Astronomers have, for the first time, discovered what seems to be a
binary set of supermassive black holes. The galactic beasts are
orbiting each other about every 100 years, and ultimately they will
collide with enough force to trip gravitational-wave detectors on Earth.
Spotting a single supermassive black hole is fairly easy, so much so
that astronomers are convinced that one lurks in the center of just
about every galaxy. Finding two supermassives locked in a binary orbit
is another story. Only about as big as a solar system--though they can
weigh as much as a billion suns--spotting them as distinct objects is
as difficult as finding the proverbial needle in the haystack.
But that's what a pair of astronomers from the National Optical
Astronomical Observatory in Tucson, Arizona, appear to have done. Todd
Boroson and Tod Lauer were combing through the Sloan Digital Sky
Survey, a collection of images and spectral information on hundreds of
thousands of galaxies, when their software flagged a quasar whose light
characteristics differed significantly from the rest of the sample.
Quasars are the most luminous objects in the universe, and their
brightness is thought to be caused by the energy from supermassive
black holes consuming tremendous amounts of surrounding gas and dust.
Tomorrow in Nature,
the two researchers report finding telltale twin hydrogen lines in the
quasar's spectrum, instead of the one line that would emanate from a
single black hole. Based on the twin lines, Boroson and Lauer calculate
that two supermassives are separated by only 0.3 light-years--one-tenth
the distance from the sun to its nearest neighbor--and are orbiting
each other at the blinding speed of 6000 kilometers per second. For
comparison, the sun's orbital speed around the galaxy's center is about
220 kilometers per second. "Sometimes discoveries depend on recognizing
something as interesting even if it wasn't what you were looking for,"
Boroson says. If this one is confirmed, he adds, it should vastly
improve what astronomers know about how supermassives merge and how
they light up quasars.
"Wow, that is certainly an interesting discovery," says astronomer
Steven Willner of the Harvard-Smithsonian Center for Astrophysics in
Cambridge, Massachusetts. Follow-up observations should reveal "a lot
more about the environment around the black holes," he says. Astronomer
Alan Marscher of Boston University says the paper does seem to show
more than one supermassive black hole, which is new but expected,
because two galaxies in the process of merging would contain two
supermassive black holes.
