The University Record, January 18, 1999

Astronomers spot cold dust in hot gas of ancient galaxies

By Sally Pobojewski
News and Information Services

U-M astronomers have detected the cool infrared signature of dust grains and silicates within superheated gas in the center of ancient elliptical galaxies 60 million light-years from Earth.

The discovery of these particles is significant, according to astronomer Joel Bregman, because it could represent the first direct observation of how mass lost by aging stars evolves in a hot, exotic environment.

Bregman and his colleagues found unexpected evidence for the existence of dust and silicates in infrared emissions from nine elliptical galaxies. Results from their research were presented Jan. 6 at the American Astronomical Society meeting. Spectral data for the study were collected by the Infrared Camera (ISOCAM) on the European Space Agency’s Infrared Space Observatory.

Since the Earth’s atmosphere blocks most infrared radiation, space observatories like ISO allow astronomers to observe objects too cold or faint to be seen in visible light. Another advantage of infrared light is that it passes undisturbed through gas and dust clouds allowing astronomers to “see” inside dense, star-forming areas in the center of galaxies.

“Elliptical galaxies contain stars that are five- to 15-billion years old, which is about as old as stars can be in the universe,” said Bregman, professor of astronomy. “Space between the stars in these galaxies is filled with superheated X-ray emitting gas that is about 10 million degrees K—one of the hottest environments in the universe.”

“Temperatures are comparable to those inside a supernova remnant, but on a galaxy-wide scale,” added graduate student Alex Athey. “Even the hottest parts of our Milky Way Galaxy rarely exceed 1 million degrees K.”

Because gas in the center of the observed galaxies is so hot, Bregman and his colleagues were startled when an excess in their infrared spectral data indicated the presence of material significantly colder than the surrounding gas.

“We expected to see the normal starlight spectrum, along with polyaromatic hydrocarbons or PAHs,” Bregman said. “When we didn’t find what we expected, we asked ESA for additional observations from ISOCAM in the six- to 15-micron wavelength band. ESA went the extra mile and gave us additional observation time with higher resolution detail.”

Although Bregman and his colleagues are still interpreting the data, they believe the source of the unusual infrared emissions is dust grains and silicates thrown off by stars interacting with superheated gas inside elliptical galaxies.

“Stars lose mass during their evolution, primarily when they are giant stars,” Bregman said. “As this gas flows away from the stars, some of the material forms into grains of dust—somewhat smaller than particles in cigarette smoke. This material is exposed to the very hot gas of the galaxy which slowly destroys the grains, and to UV light of the galaxy, which heats the dust grains causing them to emit in the mid-IR region. If this interpretation is accurate, additional analysis of infrared emissions from elliptical galaxies could provide important clues to how galaxies evolve.”

Collaborators in the study included Jesse Bregman of NASA and Pasquale Temi of the NASA Ames Research Center. Funding for the study was provided by NASA.

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