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April , 2005
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SCIENCE

5

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. Scientists find evidence of new type of black hole

super-massive black holes are millions to
billions of times as massive as the sun.
The newly discovered intermediate-
mass black hole, however, appears to be
several thousand times larger than stellar
mass black holes, but still far too small
to be classified as a super-massive black
hole. Astronomers have yet to understand
the origins of this new breed of black
holes, but the puzzling objects may give
clues to the process of galaxy formation.
Despite their name, black holes are
among the most luminous objects in the
night sky. When matter falls into a black
hole, it doesn't go without a fight. Matter
falling into the black hole tends to have
some amount of rotation. This rotation
resists gravity and causes the falling mat-
ter to form a disk of gas around the black
hole.
"We expect a disk around (the black
hole), and we expect a very high tempera-
ture in the disk," said Rackham student
Jifeng Liu, who is the lead author on the
recent paper in the March 1 issue of the
Astrophysical Journal Letters that gives
evidence for the presence of an intermedi-

the luminosity itself, you'd imagine
it's an intermediate mass black hole,"
Liu said. Other astronomers believe
that ULXs are actually stellar mass
black holes, whose X-ray emissions
are directed at the Earth like the beam
from a lighthouse.
Astronomy Prof. Joel Bregman, the
second author on the paper, said he origi-
nally believed the same theory. "For a
long time I thought beaming had to be
the answer," he said.
But the ULX in galaxy M74 was more
than just bright. It also showed a quasi-
periodic oscillation or QPO, meaning its
intensity changes in an almost regular
pattern.
"You don't really expect (oscillations)
from a jet, but you do from a disk,"
Bregman said.
Since intensity of light from the black
hole exhibited a periodic pattern, this
led the team to conclude that the object
was not simply a stellar mass black
hole beaming its X-rays directly to the
Earth. Moreover, the astronomers could
deduce the size of the black hole from

ity that astronomers have considered
is that intermediate mass black holes
formed from the first stars in the
universe, which were more. massive
than present stars. Those black holes
merged to form intermediate mass
black holes which in turn formed the
present supermassive black holes that
are the anchors for galaxies. "You
need these intermediate black holes
as a seed," Liu said. If the theory is
correct, "there should be leftovers,"
he added.
This isn't the only explanation for
mid-sized black holes. One theory
proposes intermediate mass black

holes are the result of hundreds of
stellar mass black holes merging at
the center of a star cluster.
Another suggests that they could be
the remnant cores of small galaxies that
have collided with, and are being incor-
porated into, larger galaxies.
The situation of multiple explana-
tions for a new phenomenon "often
happens in astronomy," Bregman
said. "In the end, you just have to let
nature lead you."
Other authors on the paper are Ed
Lloyd-Davies, Jimmy Irwin, Catherine
Espaillat and Patrick Seitzer, all from the
University astronomy department.


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