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
and
IMAGE CAPTIONS:
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.
