Astronomers have made the first tentative observations of a long-
speculated, but never before detected, source of natural radio waves in
interstellar space. Data from the National Science Foundation’s 140 Foot
Radio Telescope at the National Radio Astronomy Observatory in Green Bank,
W.Va., show the faint, tell-tale signals of what appear to be dust grains
spinning billions of times each second. This discovery eventually could
yield a powerful new tool for understanding the interstellar medium —
the immense clouds of gas and dust that populate interstellar space.
“What we believe we have found,” said Douglas P. Finkbeiner of Princeton
University’s Department of Astrophysics, “is the first hard evidence for
electric dipole emission from rapidly rotating dust grains. If our studies
are confirmed, it will be the first new source of continuum emission to
be conclusively identified in the interstellar medium in nearly the past
20 years.” Finkbeiner believes that these emissions have the potential
in the future of revealing new and exciting information about the
interstellar medium; they also may help to refine future studies of the
Cosmic Microwave Background Radiation.
The results from this study, which took place in spring 1999, were
accepted for publication in Astrophysical Journal. Other contributors
to this paper include David J. Schlegel, department of astrophysics,
Princeton University; Curtis Frank, department of astronomy, University
of Maryland; and Carl Heiles, department of astronomy, University of
California at Berkeley.
“The idea of dust grains emitting radiation by rotating is not new,”
comments Finkbeiner, “but to date it has been somewhat speculative.”
Scientists first proposed in 1957 that dust grains could emit radio
signals, if they were caused to rotate rapidly enough. It was believed,
however, that these radio emissions would be negligibly small — too
weak to be of any impact to current radio astronomy research, and the
idea was largely forgotten.
In the 1990s this perception began to change when scientists and
engineers designed sensitive instruments to detect the faint afterglow
of the Big Bang, which is seen in the Universe as the Cosmic Microwave
Background Radiation. While making detailed maps of this faint and cold
radiation, scientists also detected signals at approximately the same
wavelength and intensity as the background radiation, but clearly
emanating from within the Milky Way’s galactic plane. The researchers
expected to detect some emission from the Milky Way, but what they
encountered was much brighter than anticipated.
This discovery caused some concern among researchers because of the need
to have a very clear “window” on the Universe to study the background
radiation in great detail. If there were a source of radio emission in
our own galactic “back yard,” then studies of the microwave background
radiation would need to recognize these emissions and correct for them.
“We want to be clear, however, that nothing we have found invalidates
the current interpretation of the Cosmic Microwave Background
Radiation,” assured Finkbeiner. “Nobody has done anything wrong in
neglecting these signals — so far.”
Scientists considered several plausible mechanisms for this anomalous
emission, but these theories failed to explain the observed spatial
distribution of this emission across the sky. This predicament prompted
theorists to rethink the spinning dust idea, leading to a 1998 model by
Bruce Draine (Princeton University) and Alex Lazarian (University of
Wisconsin), which proposed rotational dust-grain emission as an
important mechanism. Draine and Lazarian assumed that small dust grains,
perhaps having no more than 100 atoms each, would populate many
interstellar dust clouds in the Galaxy. Each grain would have a small
electric dipole and would therefore react to the charged ions that race
through the clouds at tremendous speeds. As an ion either strikes or
passes near a dust grains, the grain would “spin up,” reaching speeds
of up to one trillion revolutions per minute, causing it to radiate.
The rate of rotation of these dust grains directly correlates to the
frequencies at which they radiate. For example, a dust grain rotating
10 billion times each second would emit radio waves at 10 gigahertz
(GHz).
In looking for this elusive signal, the researchers narrowed their
search to 10 dust clouds within the Milky Way Galaxy. These specific
clouds were selected because their location and properties would help
to eliminate other possibilities for these emissions. “Our goal was to
find those areas within the Milky Way Galaxy that would help us rule
out other sources of emission,” said Finkbeiner. “By selected these
specific targets, we believe that the signals we received are very
indicative of rapidly rotating dust grains.”
The researchers emphasize, however, that additional observations will
be required to confirm their results, and other potential emission
mechanisms have not been ruled out. Particularly, it is possible that
a portion of this radiation is due to the presence of ferro-magnetic
minerals within the dust grains. Additional studies with more
sensitive equipment will be necessary to confirm these results
conclusively.
“What we think is the most intriguing, however,” said Finkbeiner, “is
that with further advances in radio astronomy, the faint emissions from
rotating dust grains may reveal previously unknown details about the
dynamics of the interstellar medium. By detecting and understanding
this emission we also hope to give astronomers a tool to greatly refine
future studies of the Cosmic Microwave Background Radiation.”
The NSF’s 140 Foot Radio Telescope now is decommissioned after a long
and highly productive career. Research will continue on the newly
commissioned Robert C. Byrd Green Bank Telescope, which is the world’s
largest fully steerable radio telescope.The National Radio Astronomy
Observatory is a facility of the National Science Foundation, operated
under cooperative agreement by Associated Universities, Inc.
