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Space mission yields most detailed views of quasars
Posted: Jan. 18, 2000

Astronomers from around the world will gather in Japan later this week to present the most detailed images of quasars ever seen, produced with data from the Very Long Base Interferometry Space Observatory Program. Space VLBI, as this is known, is a new type of astronomy mission that uses a combination of satellite- and Earth-based radio antennas to create a telescope more than two-and-a-half times the diameter of the Earth.

As the largest astronomical instrument ever built, Space VLBI has given astronomers one of their sharpest views yet of the universe.

Astronomers plan to present a number of stunning new radio images in a January 19-21 symposium at Japan's Institute of Space and Astronautical Science (ISAS) near Tokyo. Many of these images depict quasars whose radio emission has traveled billions of light years to reach Earth.

"These images probe some of the most distant, ancient, and energetic objects in the universe, giving us a glimpse of quasars as they existed billions of years ago," said Dr. Robert Preston, U.S. Space VLBI project scientist at NASA's Jet Propulsion Laboratory (JPL), Pasadena, Calif. "These powerful objects exist at the center of many galaxies, including our own familiar Milky Way, which contains a very weak version of a quasar."

By combining data from the orbiting radio telescope Halca with radio telescopes on Earth, the space VLBI mission produces radio images with the same sharpness (although not the same sensitivity) as a single radio telescope the size of the Halca orbit. Photo: ISAS

Launched in February, 1997 by Japan's ISAS, Space VLBI uses a technique called interferometry that electronically links widely separated telescopes so that they work together as a single instrument with extraordinarily sharp "vision" or resolving power. By taking this technique into space for the first time, astronomers have approximately tripled the resolving power previously available with only ground-based telescopes. "The Space VLBI satellite system has more than 100 times greater resolving power in radio frequencies than the Hubble Space Telescope has at optical wavelengths." said Preston. "In fact, its resolving power is equivalent to being able to read a newspaper headline in Tokyo all the way from Los Angeles."

Quasars are enormously bright point-like optical objects, often shining with an intensity many hundreds of times brighter than that of an entire galaxy. However, they are so distant that they appear only as very faint points of light to optical telescopes on Earth. Scientists believe that quasars are powered by gases such as remnants of stars spiraling into black holes at the centers of galaxies. Black holes are so massive that no light or matter can escape from their immensely strong gravity and, in the case of quasars, they can have masses that are millions to billions of times that of our own sun. Although most in-rushing matter is captured forever by the black hole, some of the material is likely ejected at enormous speeds to form the observed narrow radio-emitting jets. By studying these jets, which are usually visible only at radio frequencies, astronomers hope to learn more about the black holes that power them.

  Gas spiraling
Artist's conception of gas spiraling into a massive black hole at the center of a galaxy. Photo: NASA

Key results from Space VLBI include clearly resolved individual components in the observed quasars' jets. Perhaps the most significant single result of the Space VLBI mission so far is the detection of a number of radio sources associated with quasars that are intrinsically brighter than theory generally allows for a stationary source. However, a bizarre prediction of Einstein's theory of relativity is that radiation from an object moving at near light speed will be beamed in the direction of motion. Therefore, rather than looking equally bright from all directions (like a light bulb), the source looks much brighter if it is moving rapidly toward us (like looking into the beam of a flashlight). This effect allows some sources to appear much brighter than they really are, solving the conflict between the observed and theoretically allowed brightness of the radio- emitting quasars. As a consequence, the recent observations imply that the radio-emitting plasma in these sources is actually moving toward us at nearly the speed of light in accordance with Einstein's prediction. While astronomers had suspected this outcome for many small, bright, radio sources, Space VLBI observations help prove that this idea is correct.

Fine details revealed by Space VLBI images have also been combined with observations of the same objects in other parts of the spectrum (such as infrared, optical, ultraviolet, X-ray, and gamma ray). For example, the recently launched Chandra X-ray telescope detected bright X-ray emission from the core of a distant quasar named PKS 0637-752, as well as a very unexpected source of X-ray emission coming from part of the quasar's radio jet. Space VLBI observations show the intricate radio structure in the core of this quasar with a thousand times finer detail, and measure the speed of material in the radio jet by comparing images made at different times. Knowing the speed of the jet allows astronomers to define better the physical processes responsible for generating the X-ray emission.

Virgo A
A radio image of the resulting jet as observed by the space VLBI mission. The VLBI image of the galaxy Virgo A was produced by Bill Junor (NRAO), John Biretta (STScI), Frazer Owen (NRAO), and Mitch Begelman (University of Colorado).

Not all space VLBI observations have been of very distant objects. Space VLBI observations have also helped determine the size and shape of an extremely bright radio source in a nearby star-forming region of the constellation Orion. These observations indicate that the intense, narrow-band, radio emission from water molecules in the star-forming region comes from areas with strong magnetic fields.

Space VLBI is part of a major international undertaking. Led by Japan's ISAS, the VLBI Space Observatory Program enables about 40 Earth-based radio telescopes from more than 15 countries to co-observe with the space VLBI satellite. The network spans the globe, in the northern hemisphere from the United States to Europe to Asia, and in the southern hemisphere from eastern Australia to South Africa. More than 70 scientists associated with collaborating institutions are expected to attend the conference at ISAS.

NASA's Jet Propulsion Laboratory, Pasadena, CA, a division of the California Institute of Technology, manages the U.S. portion of the Space VLBI international consortium on behalf of NASA's Office of Space Science, Washington, D.C.

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