|
New Type of
Black Hole May Turn Starburst Galaxies Inside Out
 
Figure
1
Figure 2
The stunning spiral galaxy NGC253, shown here in a black and white image
from the Digitized Sky Surveys using the Oschin Schmidt Telescope on
Palomar Mountain and the UK Schmidt Telescope. NGC253, with its furious
star production, is a Seyfert galaxy. The false-colored Chandra image
zooms in on NGC253's central region.
Credit: STScI/DSS (b/w optical), NASA/K.Weaver (colored X ray)
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.
Figure
3
Figure 4
Figure
5
The central region of NGC 253 in three different energy bands. Shown from
left to right is the 0.2-1.5 keV band, the 1.5-4.5 keV band and the 4.5-8
keV band. (Soft, Medium, Hard X rays) These bands are analogous to the
colors of the optical rainbow (where red is lower energy and blue is high
energy), only the X-ray energy range is thousands of times wider than the
optical energy range. Here, the bright X-ray point sources discussed in
the press release show up most strongly at medium to hard X-ray energies.
Credit: NASA/K.Weaver
|
