Scientists say evidence shows halo of hot gas surrounds Milky Way

Hopkins FUSE telescope supplies data suggesting `galactic fountains' exist

January 10, 2002|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

WASHINGTON - Scientists say they have found new evidence that spiral galaxies such as our own Milky Way are surrounded by halos of hot gas fed by bubbles and fountains of exhaust from stellar explosions.

Over millions of years, the gas cools and rains back down into the galaxy, providing the raw material for the next cycle of star birth.

The findings, reported this week to the annual meeting of the American Astronomical Society, were based on data gathered by the orbiting Far Ultraviolet Spectroscopic Explorer telescope, known as FUSE, built and operated for NASA by the Johns Hopkins University.

A second team of researchers using FUSE instruments told the meeting they have found the first evidence of yet another, much larger envelope of hot gas that surrounds both the Milky Way galaxy and its halo.

This larger cloud, which some scientists called a corona, may extend outward as far as the Milky Way's nearest neighboring galaxies, the Magellanic Clouds, which orbit the Milky Way at a distance of 169,000 light years.

"We didn't know about this [corona] around the Milky Way before, and any model for galaxies like our Milky Way will have to include this observation," said Kenneth Sembach, an astronomer at the Space Telescope Science Institute in Baltimore, who led the second study.

Scientists said the new findings add to understanding of how galaxies like ours assembled themselves, and how they evolved.

The Earth, our sun, and all the stars we can see in the night sky are embedded in a vast, slowly spinning spiral disk of more than 100 billion stars, called the Milky Way galaxy. And astronomers looking farther out into the universe with their most powerful telescopes can see billions more galaxies much like it.

Since the 1950s, scientists have said that such galaxies are probably enveloped in a halo of thin gas heated to more than a million degrees Fahrenheit.

In recent years, astronomers using the Hubble Space Telescope and the orbiting Chandra X-Ray Observatory have captured images of bubbles and fountains of hot gas bubbling out of the disk of a spiral galaxy called NGC 4631, about 24 million light-years away. The gas is believed to erupt from exploding stars, called supernovas, deep in the galactic disk.

(A light-year is the distance light travels in a year, about 5.9 trillion miles.)

Focusing on the same galaxy with FUSE, a team led by University of Virginia astronomer Edward M. Murphy found evidence in ultraviolet light emitted by NGC 4631 that the gas that has bubbled up into the halo eventually cools, to less than a million degrees. In time, it will fall back into the galaxy, completing a circle that Murphy likened to the cycle of water evaporation and rain on Earth.

"This was the last piece of the puzzle," Murphy said. "We believe we are actually seeing galactic fountains in this galaxy."

In the second study, Sembach and his team discovered evidence for an even larger, more distant and thinner envelope of hot gas surrounding the Milky Way.

FUSE detected it indirectly, in the ultraviolet light emitted by clouds of primordial gas falling toward the Milky Way. As these gas clouds approach the galaxy at speeds of a half-million miles per hour, Sembach said, they strike the corona, heat up to more than a million degrees and give off ultraviolet light.

"This is very similar to a meteor heating up in the atmosphere," Sembach said. Meteors reveal themselves when they strike the Earth's outer atmosphere and burn up. The falling gas seen by FUSE heats up and reveals the presence of the galactic corona.

Launched in 1999, FUSE was idled last month by the failure of reaction wheels that enable astronomers to aim it. Flight engineers continue to work on the problem.

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