A billion miles out of a gallon of gas
- From: "Uno" <uno@xxxxxxx>
- Date: Tue, 25 Apr 2006 15:46:44 GMT
(SPACE.com) -- A new study finds that supermassive black holes, located at
the heart of some galaxies, are the most fuel efficient engines in the
universe.
"If you could make a car engine that was as efficient as one of these black
holes, you could get about a billion miles out of a gallon of gas," said
study team leader Steve Allen of the Kavli Institute for Particle
Astrophysics and Cosmology at Stanford University.
"In anyone's book, that would be pretty green."
The finding, made using NASA's Chandra X-ray Observatory and announced in a
media teleconference today, is giving scientists insights into how
supermassive black holes generate energy and how they affect the galaxies
where they make their homes.
'Quiet' black holes
Black holes are regions of space where gravity is so strong that matter and
light can't escape once they pass the event horizon, a spherical boundary
surrounding the black hole.
However, inflowing matter that hasn't yet passed this point of no return
can -- through friction and interaction with the black hole's strong
magnetic field -- release energy in the form of either diffuse light or
focused jets of energy.
"Once gas comes within a distance about a million times larger than the
event horizon of the black hole, it becomes gravitationally captured," Allen
explained. "At this point the gas becomes fuel for the black hole engine."
The new study looked at nine supermassive black holes at the centers of
elliptical galaxies; each one was about a billion times more massive than
our sun. The black holes were relatively old and generated much less energy
than the fiercely luminous and rapidly growing supermassive black holes
known as "quasars."
The researchers found that these "quiet" black holes released about 1,000
times more energy as jets than as light. The reasons for this are still
unclear.
"That's a mystery, how these black holes selectively put that much energy
into the jets without producing much light," study team member Christopher
Reynolds from the University of Maryland told SPACE.com.
Space bubbles
Most of the energy in the jets is being emitted as radio waves, but in at
least one of the black holes studied, the energy was in the form of more
energetic X-rays.
"The energy in these jets is absolutely huge, about a trillion trillion
trillion watts," Allen said.
As they race outwards from their parent black holes at nearly light speed,
the jets carve out enormous cavities, or "bubbles," in the surrounding gas
environment; some of these bubbles can be tens of thousands of light years
across.
Bubbles can also form in the aftermath of stellar explosions called
supernovas; our own solar system is enveloped by such a structure, called
the "Local Bubble," which was formed during an explosion long ago.
The researchers used these bubbles to figure out the fuel efficiency of the
black holes. Using Chandra images, they first calculated how much fuel in
the form of gas was available to each black hole. They then estimated the
power required to produce the bubbles that were observed.
Preventing galactic sprawl
The finding could have implications for other types of black holes as well,
including much smaller, stellar-mass black holes, the researchers say.
"We already knew that powerful quasars are very efficient at making light.
Now we know that black holes in elliptical galaxies are efficient as well,"
said Kim Weaver, a researcher at NASA's Goddard Space Flight Center who was
not involved in the study. "This suggests that being green is a trait that
all black holes may have in common."
The scientists think the supermassive black holes are green in another way,
too. The energy that each black hole emits as jets warms the surrounding
environment. This prevents gas from cooling and coalescing into billions of
new stars and places an upper limit on how large a galaxy can grow.
"In an environmental sense, the black holes are actually preventing galactic
sprawl from taking over the neighborhood," Weaver said.
The study will be published in an upcoming issue of the Monthly Notices of
the Royal Astronomical Society.
.
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