Astronomers Opening a New Window on the Universe
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Recent technological advances are about to open one of the most poorly explored areas of astronomy, providing scientists with critical new insights about objects such as galaxy clusters, pulsars, and supernova explosions and perhaps about extrasolar planets and the first stars and black holes ever to have formed in the Universe, according to astronomers at the Naval Research Laboratory in Washington, DC. The scientists are planning a next-generation, long wavelength radio telescope that will remove certain technical obstacles to provide unique information about celestial objects.
"With our new telescope, the Long Wavelength Array (LWA), we will be opening an entirely new window on the Universe," said Dr. Namir Kassim, a radio astronomer in NRL's Remote Sensing Division, in a presentation to the 205th meeting of the American Astronomical Society in San Diego, CA. The plans will be carried out by a collaboration of astronomers at NRL, the University of New Mexico (UNM), the Applied Research Laboratories of the University of Texas at Austin (ARL:UT), and the Los Alamos National Laboratory (LANL).
Ironically, the wavelengths for which the LWA is being designed to work, between 15 and 3.75 meters (or 20 and 80 Megahertz [MHz]) are the wavelengths at which the first radio astronomy observations occurred. Karl Jansky made the discovery of radio emission from celestial objects in 1932 at the wavelength of 15 meters (or frequency of 20 MHz). Long wavelength radio astronomy in the 1950s and 1960s produced landmark discoveries responsible for much of modern astrophysics, such as quasars and pulsars.
However, in their quest to make more detailed radio images, astronomers soon moved to shorter wavelengths, where technical factors produced much better results. That has left the longer wavelength radio emissions as a largely unexplored view of the Universe. Now, a series of new observing and analysis techniques allow the long wavelength radio region to become a productive observational target again.
The primary difficulty in producing detailed images at these wavelengths, says Dr. Joseph Lazio, an NRL astronomer, has been the effect of Earth's ionosphere, a region of charged particles between about 50 and 600 miles above the surface. The ionosphere, which can "bend" radio waves to produce long-distance reception of AM and short-wave radio signals also causes distortions in radio-telescope
images in much the same way that atmospheric irregularities cause twinkling of stars and distortions in images produced by ground-based visible-light telescopes. In addition, human-generated radio interference and the computational requirement to produce images from long wavelength radio telescopes have posed further challenges.
Radio astronomers began tackling these difficulties during the 1980s and 1990s, applying new technical advances as they became available. These efforts culminated in a 4-meter wavelength (or 74-MHz) receiving system built by NRL and installed on the National Science Foundation's Very Large Array (VLA) radio telescope at the National Radio Astronomy Observatory in New Mexico. A similar system has been deployed on the Giant Metrewave Radio Telescope (GMRT) in India.
"This current generation of long wavelength radio telescopes is revolutionary. For the first time we are able to obtain high-quality images of the sky at these wavelengths," Kassim said. "However, they are only the first, baby steps towards exploring this wavelength regime. New and more powerful long wavelength telescopes are needed." He continued, "Throughout history, great discoveries in astronomy have been catalyzed by technological breakthroughs that enable the opening of new regions of the electromagnetic spectrum. The LWA will capitalize on the technological breakthrough in ionospheric removal first demonstrated at the 74 MHz VLA, and state-of-the-art computers to explore the last of these regions that can be observed from the Earth's surface, and hence the discovery potential is very high."
"Jansky's work resulted from a telecommunications revolution early in the last century; we are using the 21st-century telecom and computer revolutions to return to the roots of radio astronomy and explore this here-to-fore abandoned, but fertile field," added Lazio.
The radio astronomer's efforts have been motivated by the both unique and complementary astronomical information that long radio wavelengths offer. Detection of sources such as distant galaxies containing supermassive black holes in their cores, rapidly spinning pulsars, and possibly planets in other solar systems can be optimized at long wavelengths. Coupled with X-ray observations, long wavelength observations will provide insights into the debris of massive star explosions called supernova remnants, and the effects of Dark Matter and Dark Energy on the evolution of clusters of galaxies. Coupled with gamma-ray observations, long wavelength observations will improve our knowledge of the distribution and origin of the enigmatic cosmic rays in our Milky Way Galaxy and may provide observations of the formation of the first stars in the Universe.
One of the best recent examples of the re-emergence of long wavelength radio astronomy has been images of powerful radio-emitting galaxies such as Hydra A. Sitting near the middle of a large cluster of galaxies, Hydra A has long been studied at shorter radio wavelengths. However, recent long wavelength radio observations reveal that the object is significantly bigger than recognized previously. A central supermassive black hole is thought to power the radio emission seen in Hydra A and similar galaxies. The much larger size of Hydra A means that supermassive black holes may have a much larger impact on their surroundings than has been recognized previously. Moreover, Hydra A is brighter at longer wavelengths than at shorter wavelengths, suggesting that long wavelength radio observations may be an efficient way to find the first, most distant radio galaxies in the Universe.
While spectacular, these pioneering VLA and GMRT efforts only scrape the surface of the potential capabilities of long wavelength radio astronomy. Both the VLA and the GMRT combine a relatively small number of telescopes (about 30) over a small area (about 30 km [18 mi.]) to produce their images. The LWA will employ more telescopes (approximately 50) over a much larger area (about 400 km [250 mi.]) to produce more detailed and sensitive images.
One of the key requirements for the LWA will be to identify a relatively unpopulated region of the country with enough space to accommodate the many telescopes. With scientists at UNM, ARL:UT, and LANL, the NRL astronomers are working to find telescope sites in the southwestern U.S. These four institutions represent the newly formed Southwest Consortium. A key goal of the Southwest Consortium is to construct and operate the LWA in such a way as to engage students and researchers in the academic community and enhance university-based radio astronomy in the U.S.
Basic research in radio astronomy at NRL is supported by the Office of Naval Research. For additional information and 600 dpi images, see http://lwa.nrl.navy.mil/.
|Images of Hydra A (3C 218), a giant radio galaxy near the middle of a large cluster of galaxies. At short radio wavelengths (6 centimeters or 4635 MHz; left) the radio galaxy appears fairly small, with an extent of about 1.5 arcminutes (or about 1/20 the diameter of the full Moon). At long radio wavelengths (4 meters or 74 MHz; right), the size of the radio galaxy is substantially larger, about 8 arcminutes or nearly 5 times larger, with extensive outer radio lobes are revealed. The radio emission results from highly relativistic electrons, moving at nearly the speed of light, which are ejected from the central part of the radio galaxy, presumably in the local environment of a supermassive black hole. The long wavelength radio observations clearly provide a more accurate measure of the amount of material ejected from central regions of the galaxy. In turn, this provides a means of measuring the influence of the supermassive black hole upon its environment and the interaction of the radio galaxy with the cluster of galaxies in which it is located.|
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