Massive star clusters swaddled in huge cocoons during infancy

June 03, 2001

New observations with the Gemini North Telescope in Hawaii indicate three of the youngest massive star clusters yet detected each are swaddled in dust cocoons at least 600 trillion miles across, providing new clues to the evolution of the early universe.

Globular star clusters are among the most ancient objects in the universe, forming about the same time as the first galaxies, said Kelsey Johnson, a doctoral student at the University of Colorado at Boulder. "While their formation remains somewhat of a mystery, they clearly require conditions that are uncommon in the universe today."

Astronomers estimate the baby globular star clusters observed in the starburst galaxy Henize 2-10 are less than 1 million years old - analogous to the first day of life for a human. "If we can study objects like globular clusters as they are forming, then we should be able to learn more about the conditions in the early universe," said Johnson, a doctoral student in the astrophysical and planetary sciences department and a researcher at JILA.

Johnson and her collaborators Bill Vacca of the Max Planck Institute in Garching, Germany, and CU-Boulder Professor Peter Conti presented a paper on the subject at 197th meeting of the American Astronomical Society June 3-7 in Pasadena.

The birthing massive star clusters were first observed two years ago by Johnson and University of Wisconsin, Madison astronomer Henry Kobulnicky with the Very Large Array Telescope in Socorro, N.M.

The new images taken by the three astronomers using Gemini North revealed three infrared objects deep within the heart of the Henize 2-10 galaxy. Because the infrared emissions from the three infant star clusters comprise the bulk of all wavelength emissions, Johnson and her colleagues believe the three clusters must be "the engine" that powers most of the energy radiated by the galaxy.

Johnson said the Gemini North telescope observations indicated the baby star clusters were emitting between 60 percent and 100 percent of the infrared emissions emanating from Henize 2-10. "Given the vigorous star formation throughout Henize 2-10 as seen in optical light, it is remarkable that only a handful of these objects could be responsible for all the infrared light we see," she said.

The youngest phases of massive star clusters require behemoth "birthing clouds" of dust, which are extremely efficient at absorbing optical and ultraviolet light and "re-radiating it as infrared light," said Vacca. The researchers estimated the temperature of the dust shells surrounding the infant clusters at a minus 200 degrees Fahrenheit.

"This may sound cold, but compared to the rest of the universe - which is just above absolute zero due to the cosmic microwave background - the temperatures of the dust shells surrounding infant stars really are quite balmy."

Henize 2-10 is located some 32 million light-years from Earth in the constellation Pyxsis and contains several star clusters harboring as many as a million stars each. Packed into a relatively small area of space, each cluster is believed to evolve over billions of years into globular clusters like those orbiting the Milky Way.

Such "super star clusters" in galaxies like Henize 2-10 seem to undergo a violent, short-lived phase, producing prolific numbers of stars in specific star-birth regions at extremely rapid rates. It is not known exactly what triggers these outbursts, but it appears that a catastrophic event such as the collision or merger of two galaxies is required, said Vacca.

Galaxies like Henize 2-10 are known to radiate as much as 10 times more energy in the infrared than in optical and UV wavelengths, said Vacca. This suggests there is not only a large amount of newly formed stars and star clusters present, but also a vast amount of dust in these massive objects.
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Located on Mauna Kea, The Gemini North Telescope has an 8.1-meter mirror and sophisticated optics that make it the most powerful ground-based telescope in the world. An identical observatory, the Gemini South Telescope, is now being completed in Chile.

The two identical Gemini telescopes are an international project funded in the United States by the National Science Foundation as well as funds from the United Kingdom, Canada, Chile, Australia, Argentina and Brazil.

Peter Conti, 303-492-8497 Jim Scott, 303-492-3114

Note to Editors: Contents embargoed until 10 a.m. PDT on Monday, June 4. The AAS Press Room numbers are 626-844-6037, 844-6038 and 844-6039.

University of Colorado at Boulder

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