New Type of Black Hole May Turn Starburst Galaxies Inside Out
Starburst galaxies -- those distant gems set aglow in a colorful lifecycle of star birth, death and renewal -- may be the stepping stone to a far brighter phenomenon: a quasar-type galaxy with a supermassive black hole at its core.
The Chandra X-ray Observatory
is finding that starburst galaxies have a proportionally
higher number of what appears to be intermediate-size black
holes, a new type of object announced last year. Chandra
images also suggest that these black holes may be sinking to
the center of one particular starburst galaxy, where they
would merge with each other to create an engine that could
illuminate a core millions of times brighter than the entire
Milky Way galaxy.
Dr. Kimberly Weaver of NASA Goddard Space Flight Center
in Greenbelt, Md., speaks of the implications of new Chandra
observations of starburst galaxies in a press conference today
at the 198th meeting of the American Astronomical Society in
Pasadena, California.
"With Chandra, nearly every day we are finding more of these
point-like sources that appear to be hundreds or thousands of
times more massive than the 'ordinary' type of stellar black
hole formed when massive stars collapse," says Weaver. "The
big questions are: What are these objects, and what role do
they play in galaxy evolution?"
Scientists call the newly found objects "ultraluminous X-ray
sources"; they also have been referred to over the past year as
mid-mass and intermediate-size black holes. These objects
are found outside of the core of many types of galaxies. In
starburst galaxies, Weaver says, these sources may take on a
particular significance.
Starburst galaxies are known for their brightness caused by a
high concentration of young, massive stars and supernova
explosions. The bulk of a starburst galaxy's luminosity is from
outside of the core region.
Other types of bright galaxies -- such as quasars, blazars and
Seyferts -- emit most of their light from the core region, a
phenomenon called an Active Galactic Nucleus, or AGN. The
engine of the AGN is thought to be a supermassive black hole,
which could contain the mass of billions of suns confined to a
region about the size of our solar system.
Weaver is finding that starbursts may evolve into AGN. Gas
expelled from numerous star explosions may collide and
collapse into intermediate-size black holes. These smaller
black holes may sink to the center of the galaxy to form a single
supermassive black hole. The supermassive black hole, in
turn, would grow larger and release fantastic amounts of
energy as it pulls in more and more gas from the galaxy core.
This would then turn the light source of the galaxy "inside out,"
making the core bright instead of the disk.
With Chandra, Weaver studied the galaxy NGC 253, a
relatively nearby, classical starburst galaxy with no prior
evidence of an X-ray-emitting AGN. This galaxy has at least
ten ultraluminous point sources, and three of them are located
within about 3,000 light years from the galaxy core. That's
close, and it may imply that the ultraluminous objects -- often
found slightly farther out -- are gravitating towards the center of
the galaxy.
In the core and farther back in the disk of NGC 253, Weaver
has also found a type of X-ray light from hot iron atoms that
may be from dense molecular clouds lit up by a budding AGN.
"Could it be that this starburst galaxy is transforming itself into
a quasar-like galaxy as we watch," asks Weaver. "We have
known for several years that starburst activity can be
associated with AGN activity. In NGC 253, Chandra may have
found a causal connection."
Weaver's collaborators on this observation were Drs. David
Strickland and Timothy Heckman of Johns Hopkins University.
The team observed NGC 253, approximately 10 million light
years from Earth, for 3.5 hours with Chandra's Advanced CCD
Imaging Spectrometer (ACIS).
The ACIS X-ray camera was developed for NASA by Penn
State and MIT. NASA's Marshall Space Flight Center in
Huntsville, Ala., manages the Chandra program. TRW, Inc.,
Redondo Beach, Calif., is the prime contractor for the
spacecraft. The Smithsonian's Chandra X-ray Center controls
science and flight operations from Cambridge, Mass. |