LONG BEACH,
Calif. - Supermassive black holes are thought to lurk at the heart of
essentially all galaxies bigger than our own. Their powerful gravity should be luring
in galactic matter, feeding the black holes' voracious appetites.
However, while
plenty of gas is available for these black
holes to feast upon, few of them have been observed to actively
accrete gas from their home galaxy, presenting astronomers with a puzzle as
to why these black holes aren't eating. Something must be preventing the black
holes from accreting gas, though no one has known exactly what that was.
"This
has been a longstanding problem," said Q. Daniel Wang of the University of
Massachusetts at Amherst.
Now, Wang
and his colleagues have some possible suspects behind the starving black holes:
exploding stars, or supernovas.
Wang and
his team investigated the starvation of the supermassive black holes at the
center of two galaxies, M31 (aka the Andromeda Galaxy, our nearest galactic
neighbor) and NGC 5866. They presented their findings here this week at the
213th meeting of the American Astronomical Society.
With both
of these galaxies, and others, the precise clue that the galaxies aren't
feeding is the lack of large amounts of radiation coming from the nucleus of
the galaxy, which astronomers would expect to detect from an actively eating
black hole. What makes the lack of radiation most perplexing is that plenty of
gas should be expelled by older stars and their remnants, such as planetary nebulas, and accumulating in
the galactic bulges of the galaxies (the tightly-packed group of stars found in
the center of galaxies, where the black hole resides).
What was
happening to that gas has been a mystery. Astronomers had surmised that the gas
"has to be removed continuously from the bulge," Wang said, otherwise
the black holes would be feasting on it.
Some
astronomers thought gravitational influences from nearby galaxies could be
sucking away the gas. But Wang's study suggests it's actually an internal
problem generated by powerful supernova explosions.
These
explosions occur when a massive star's core stops generating energy and
collapses in on itself, releasing energy that heats and expels the star's outer
layers - the star goes supernova.
Supernovas
come in slightly varying types.
Type 1a supernovas
are constantly exploding throughout a given galaxy. These exploding stars send
out a shockwave - what Wang calls an "interstellar tsunami" - that
propagates throughout the gas in the galaxy. Wang and his team simulated the
effect of these shockwaves on the gas accumulation around the galaxy's center.
The
interstellar tsunami works in a similar manner to tsunamis on Earth: The
shockwave generated by an earthquake below the ocean has little effect where
the ocean is deep and can absorb the energy, but when that wave reaches shallow
water, it forms the characteristic enormous wave that slams onto the coast.
Likewise,
the hot gas in the galaxy can absorb the supernova's shock, but when the wave
reaches the cool gas expelled by dying stars, it steepens and pounds the
central disk of the galaxy, evaporating the gas.
Because
these supernova explosions are happening all the time, they continue to pound
away at the disk; the gravity of a less massive galaxy can't counteract the
evaporating energy of the supernovae, so the gas can't accumulate, and the
black hole starves.
The
interruption of the gas accumulation also affects the evolution of the galaxy,
Wang noted.
More
massive galaxies, on the other hand, have a bigger gravitational pull that
keeps the gas from leaving.
"It's
a much more difficult escape," Wang told SPACE.com.
"Eventually gravity wins."
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