VLBA Reveals Closest Pair of Supermassive Black Holes (Forwarded)

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May 1, 2006

VLBA Reveals Closest Pair of Supermassive Black Holes

Astronomers using the National Science Foundation's Very Long Baseline
Array (VLBA) radio telescope have found the closest pair of supermassive
black holes ever discovered in the Universe -- a duo of monsters that
together are more than 150 million times more massive than the Sun and
closer together than the Earth and the bright star Vega.

"These two giant black holes are only about 24 light-years apart, and
that's more than 100 times closer than any pair found before," said
Cristina Rodriguez, of the University of New Mexico (UNM) and Simon
Bolivar University in Venezuela. Black holes are concentrations of mass
with gravity so strong that not even light can escape them.

The black hole pair is in the center of a galaxy called 0402+379, some
750 million light-years from Earth. Astronomers presume that each of the
supermassive black holes was once at the core of a separate galaxy, then
the two galaxies collided, leaving the black holes orbiting each other.
The black holes orbit each other about once every 150,000 years, the
scientists say.

"If two black holes like these were to collide, that event would create
the type of strong gravitational waves that physicists hope to detect
with instruments now under construction," said Gregory Taylor, of UNM.
The physicists will need to wait, though: the astronomers calculate that
the black holes in 0402+379 won't collide for about a billion billion years.

"There are some things that might speed that up a little bit," Taylor
remarked.

An earlier VLBA study of 0402+379, an elliptical galaxy, showed the pair
of radio-wave-emitting objects near its core. Further studies using the
VLBA and the Hobby-Eberly Telescope in Texas, revealed that the pair of
objects is indeed a pair of supermassive black holes.

"We needed the ultra-sharp radio 'vision' of the VLBA, particularly at
the high radio frequencies of 22 and 43 GigaHertz, to get the detail
needed to show that those objects are a pair of black holes," Taylor
said. The VLBA is a continent-wide system of ten radio-telescope
antennas. It provides the greatest ability to see fine detail, called
resolving power, of any telescope in astronomy.

"Astronomers have thought for a long time that close pairs of black
holes should result from galaxy collisions," Rodriguez said. Still,
finding them has proven difficult. Until now, the closest confirmed
pairs of supermassive black holes were at least 4,500 light-years apart.
Pairs of smaller black holes, each only a few times the mass of the Sun,
have been found in our own Milky Way Galaxy, but 0402+379 harbors the
pair of supermassive black holes that are the closest to each other yet
found.

Galactic collisions are common throughout the Universe, and astronomers
think that the binary pairs of supermassive black holes that result can
have important effects on the subsequent evolution of the galaxies. In a
number of predicted scenarios, such giant pairs of black holes will
themselves collide, sending gravitational waves out through the
Universe. Such gravitational waves could be detected with a proposed
joint space mission between NASA and the European Space Agency, the
Laser Interferometer Space Antenna.

"Such black-hole collisions undoubtedly are important processes, and we
need to understand them. Finding ever-closer pairs of supermassive black
holes is the first step in that process. Even finding one such system
has dramatically changed our expectations, and informed us about what to
look for," Taylor said. Taylor and his collaborators are currently using
the VLBA to carry out the largest survey of compact radio-emitting
objects ever undertaken, in the hope of finding more such pairs.

Rodriguez and Taylor worked with Robert Zavala of the U.S. Naval
Observatory, Allison Peck of the SubMillimeter Array of the Harvard-
Smithsonian Center for Astrophysics, Lindsey Pollack of the University
of California at Santa Cruz, and Roger Romani of Stanford University.
Their results have been accepted for publication in the Astrophysical
Journal.

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