Astronomers using NASA's Chandra X-ray Observatory discovered the
first pair of supermassive black holes in a spiral galaxy similar to
the Milky Way. Approximately 160 million light-years from Earth, the
pair is the nearest known such phenomenon.
The black holes are located near the center of the spiral galaxy NGC
3393. Separated by only 490 light-years, the black holes are likely
the remnant of a merger of two galaxies of unequal mass a billion or
more years ago.
"If this galaxy weren't so close, we'd have no chance of separating
the two black holes the way we have," said Pepi Fabbiano of the
Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Mass.,
who led the study that appears in this week's online issue of the
journal Nature. "Since this galaxy was right under our noses by cosmic
standards, it makes us wonder how many of these black hole pairs we've
Previous observations in X-rays and at other wavelengths indicated
that a single supermassive black hole existed in the center of NGC
3393. However, a long look by Chandra allowed the researchers to
detect and separate the dual black holes. Both black holes are
actively growing and emitting X-rays as gas falls towards them and
When two equal-sized spiral galaxies merge, astronomers think it
should result in the formation of a black hole pair and a galaxy with
a disrupted appearance and intense star formation. A well-known
example is the pair of supermassive black holes in NGC 6240, which is
located about 330 million light-years from Earth.
However, NGC 3393 is a well-organized spiral galaxy, and its central
bulge is dominated by old stars. These are unusual properties for a
galaxy containing a pair of black holes. Instead, NGC 3393 may be the
first known instance where the merger of a large galaxy and a much
smaller one, dubbed a "minor merger" by scientists, has resulted in
the formation of a pair of supermassive black holes.
In fact, some theories say that minor mergers should be the most
common way for black hole pairs to form, but good candidates have been
difficult to find because the merged galaxy is expected to look so
"The two galaxies have merged without a trace of the earlier
collision, apart from the two black holes," said co-author Junfeng
Wang, also from CfA. "If there were a mismatch in size between the two
galaxies it wouldn't be a surprise for the bigger one to survive
If this were a minor merger, the black hole in the smaller galaxy
should have had a smaller mass than the other black hole before their
host galaxies started to collide. Good estimates of the masses of both
black holes are not yet available to test this idea, although the
observations do show that both black holes are more massive than about
a million Suns. Assuming a minor merger occurred, the black holes
should eventually merge after about a billion years.
Both of the supermassive black holes are heavily obscured by dust and
gas, which makes them difficult to observe in optical light. Because
X-rays are more energetic, they can penetrate this obscuring material.
Chandra's X-ray spectra show clear signatures of a pair of
supermassive black holes.
The NGC 3393 discovery has some similarities to a possible pair of
supermassive black holes found recently by Julia Comerford of the
University of Texas at Austin, also using Chandra data. Two X-ray
sources, which may be due to supermassive black holes in a galaxy
about two billion light-years from Earth, are separated by about 6,500
As in NGC 3393, the host galaxy shows no signs of disturbance or
extreme amounts of star formation. However, no structure of any sort,
including spiral features, is seen in the galaxy. Also, one of the
sources could be explained by a jet, implying only one supermassive
black hole is located in the galaxy.
"Collisions and mergers are one of the most important ways for
galaxies and black holes to grow," said co-author Guido Risaliti of
CfA and the National Institute for Astrophysics in Florence, Italy.
"Finding a black hole pair in a spiral galaxy is an important clue in
our quest to learn how this happens."
NASA's Marshall Space Flight Center in Huntsville, Ala., manages the
Chandra program for NASA's Science Mission Directorate in Washington.
The Smithsonian Astrophysical Observatory controls Chandra's science
and flight operations from Cambridge, Mass.
Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for
Astrophysics (CfA) is a joint collaboration between the Smithsonian
Astrophysical Observatory and the Harvard College Observatory. CfA
scientists, organized into six research divisions, study the origin,
evolution and ultimate fate of the universe.
David A. Aguilar
Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
Christine PulliamPublic Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
Chandra X-ray Center, Cambridge, Mass.