Combining the best features of the National Science Foundation’s
(NSF) new
Robert C. Byrd Green Bank Telescope (GBT) in West Virginia
with those of the NSF’s
Very Large Array (VLA) in New Mexico,
astronomers have produced a vastly improved radio image of the
Orion Nebula and developed a valuable new technique for studying
star formation and other astrophysical processes.

“Our GBT image of the Orion Nebula is the best image ever produced
with a single-dish radio telescope and it illustrates the superb
performance of this new telescope,” said Debra Shepherd, of the
National Radio Astronomy Observatory (NRAO) in Socorro, NM. “By
combining data from the GBT with that from the VLA, we get an
image that reflects reality far better than images from the
separate telescopes could do,” she added. Shepherd worked with
Ron Maddalena from NRAO in Green Bank and Joe McMullin, from NRAO
in Socorro. The astronomers presented their work to the American
Astronomical Society meeting in Washington, DC.

Single-dish radio telescopes such as the GBT, dedicated in 2000,
are able to capture the large-scale structure of objects such as
the Orion Nebula. However, they are unable to discern the fine
detail revealed by multi-antenna arrays such as the VLA. Conversely,
a VLA-like array is “blind” to the larger-scale structures. Combining
the data from both types of radio telescopes to produce an image
showing both large- and small-scale structures in the same celestial
object has been a difficult, laborious task.

“We are developing new observing techniques and software to make this
task much easier and quicker,” said McMullin. “We now have achieved
in hours what used to take months or even longer to do, but we are
producing an observational tool that will allow astronomers to make
much higher-fidelity images that will greatly improve our understanding
of several important astronomical processes,” McMullin added. For this
observation, both the individual images from each telescope as well
as the combined image were produced using the
AIPS++ (Astronomical
Information Processing System) software, developed, in part, by
NRAO. The observers worked with Tim Cornwell, NRAO’s Associate
Director for Data Management, to develop the techniques used to
combine the images.

The Orion Nebula, easily visible in amateur telescopes, is a giant cloud
of gas some 1,500 light-years away in which new stars are forming. The
GBT-VLA radio image, Shepherd said, shows new details that will allow
scientists to better understand how ionized gas near the young, hot
stars at the nebula’s center flows outward toward the edge of the nebula.

The ability to produce combined GBT-VLA images also may revise scientists’
understanding of other objects. For example, says NRAO Director Paul
Vanden Bout, “Astronomers have seen many pockets of ionized Hydrogen gas
in star-forming clouds with the VLA that are thought to be ultra-compact.
It may be that they are, in fact, larger than thought and, using the GBT
in addition to the VLA will show us the true picture.”

The importance of this observing technique lies in its ability to
greatly improve the fidelity of images. “By fidelity we mean how closely
the image actually reflects reality. We now have a powerful new tool for
improving the fidelity of our images when we look at objects that are close
enough to appear relatively large in the sky but which also contain fine
detail within the larger structure,” Shepherd said. “This will have a big
impact on a number of research areas such as star formation in our Galaxy,
planetary nebulae, supernova remnants, as well as dynamics and star
formation in near-by galaxies,” she added.

The new technique also paves the way for effective use of the
Expanded
VLA, which will incorporate state-of-the-art electronics and digital
equipment to replace now-aging technologies dating from the VLA’s
construction in the 1970s. In addition, the new capabilities can be
used with the
Atacama Large Millimeter Array (ALMA), a millimeter-wave
observatory to be constructed in Chile as a partnership among North
American, European and Japanese astronomers.

The combined GBT-VLA image was produced from observations made at a
radio frequency of 8.4 GHz. The VLA observations were made in 2000 and
the GBT observations in November of 2001.

The
National Radio Astronomy Observatory is a facility of the
National Science Foundation, operated
under cooperative agreement by
Associated Universities, Inc.

Graphics to Accompany This Story

GBT Image of Orion Nebula
VLA Image of Orion Nebula
GBT+VLA Image of Orion Nebula