AUSTIN, Texas — A half-dozen hefty black holes
hide out where they are least expected, in relatively skinny galaxies. The discovery
implies galaxies don't need bulging bellies to harbor monstrous black holes.
Until now,
astronomers had thought central concentrations of stars called galactic bulges
were required for black holes to grow. Our own Milky Way, like most spiral
galaxies, has such a bulge. In many large galaxies, the bulge feeds binge
eating by the
black hole, creating a chaotic scene of high-speed consumption and copious
radiation.
Some
smaller and thinner galaxies — those without bulges — lack evidence for
supermassive black holes.
New
observations from the Spitzer Space Telescope, however, show galactic obesity
isn't the only path to black hole generation.
"This
finding challenges the current paradigm," said Shobita Satyapal of the George Mason University in Fairfax, Va. "The fact that galaxies without bulges have black
holes means that the bulges cannot be the determining factor." Satyapal
presented the research here at a meeting of the American Astronomical Society.
Instead,
they think dark matter, an invisible substance thought to account for about 85
percent of all matter in the universe, might play a role in the early
development of supermassive black holes.
Bulge or
no bulge
Past
research has shown the more massive the bulge, the more massive the associated
black hole. That led astronomers to think a galaxy's
bulge and its supermassive black hole were intimately related.
For
instance, our Milky Way is a massive disk of stars as seen from above. But if
viewed edge-on it would appear like the shape of an airplane viewed head-on
with its wings extended. The wings represent the galaxy's disk, and the bulbous
fuselage would be the bulge.
The mostly
dormant black hole at the center of our galaxy weighs several million times the
mass of the sun. In more active galaxies, black holes can surpass a billion
solar masses. (The record-setter,
at 18 billion solar masses, was announced this week.)
"Scientists
reasoned that somehow the formation and growth of galaxy bulges and their
central black holes are intimately connected," Satyapal said.
A few
recent studies have caused astronomers to question the conventional thinking.
In 2003, scientists discovered a relatively "lightweight"
supermassive black hole in a bulge-less galaxy. And even more recently,
Satyapal and her team spotted another supermassive black hole lurking in a
similarly svelte galaxy.
Now,
Satyapal and her colleagues have turned up six more monstrous black holes in
thin galaxies with minimal bulges.
Veiled
by dust
Satyapal
suggests these black holes were only recently detected because they have been
shrouded by their
dusty abodes. Galaxies with such minimal midsections tend to be extremely
dusty. Infrared light can penetrate the dust, meaning Spitzer could unmask the
black holes with its infrared capabilities.
"A
feeding black hole spits out high-energy light that ionizes much of the gas in
the core of the galaxy," Satyapal said.
Perhaps,
Satyapal speculates, the missing piece of the black-hole puzzle is dark matter.
Early on in the galaxy's life, this invisible matter might somehow set the mass
of the black hole. Other theorists have figured in recent years that dark
matter was integral to galaxy formation.
"Maybe
the bulge was just serving as a proxy for the dark matter mass," Satyapal
said. So the amount of dark matter is "the real determining factor behind
the existence and mass of a black hole in a galaxy's center."
The study
will be published in the April 10 issue of the Astrophysical Journal.
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