Researchers
finally have strong evidence for the existence of a binary black hole system, a
long-theorized result of galactic mergers that features two black holes
orbiting around each other at the center of large galaxy.
The black
holes are expected
to merge in what astronomers figure would be one of the most energetic
events in the universe.
The new
evidence comes from a study of the light signatures of 17,500 quasars —
extremely bright features at the center of galaxies, thought to be powered by black holes — taken by
the Sloan Digital Sky Survey and detailed in the March 5 issue of the journal Nature.
It has long
been accepted that most large galaxies harbor black holes at their centers.
Astronomers also think that mergers between galaxies are common — our own Milky
Way is on
a collision course with our nearest neighbor, the Andromeda galaxy (though
this merger won't happen for several billion years).
By
combining these tenets of astrophysics, it's a logical leap to the eventual
merger of each galaxy's black hole.
"They
should merge, and we expect them to," said study team member Tod Lauer of
the National Optical Astronomy Observatory (NOAO) in Tucson, Ariz.
Best
evidence
Lauer
likened galaxies to fluffy masses and the black holes at their centers to rocks
— eventually through interactions with the fluff (the stars of the galaxy), the
black holes lose orbital energy and fall to the center of the newly-merged
galaxy.
But once
they've migrated to the center, interactions with stars are rare, "and the
black holes orbit each other in a small tightly bound orbit, unable to loose
enough momentum so that they could actually merge," Lauer explained.
It was at
this stage that Lauer and study leader Todd Boroson, also of the NOAO, think
they detected the signature of two separate black holes in the spectrum of the
quasar known as SDSS J153636.22+044127.0.
The black
holes appear to be separated by a mere 0.1 parsec, 13 times shorter than the distance
between our sun and its nearest neighbor, Alpha Centauri. The researchers
roughly estimated the masses of the pair, coming up with 10 million solar
masses for the smaller black hole and a little less than 1 billion solar masses
for its larger partner. They also estimated that the black holes orbit each
other about every 100 years.
Previous
studies, including one of the
galaxy NGC 6240, have shown potential evidence of black holes heading for a
merger, "but the case presented by Boroson and Lauer is special because
the pairing is tighter and the evidence much stronger," wrote astronomer
Jon Miller of the University of Michigan in an accompanying Nature analysis.
Miller was not involved in the new study.
"We
think we have the best one so far," Lauer told SPACE.com.
To catch
in the act
While this
evidence shows possible binary black holes squeezed closer together than any
yet found, Boroson and Lauer still need solid proof that this is a binary
system.
"The
issue is that this is a good candidate for a binary, but we haven't proved it
beyond doubt," Lauer said.
Once the
black holes have reached the gravitational center of the merged galaxy and come
as close as they can through interactions with the surrounding stars, they need
an extra push to fully merge. If gas later collects at the center of the
galaxy, it could absorb some of the remaining orbital momentum of the black
holes and provide that extra oomph to push them into a merger.
Black holes
that have reached the point of merging are expected to give off gravitational
waves, or ripples in the fabric of space-time predicted by Einstein's
General Relativity. Scientists hope that the Laser Interferometer Space Array
(LISA), currently in development and designed to detect these waves, will be
able to catch black holes actually in the act of merging.
In the
meantime, Lauer and Boroson will continue observing their quasar to see if
changes in the spectrum over time (as the black holes orbit one another)
further support the existence of a binary black hole and rule out the
possibility that they are two separate objects super-imposed. Lauer thinks this
is unlikely because "the spectrum is difficult to explain with two
different objects."
The paucity
of signatures of black hole mergers and binary black holes so far could mean
that these are much rarer events than scientists had thought.
"We
looked through 17,000 of these and found this one beast," Lauer said.
"They should be very common."
But it's
possible that LISA
could provide a wealth of evidence once it gets off the ground. It's also
possible that binaries are only detectable when actively accreting gas and that
most reside in so-called normal galaxies that are not in an actively accreting
quasar phase, Miller noted in his editorial.
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