supermassive black holes that accompanies the merger of galaxies.
But the astronomical community has not had definitive proof that
these black holes are actually coming together. For the first
time, astronomers have now produced a convincing mathematical
model that offers the strongest support to date for the idea
that the black holes merge when their host galaxies do.

David Merritt of Rutgers, The State University of New Jersey,
and Ron Ekers of the Australia Telescope National Facility in
Sydney, Australia, have published a paper online in Science
Express that supports this interpretation.

Their calculations demonstrated that when two black holes merge,
the interaction will realign the larger one. They showed for the
first time that a smaller hole could knock a bigger one, with
five times the mass, out of kilter.

The realignment takes place with a sudden flip in the spin axis
of the larger hole. It shows up, said Merritt and Ekers, as a
sudden switch in direction of the jets of particles that shoot
out along the black hole’s spin axis. Images made with a radio
telescope show both the old and the new paths, and the galaxy
appears X-shaped.

Supermassive black holes have been found in the cent°Ï¥Wf almost
every galaxy where astronomers have looked. From a few million
to a few billion times the size of our sun (or solar masses),
they are thought to have formed from giant gas clouds or from
the collapse of clusters of immense numbers of stars shortly
after the Big Bang when the universe began.

Merritt, who leads the Supermassive Black Hole Research Group
at Rutgers, is a theorist who has worked extensively on the
interaction of black holes with galaxies. Ekers, a prominent
radio astronomer, is the president-elect of the International
Astronomical Union (IAU) and director of the Australia
Telescope National Facility.

"Supermassive black holes may have collided in a surprisingly
large number of galaxies, leaving their signatures plain to
read," reported Merritt and Ekers. About 7 percent of known
radio-emitting galaxies show their jets in this characteristic
X-shaped pattern. Merritt and Ekers calculated that a large
galaxy has the probability of being involved in a collision
once every billion years. Based on this calculation, one of
these spectacles is bound to take place somewhere in the
universe each year.

"We have known about X-shaped galaxies for a long time, but
until now we have never had a convincing explanation for them,"
said Merritt. "Most astronomers were fairly sure that black
holes coalesce, but we now regard the X-shaped galaxies as
the first ‘smoking-gun’ evidence. Our model demonstrates that
these constitute solid evidence that the black hole mergers
actually take place."

Additional images can be downloaded from the Web at

NGC326 showing the X-shaped radio source can be downloaded from
the Web at (269KB)
A higher resolution inset showing the nucleus and jet is
superimposed on a Hubble Space Telescope image of the two
interacting galaxies.