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 Observatory of 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 workingto 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.
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. 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.
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.
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![330 MHz [1 meter wavelength] image](gc-1m-300.jpg){kind=link}
![74 MHz [4 meter] image](gc-4m-300.jpg){kind=link}