An intensive study of a neighboring dwarf galaxy has surprised
astronomers by showing that most of its molecular gas — the
raw material for new stars — is scattered among clumps in the
galaxy’s outskirts, not near its center as they expected.

“This tells us that the galaxies we call dwarf irregulars
are even more irregular than we thought,” said Fabian Walter,
of the
National Radio Astronomy Observatory (NRAO) in
Socorro, NM. “Our new work also shows that these galaxies
probably are useful ‘laboratories’ for studying how stars
were formed when the Universe was young,” Walter added.

Walter worked with Christopher Taylor of the
University of Massachusetts
and Nick Scoville of
Caltech. The
scientists presented their results at the American
Astronomical Society’s meeting in Seattle, WA.

Using the millimeter-wave interferometer at Caltech’s

Owens Valley Radio Observatory, the astronomers combined
15 smaller images into a single mosaic to produce an
image showing the location of Carbon Monoxide (CO) gas
throughout a galaxy called IC 10, some 2.5 million
light-years away. IC 10 is one of the Local Group of
galaxies of which our own Milky Way is part. The
telescope system was tuned to a frequency near 115
GigaHertz, where the CO molecule naturally emits radio
waves.

“We found the clumps of CO gas far from the galaxy’s
center, and not near the regions of current star formation,”
Walter said. “This tells us that stars may, in fact, form
way out there in the outskirts of the galaxy, where we
didn’t expect,” he added.

Most of the galaxy’s gas is atomic Hydrogen, composed of
single Hydrogen atoms. Most of the galaxy’s molecular gas
is composed of Hydrogen molecules with two atoms each.
Atomic Hydrogen can be seen with radio telescopes because
it naturally emits at a radio frequency of 1420 MegaHertz.
However, cold molecular Hydrogen cannot be observed with current
telescopes. Instead, astronomers look for CO, which emits
at several radio frequencies, and then estimate the amount
of molecular Hydrogen based on how much CO they see.

Based on the new observations of CO, the astronomers
concluded that IC 10 has much less molecular gas than
previously thought and apparently has a much smaller
percentage of molecular gas than our Milky Way. The
astronomers add that dwarf galaxies in general are found
to have less of the heavy elements than larger, spiral
galaxies. They are thus probably more similar to galaxies
in the early Universe when there had been less time for
stars to produce the heavy elements and then return them
to their surroundings through supernova explosions.

Studies of a dwarf irregular galaxy like IC 10 therefore
give astronomers new insights about how stars formed in the
distant past. In addition, many astronomers believe dwarf
galaxies are the “building blocks of the Universe,” from
which larger galaxies were assembled through mergers.

“The beauty of this is that dwarf irregulars are the most
numerous type of galaxy, and many, like IC 10, are relatively
nearby. That means we can learn about star formation in
such extreme environments by studying nearby dwarf galaxies.
That’s fortunate, because we cannot observe extremely distant
galaxies with sufficient detail,” Walter said.

Studies of molecules in galaxies also will benefit from
the completion of the Atacama Large Millimeter Array (ALMA),
an international millimeter-wave telescope project to be
located in the high plains of northern Chile. With ALMA,
astronomers will be able to study galaxies with greater
detail and sensitivity to learn more about the nature
of the building blocks of the Universe.

Research with the Owens Valley Radio Telescope, operated
by the
California Institute of Technology, is supported by
NSF grant AST96-13717. The
National Radio Astronomy Observatory is a facility of the
National Science Foundation,
operated under cooperative agreement by

Associated Universities, Inc.