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. Back
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