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This optical image shows the Was 49 system, which consists of a large
disk galaxy, Was 49a, merging with a much smaller "dwarf" galaxy Was
49b. Image credit: DCT/NRL.› Full image and caption
A supermassive black hole inside a tiny galaxy is challenging scientists' ideas about what happens when two galaxies become one.
Was 49 is the name of a system consisting of a large disk galaxy,
referred to as Was 49a, merging with a much smaller "dwarf" galaxy
called Was 49b. The dwarf galaxy rotates within the larger galaxy's
disk, about 26,000 light-years from its center. Thanks to NASA's Nuclear
Spectroscopic Telescope Array (NuSTAR) mission, scientists have
discovered that the dwarf galaxy is so luminous in high-energy X-rays,
it must host a supermassive black hole much larger and more powerful
"This is a completely unique system and runs contrary to what we
understand of galaxy mergers," said Nathan Secrest, lead author of the
study and postdoctoral fellow at the U.S. Naval Research Laboratory in
Data from NuSTAR and the Sloan Digital Sky Survey suggest that the
mass of the dwarf galaxy's black hole is huge, compared to similarly
sized galaxies, at more than 2 percent of the galaxy's own mass.
"We didn't think that dwarf galaxies hosted supermassive black holes
this big," Secrest said. "This black hole could be hundreds of times
more massive than what we would expect for a galaxy of this size,
depending on how the galaxy evolved in relation to other galaxies."
The dwarf galaxy's black hole is the engine of an active galactic
nucleus (AGN), a cosmic phenomenon in which extremely high-energy
radiation bursts forth as a black hole devours gas and dust. This
particular AGN appears to be covered by a donut-shaped structure made of
gas and dust. NASA's Chandra and Swift missions were used to further
characterize the X-ray emission.
Normally, when two galaxies start to merge, the larger galaxy's
central black hole becomes active, voraciously gobbling gas and dust,
and spewing out high-energy X-rays as matter gets converted into energy.
That is because, as galaxies approach each other, their gravitational
interactions create a torque that funnels gas into the larger galaxy's
central black hole. But in this case, the smaller galaxy hosts a more
luminous AGN with a more active supermassive black hole, and the larger
galaxy's central black hole is relatively quiet.
An optical image of the Was 49 system, compiled using observations
from the Discovery Channel Telescope in Happy Jack, Arizona, uses the
same color filters as the Sloan Digital Sky Survey. Since Was 49 is so
far away, these colors are optimized to separate highly-ionized gas
emission, such as the pink-colored region around the feeding
supermassive black hole, from normal starlight, shown in green. This
allowed astronomers to more accurately determine the size of the dwarf
galaxy that hosts the supermassive black hole.
The pink-colored emission stands out in a new image because of the
intense ionizing radiation emanating from the powerful AGN. Buried
within this region of intense ionization is a faint collection of stars,
believed to be part of the galaxy surrounding the enormous black hole.
These striking features lie on the outskirts of the much larger spiral
galaxy Was 49a, which appears greenish in the image due to the distance
to the galaxy and the optical filters used.
Scientists are still trying to figure out why the supermassive black
hole of dwarf galaxy Was 49b is so big. It may have already been large
before the merger began, or it may have grown during the very early
phase of the merger.
"This study is important because it may give new insight into how
supermassive black holes form and grow in such systems," Secrest said.
"By examining systems like this, we may find clues as to how our own
galaxy's supermassive black hole formed."
In several hundred million years, the black holes of the large and small galaxies will merge into one enormous beast.
NuSTAR is a Small Explorer mission led by Caltech and managed by JPL
for NASA's Science Mission Directorate in Washington. NuSTAR was
developed in partnership with the Danish Technical University and the
Italian Space Agency (ASI). The spacecraft was built by Orbital Sciences
Corp., Dulles, Virginia. NuSTAR's mission operations center is at UC
Berkeley, and the official data archive is at NASA's High Energy
Astrophysics Science Archive Research Center. ASI provides the mission's
ground station and a mirror archive. JPL is managed by Caltech for