Presented at the American Astronomical Society Meeting Albuquerque, NM on June 5, 2002
High-resolution visuals available below.
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 yielding unprecedented images of the first stars and galaxies ever formed in the Universe, according to astronomers at the Naval Research Laboratory (NRL) in Washington, DC. The scientists are planning a next-generation, low-frequency radio telescope that will remove certain technical obstacles to provide unique information about celestial objects.
"With our new telescope, called the Low Frequency Array (LOFAR), we will be opening an entirely new window on the Universe," said Dr. Namir Kassim, a radio astronomer in NRL's Remote Sensing Division, and Dr. Joseph Lazio, also an NRL astronomer, in a presentation to the 200th meeting of the American Astronomical Society in Albuquerque, NM. The two represent a consortium of astronomers at NRL, the Haystack ObservatohXXXf the Massachusetts Institute of Technology (MIT/HO), and the Netherlands Foundation for Research in Astronomy (ASTRON).
Ironically, the radio frequencies at which LOFAR is being designed to work, between 10 and 240 Megahertz (MHz), are the frequencies where the first radio astronomy observations occurred. Karl Jansky made the first discovery of radio emission from celestial bodies in 1932 at the frequency of 20 MHz. Low-frequency radio astronomy in the 1950s and 1960s produced the landmark discoveries of quasars and pulsars.
However, in their quest to make more detailed, or higher-resolution images, radio astronomers soon moved to higher radio frequencies, where technical factors produced much better results. That has left the lower-frequency radio emissions as a largely unexplored area of research. Now, a combination of new analysis techniques and the explosion of computing power are allowing the low-frequency radio region to again become a productive observational target.
The primary difficulty in producing high-resolution images at these frequencies, say the scientists 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, causes distortions in radio-telescope images in much the same way that atmospheric turbulence causes twinkling of stars and distortions in images produced by ground-based visible-light telescopes. In addition, human-generated radio interference and the huge computational requirements of producing images from low-frequency 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 74-MHz receiving system built by NRL and installed on the National Science Foundation's Very Large Array (VLA) radio telescope in New Mexico, and in the Giant Metre-wave Radio Telescope (GMRT) in India.
"This current generation of low-frequency radio telescopes is revolutionary. For the first time we are able to obtain high-quality pictures of the sky at these frequencies," Kassim said.
This success led the astronomers to conclude that the advances in computing power and consumer electronics enable them to build a next-generation low-frequency radio telescope that can produce much higher-quality images at these frequencies. The new technologies also allow this telescope to be built at a relatively low cost.
"Jansky's work resulted from a telecommunications revolution early in the last century; we are using the 21st-century telecom revolution to return to the roots of radio astronomy," Lazio said.
Their efforts have been motivated by the both unique and complementary astronomical information that low radio frequencies offer. Detection of sources such as distant galaxies, rapidly spinning pulsars, and possibly planets in other solar systems can be optimized at low frequencies. Coupled with X-ray observations, low frequency observations will provide important insights into clusters of galaxies and massive star explosions called supernova remnants; coupled with gamma-ray observations, low-frequency observations will improve our knowledge of the distribution and origin of high-energy cosmic rays in the Galaxy. Low-frequency observations may provide our first pictures of the first stars and galaxies in the Universe.
One of the best recent examples of the re-emergence of low-frequency radio astronomy has been low-frequency images of the Milky Way Galaxy's center. These spectacular images represent the state of the art in low frequency imaging today. They not only inspire the imagination of scientists and non-scientists alike, but also are proving scientifically valuable by uncovering a variety of new and exotic Galactic center sources. These include supernova remnants and new nonthermal filaments, mysterious objects whose true nature is not known even 20 years after their initial discovery.
While spectacular, these pioneering VLA and GMRT efforts only scrape the surface of the potential capabilities of low-frequency 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. LOFAR will employ many more telescopes (approximately 100) over a much larger area (about 300 km [180 mi.]) to produce much higher fidelity images. A key aspect of LOFAR will be identifying a region with enough space to accommodate the many telescopes. A consortium of universities in the Southwestern US, led by the University of New Mexico, is working to identify telescope sites in the Southwestern US and is preparing a bid to host LOFAR. Other bids to host LOFAR are expected from organizations in The Netherlands and Australia. It is planned that LOFAR will become operational in 2006.
Basic research in radio astronomy at the NRL is supported by the Office of Naval Research. For additional information see
Low Frequency Array
Forever hidden behind a thick veil of dust and gas, the center of our Milky Way Galaxy cannot be seen in the visible light that our eyes see. In order to study the center of our Galaxy, astronomers must turn to other wavelengths of light, like radio. These panoramic views of the Galactic center are at radio frequencies of 330 and 74 MHz, respectively, and were produced by Michael Nord (University of New Mexico/Naval Research Laboratory) and collaborators at NRL and the National Radio Astronomy Observatory.
The concentration of sources along a diagonal line through the images reveal the disk-like shape of the Milky Way viewed edge-on.
330 MHz [1 meter wavelength] image [http://www.pao.nrl.navy.mil/rel-02/gc-1m-300.jpg (1.56MB)] The most prominent source in the image is Sgr A. (Its name derives from the fact that the Milky Way's center is in the direction of the constellation Sagittarius, abbreviation Sgr.) Deep within Sgr A is the source Sgr A*, which astronomers have identified as being a black hole with a mass millions of times that of the Sun.
Other sources include prominent regions of star formation where hot young stars are heating the surrounding gas, and supernova remnants, the remains of massive explosions after hot stars run out of fuel. Within the debris of a supernova remnant are high-speed electrons spiraling around magnetic fields. In addition, this spiraling or synchrotron radiation seems to be responsible for a collection of enigmatic sources known as the Galactic center arc, filaments, and threads. The true nature of these filamentary structures remains a mystery, though it is clear that their emission, orientation, and structure provide important clues to the magnetic field in the Galactic center.
74 MHz [4 meter] image [http://www.pao.nrl.navy.mil/rel-02/gc-4m-300.jpg (471KB)] This image is marked by "dark "patches, resulting, surprisingly enough, from star formation regions. The total brightness of the synchrotron radiation from other sources in the Milky Way Galaxy (including the Galaxy itself) is more than that of the regions of the star formation, so the star formation regions appear dark. By determining the amount of contrast between the star formation regions and their surroundings, astronomers can probe the synchrotron radiation coming from the Galaxy itself.