 An
emerging class of black holes is a thousand times more massive,
yet nearly a million times smaller than others. The strongest
emission detected in Galaxy NGC 4395 is from the central region,
produced by a mid-mass black hole at its core.
Sometimes
big things come in little packages. Astronomers have found a relatively
tiny “supermassive” black hole that pulls in as much
matter and radiates as many X-rays as its larger, lazier cousins.
The observation may imply that the central engine behind the dimmer
examples of the universe's brightest galaxies might often be a
smaller black hole giving all it's got, instead of a larger black
hole accreting matter at a slower rate.
The newly discovered object, located in the core of galaxy NGC
4395, is part of an emerging class of mid-mass black holes, a
thousand times more massive than stellar black holes yet a thousand
to a million times smaller than the largest variety.
Kazushi Iwasawa and Prof. Andrew Fabian at the Institute of Astronomy
in Cambridge in the United Kingdom, along with other astronomers
from the United Kingdom and Japan, identified the mid-mass black
hole using the Advanced Satellite for Cosmology and Astrophysics
(ASCA), an X-ray telescope built by Japan. Their results have
been accepted for publication in the Monthly Notices of the Royal
Astronomy Society.
“Recent studies with the Hubble and other telescopes show
that massive, dark objects appear ubiquitous at the centers of
galaxies,” said Iwasawa. “These dark objects are likely
black holes, and we are beginning to learn that they come in a
variety of sizes.”
Supermassive black holes contain the mass of millions to billions
of suns confined to a region no larger than our solar system.
They are thought
to be the engines behind the bright, concentrated emissions in
the cores of many types of galaxies, such as quasar galaxies.
The tremendous force of their gravity pulls in gas, dust and whole
stars with such fury that the in-falling matter heats to millions
of degrees and radiates brilliantly across the spectrum, particularly
in X-rays.
In galaxies
lacking a bright core, such as our own Milky Way, scientists speculate
that the central black hole has either run out of fuel—that
is, has already accreted all the matter within its gravitational
clutches—or, for whatever reason, is accreting matter at
a slower rate. The theory of Advection-Dominated Accretion Flow
(ADAF) states that matter flow into the black hole at an expected
rate, but that radiation is emitted inefficiently, perhaps being
pulled into the black hole itself.
Within the past year, scientists have uncovered other mid-mass
black holes. These, such as the one in NGC 4395, contain the mass
of 10,000 to 100,000 suns. A black hole of this mass could explain
the type of emissions from low-luminosity Seyfert galaxies. These
are galaxies with active cores and bright emissions, albeit much
dimmer than quasars or other extremely bright, active galaxies.
The X-ray emission from NGC 4395 had been a mystery. Earlier observations
suggested that perhaps an extremely bright group of stars was
the cause for the emission. Yet the source was too bright for
this theory to be correct. Likewise, the type of fluctuation in
the X-ray signals from the source was inconsistent with the ADAF
black hole model.
“We now see that the nuclear source in NGC 4395 is a scaled-down
version of a black hole found in the most luminous of galaxies,”
said Fabian. “Everything is the same, only it is smaller.”
The apparent small black hole mass of NGC 4395 suggests that X-ray
variability in active galaxies correlates with mass and not directly
with luminosity, as previously thought, Iwasawa said. This suggests
that some active galaxies with low luminosity may have mid-mass
black holes.
“We do not know how common galaxies like NGC 4395 are, but
it certainly wouldn't be surprising if there were many out there,”
said Iwasawa. “NGC 4395 is the only one as far as we know.
They might be just difficult to find because they tend to reside
in tiny galaxies. On the other hand, many 'big' galaxies with
low luminosity have been found to have big black holes and for
some reason, they accrete in an inefficient mode.”
Scientists contributing to this work include Omar Almaini, Paulina
Lira and Prof. Andrew Lawrence at the Institute for Astronomy
at University of Edinburgh, Kiyoshi Hayashida of Osaka University
and Prof. Hajime Inoue of Japan's Institute of Space and Astronautical
Science. Lira has since joined Leicester University.
ASCA, launched in February 1993, is Japan's fourth cosmic X-ray
astronomy mission, and the second for which the United States
has provided part of the scientific payload. The satellite is
operated through NASA's Goddard Space Flight Center in Greenbelt,
Maryland.
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