archive: SETI [ASTRO] Nearby Quasars Result From Galactic Encounters, VLA

SETI [ASTRO] Nearby Quasars Result From Galactic Encounters, VLA

Larry Klaes ( )
Wed, 30 Dec 1998 11:59:20 -0500

>X-Authentication-Warning: majordom set sender
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>Date: Wed, 30 Dec 1998 16:25:54 GMT
>From: Ron Baalke <>
>Subject: [ASTRO] Nearby Quasars Result From Galactic Encounters, VLA
Studies Indicate
>Reply-To: Ron Baalke <>
>National Radio Astronomy Observatory
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>Socorro, New Mexico 87801
>Dave Finley, Public Information Officer
>(505) 835-7302
>FOR IMMEDIATE RELEASE: December 29, 1998
>Nearby Quasars Result From Galactic Encounters, VLA Studies Indicate
>Astronomers using the National Science Foundation's Very Large Array (VLA)
>radio telescope have found previously unseen evidence that galaxy collisions
>trigger energetic quasar activity in relatively nearby galaxies. New radio
>images of galaxies with bright quasar cores show that, though the galaxies
>appear normal in visible-light images, their gas has been disrupted by
>encounters with other galaxies.
>"This is what theorists have believed for years, but even the best images
>from optical telescopes, including the Hubble Space Telescope, failed to
>show any direct evidence of interactions with other galaxies in many cases,"
>said Jeremy Lim, of the Academia Sinica Institute of Astronomy &
>Astrophysics in Taipei, Taiwan. Lim, along with Paul Ho of the
>Harvard-Smithsonian Center for Astrophysics in Cambridge, MA, reported their
>findings in the January 1 issue of Astrophysical Journal Letters.
>Quasars are among the most luminous objects in the universe, and generally
>are believed to be powered by material being drawn into a supermassive black
>hole at the center of a galaxy, releasing large amounts of energy.
>Many quasars are found at extremely great distances from Earth, billions of
>light-years away. Because the light from these quasars took billions of
>years to reach our telescopes, we see them as they were when they were much
>younger objects. These distant quasars are thought to "turn on" when the
>host galaxy's central black hole is "fueled" by material drawn in during an
>early stage of the galaxy's development, before the galaxy "settles down" to
>a more sedate life.
>However, other galaxies with quasar cores are much closer, and thus are
>older, more mature galaxies. Their quasar activity has been attributed to
>encounters with nearby galaxies -- encounters that disrupt material and
>provide new "fuel" to the black hole. The problem for this scenario was the
>lack of evidence for such galactic encounters in optical images of many
>nearby quasars.
>"Our VLA studies are the first to image the neutral atomic hydrogen gas in
>nearby quasar galaxies," said Ho. "This is important, because, in any
>galactic encounter, the gas is more easily disrupted than the stars in the
>galaxies, and the gas takes longer to return to normal after the encounter.
>This means we have a better chance of finding evidence of galactic
>encounters by imaging the gas using radio telescopes."
>The VLA can image the gas in such galaxies because it is particularly
>sensitive to the radio waves naturally emitted by hydrogen atoms.
>The researchers chose three quasars at distances of 670 million to 830
>million light-years. The three galaxies surrounding these quasars had
>different appearances in optical images: one showed evidence of mild
>interaction with a neighboring galaxy; one appeared undisturbed but had a
>nearby neighbor; and the third appeared undisturbed and alone. When imaged
>with the VLA, all three showed strong evidence that their gas had been
>disrupted by an encounter with another galaxy.
>"This shows how well such radio images of the gas distribution in galaxies
>can reveal evidence of galactic interactions," Lim said. "We hope to make
>further studies and learn more about how these galaxy mergers actually
>stimulate the quasar activity."
>Quasars are among the most enigmatic objects in the universe. Though they
>appear on photographic plates made by astronomers more than a century ago,
>they looked like ordinary stars, and raised no curiosity. When radio
>telescopes were first used to make detailed maps of the sky in the 1950s,
>many strong sources of radio emission seemed to have no counterparts in
>visible light. In 1960, one of these bright radio-emitting objects was
>identified as a faint, bluish-looking "star" by astronomers using the
>200-inch telescope on Palomar Mountain in California.
>That first quasar and others identified later puzzled astronomers because,
>when their light was analyzed to find the characteristic "signature" of
>emission at specific wavelengths shown by particular atoms, the pattern was
>at first indecipherable. In 1963, Maarten Schmidt of Caltech realized that
>the pattern made sense if the light's wavelength had been shifted through
>the Doppler effect by the object's motion away from Earth at greater
>velocities than had yet been seen.
>Because the universe is expanding, objects are moving away from Earth with
>greater speed at greater distances. The speeds seen in the quasars indicated
>that they were the most distant objects yet found, and, because they appear
>bright even at those great distances, must be extremely energetic.
>The idea that the tremendous amounts of energy released by quasars results
>from material being drawn into a black hole at the center of a galaxy
>quickly rose as the leading explanation. Galactic interactions were first
>proposed as an explanation for nearby quasar activity in 1972. Today,
>quasars are thought to be one of several types of active galactic nuclei,
>all of which are powered by central black holes.
>The VLA is an instrument of the National Radio Astronomy Observatory, a
>facility of the National Science Foundation, operated under cooperative
>agreement by Associated Universities, Inc.
>A combined optical-radio image of the quasar IRAS 17596+4221 and a companion
>galaxy. The orange areas are the hydrogen gas imaged by the VLA. In the
>optical image, there is no direct evidence for an interaction between the
>galaxy hosting the quasar and the companion galaxy. The extensions in the
>hydrogen gas, however, are a clear indication of disruption resulting from
>an interaction between the two galaxies.
>Credit: Jeremy Lim and Paul Ho; National Radio Astronomy Observatory and
>Associated Universities, Inc.