Jan. 9, 2002

Rollin' down the river: Arecibo Observatory captures evidence of turbulence and thick gas as a possible precursor to galactic evolution

WASHINGTON, D.C. -- In a huge river of primordial hydrogen flowing from the neighboring Magellanic Clouds into our own Milky Way galaxy, astronomers have discovered the first evidence of turbulence and concluded that the invisible, hot mass of gas surrounding our galaxy is much thicker than physicists previously thought.

Galactic turbulence, an ingredient in cosmic cloud and star formation, has never before been seen in starless areas of the cosmos. "What causes turbulence in a star-free cosmic stream is unclear, but this finding could be important in understanding the cosmic-cloud and star-formation processes," says Snezana Stanimirovic, an astronomer at the National Astronomy and Ionosphere Center at Arecibo, Puerto Rico, which is operated by Cornell University in a cooperative agreement with the National Science Foundation.

The Large Magellanic Cloud and the Small Magellanic Cloud are the Milky Way galaxy's closest galactic neighbors. Like the Milky Way, the Magellanic Clouds contain millions of stars and possibly planets. The Magellanic Stream runs through the Magellanic Clouds and into the Milky Way. Ultra-rich in hydrogen gas and surrounded by very hot (about 1 million degrees Kelvin) gas from the Milky Way's Galactic Halo, the Magellanic Stream is thought to be about 160,000 light years long. This means that it would take 160,000 years -- travelling at the speed of light -- to go from one end of the stream to the other.

Using the radio telescope at Arecibo, astronomers have made rare, direct observations of the Magellanic Stream, which dips into the Galactic Halo, a thin veil of gas surrounding the Milky Way that is visible only through the radio spectrum. Only the Magellanic Stream's northern portion can be seen by the Arecibo Observatory. The astronomers analyzed the hydrogen emission from the stream sections accessible to the Northern Hemisphere and found numerous cometlike clumps and filaments of gas, suggesting turmoil in the stream. "While this is new research and the implications not yet fully understood, thanks to the Arecibo radio telescope, we are seeing this hydrogen gas in a sharper picture than ever before. Because of this, we've managed to derive the density of the Galactic Halo. This will be a factor in understanding the billion-year processes of galactic evolution," says Stanimirovic.

John Dickey University of Minnesota professor of astronomy, who earned his Ph.D. at Cornell 1977; Marko Krco, an undergraduate student at Colgate University; and Stanimirovic will present the poster, "Magellanic Stream Probes Density of the Galactic Halo," Jan. 9 at the annual meeting of the American Astronomical Society at the Hilton Washington and Towers in Washington, D.C.

While the Magellanic Clouds are quite visible from Earth's Southern Hemisphere, the Magellanic Stream -- virtually invisible to the naked eye -- can be observed only through radio telescopes from the Southern Hemisphere and from tropical regions of the Northern Hemisphere. Observers in those regions, without telescopes, can see the Large Magellanic Cloud at about zero magnitude, which is bright. The Small Magellanic Cloud is at second magnitude, which is dim but bright enough to see in a dark sky. The famed supernova 1987A, which surprised astronomers on Feb. 24, 1987, occurred in the Large Magellanic Cloud.

This research was funded in part by the National Science Foundation.

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