Observations of emission from the SMTO 10-m telescope on Mt. Graham, AZ. The CO emission is shown as white contours over top a false color image of H-alpha emission, which shows the locations of massive young stars. Photo: UMass

A team of researchers led by a University of Massachusetts astronomer has produced the first complete maps of carbon monoxide emission in the interstellar medium of a nearby dwarf irregular galaxy. These observations may provide a unique insight into the formation of stars in the early universe. Dwarf irregular galaxies lack an apparent structure or shape, and are significantly smaller than their counterparts.

The research was conducted by Christopher L. Taylor of the Five College Radio Astronomy Observatory (FCRAO) at UMass, along with Wilfred Walsh of the Max-Planck Institut in Bonn, Germany, and Susanne Huttemeister and Thomas Fritz of the University of Bonn, Germany. The team presented its findings on June 6 at the American Astronomical Society meeting in Rochester, N.Y.

"These observations will be important in understanding how galaxies form and evolve," said Taylor. "The light from distant galaxies can take billions of years to reach the Earth, and shows astronomers how those galaxies looked when the light first started its journey billions of years ago." Thus by looking to ever-more distant galaxies, astronomers look back in time to when the universe was young. The astronomers observed IC 10, a nearby dwarf irregular galaxy only 2.7 million light years (0.82 megaparsecs) distant.

"IC 10 will become a Rosetta stone, helping us interpret the observations of extremely distant galaxies which will become possible with the next generation of millimeter radio telescopes," said Taylor.

Most of the heavy elements in the universe, such as carbon, nitrogen, and oxygen, are created by fusion in the centers of stars. Galaxies in the early universe had not had enough time to create all the heavy elements that exist now. Unfortunately, these distant galaxies are also extremely faint and hard to observe. Because of the deficiency of heavy elements, dwarf irregular galaxies such as IC 10 mimic the conditions in the distant, very young galaxies, but are close enough to be observed in great detail.

"Stars are formed in molecular gas," explained Taylor. "Carbon monoxide is the most easily observable molecule, so we use it to study the temperature and density of the molecular gas, to understand the conditions needed for stars to form." However, because dwarf irregular galaxies are deficient in carbon and oxygen, observations of carbon monoxide in them are demanding, requiring the best receivers and telescopes, and most of all, a large amount of observing time.

The CO emission is shown as black contours over top a false color image of atomic hydrogen emission, which shows the density of warm atomic gas in the interstellar medium. Photo: UMass

Taylor and his collaborators observed IC 10 at five different wavelengths, including 2.7 mm, 1.3 mm and 0.9 mm, with two different radio telescopes. Carbon monoxide emits at different wavelengths depending upon the density and temperature of the gas, so by combining all the data they collected, the astronomers will be able to determine how the physical conditions in the gas vary at different locations in IC 10. The research team has compared their data to an optical image showing sites of recent star formation, as well as to a map of atomic hydrogen gas. They discovered that the molecular gas generally lies close to the regions of densest atomic hydrogen, as well as near young, recently formed stars. They did find a region with molecular gas that lacks young stars. The astronomers suspect this may be a location for the formation of future generations of stars.

Observations at 2.7 mm were carried out at the FCRAO 14-meter (46-foot) radio telescope in New Salem, in the Quabbin Reservoir watershed. This telescope features a unique detector with 16 receivers, allowing 16 simultaneous observations. This allowed the astronomers to observe, for the first time, the entire galaxy, rather than just a few locations within the galaxy. Without this capability, some of the molecular gas might have been missed. The 1.3-mm and 0.9-mm observations were carried out at the 10-meter (33 feet) Heinrich Hertz Telescope (HHT) of the Submillimeter Telescope Observatory on Mt. Graham, Ariz. Submillimeter observations require excellent weather and can only be performed at dry, high-altitude locations like Mt. Graham. Because of the short wavelengths observed, a submillimeter telescopes requires a very precise antenna. The HHT is the most accurate radio telescope in the world, with no surface irregularities larger than the thickness of a human hair.

FCRAO is operated with the support of a grant from the National Science Foundation.