Energized Heartbeats May Explain Why Galaxies|
Are Continually Stirred Up, UCSD Researchers Say
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Article in Astrophysical Journal Letters is available at
Until now, astronomers haven't been able to offer a full explanation for
why the Milky Way and other galaxies produce new stars at a relative
snail's pace. While they have known for decades that high turbulence
keeps huge clouds of hydrogen gas from condensing into stars, they
have yet to identify all of the causes of the galactic perturbations.
In a report to be published in the November 20 issue of Astrophysical
Journal Letters, researchers at the University of California, San Diego
have discovered that a well-known, but overlooked source of
heating, regular outbursts of ultraviolet radiation from clusters of
very large, bright stars, may play a significant role in keeping the Milky
Way's gas continually stirred up.
In a rapid-release online publication of the report today, Alexei G.
Kritsuk, a visiting researcher at UCSD, and Michael L. Norman, a
professor of physics at UCSD and a senior fellow at UCSD's San Diego
Supercomputer Center, detailed their findings, which were made with a
sophisticated hydrodynamic computer simulation.
"The most massive and brightest stars in the spiral arms of our galaxy
emit lots of ultraviolet radiation in regular cycles," said Norman. As
clouds of cold, dense gas condense into very bright stars, the
ultraviolet radiation they emit dissipates the remaining clouds of gas,
which causes further star formation to slow considerably. "We think this
radiation may act as an important regulation mechanism, providing a
feedback effect from star-formation that may inhibit further star
formation," he said. Light with wavelengths shorter than the human eye can see is called
ultraviolet (beyond violet) light, and its energy is readily transferred
to gas clouds in galaxies. (The sun also produces ultraviolet light,
which is absorbed by the ozone layer in Earth's atmosphere or reflected
back into space.)
Astrophysicists agree that some form of energy must be supplied to gases
in the so-called interstellar medium of galaxies to sustain the
turbulence that astronomers have noted in beautiful images of the spiral
arms of the Milky Way and nearby galaxies. Researchers have identified a
variety of potential energy sources that in some situations could
maintain the turbulence, such as violent stellar explosions called
supernovae, or blasts of supersonic "winds" given off by massive newborn
"Astronomers have been thinking about these mechanical effects for
several decades, but not really considering the radiation effects, which
travel at the speed of light and exert a heating effect much farther
away than a supernova shock or a stellar wind," said Norman.
"Ultraviolet radiation may not be the complete answer to all the
galactic turbulence, but I'm confident that it plays some role in what
astronomers call the interstellar medium."
In a research paper published online this year at
http://lanl.arXiv.org/abs/astro-ph/0202196, a group of researchers led
by Venezuelan astrophysicist Antonio Parravano reported that large,
bright stars generate large, bursts of ultraviolet radiation on
timescales of about 10 million years. Norman and Kritsuk used
Parravano's findings as the starting point for their simulation studies.
"The ultraviolet radiation in a huge section of the Milky Way's disk
goes up by a factor of two to 10 and then, falls, and then repeats like
heartbeats," said Kritsuk. "Our numerical simulations show that this
time-dependent heating of the interstellar medium would be a good
driving force for the filaments and blobby sheets of gas actually
observed by astronomers in the regions of the galaxy where background
ultraviolet radiation is the main source of energy."
Kritsuk and Norman used supercomputers at the San Diego Supercomputer
Center at UCSD and the National Center for Supercomputing Applications
at the University of Illinois at Urbana-Champaign. The machines are
provided by the National Science Foundation to enable computationally
intensive approaches to astrophysical research by scientists at dozens
of colleges and universities in the United States.
Alexi G. Kritsuk
Professor Michael Norman
UCSD Media Contacts:
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Rex Graham (858) 822-5408;
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University of California-San Diego