Astronomers find proof that Milky Way has hot corona

Jan. 12, 2000

by Terry Devitt

With the help of a new satellite capable of finding the telltale, superheated gas created by stars that exploded long ago, scientists have confirmed a four-decade-old theory that the Milky Way is swathed in a corona of hot gas.

First frame of animation
Exploding stars or supernovas in the Milky Way are thought to be the primary mechanism by which hot gas is blasted from the plane of the galaxy into its halo or corona. The idea that our galaxy is swathed in gas as hot as half-a-million degrees Fahrenheit has been around for about 45 years, but was only recently firmed up with evidence obtained by UW-Madison astronomers using NASA's new FUSE satellite.
(Animation courtesy of NASA)
View a Quicktime animation 17.6Mb

The corona or halo of the Milky Way contains extremely hot ionized oxygen that was probably blasted from the star-rich plane of the galaxy by exploding stars or supernovas. This image shows the corona of the Milky Way and the irregularly distributed clouds of superhot oxygen.
(Image courtesy of NASA)
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The discovery is important because it provides strong evidence that supernovas continuously pump extremely hot gas atoms high into the galactic halo, a nebulous region far above the plane of the galaxy. It fleshes out astronomers' ideas of how the material that makes up all the stars in the galaxy is recycled and confirms a hypothesis that the galaxy has a hot corona or halo, an idea first postulated nearly 45 years ago by renowned astronomer Lyman Spitzer.

The new finding was based on data from NASA's new Far Ultraviolet Spectroscopic Explorer (FUSE) satellite and was presented in Atlanta today at a meeting of the American Astronomical Society by Blair D. Savage, an astronomer from UW-Madison.

The existence and origin of hot gas in the halo of the Milky Way, said Savage, has been the subject of polite debate among astronomers since Spitzer proposed the idea in a pioneering paper in 1956. Previous observations hinted at the presence of the gas, but convincing evidence has now emerged with observations from FUSE, which show the presence in the corona of an oxygen atom known as oxygen VI.

With FUSE's ability to sample ultraviolet light in a nearly unexplored region of the electromagnetic spectrum, Savage and his collaborators captured light from distant quasars and other energetic extragalactic sources that had passed through the clouds. With this technique, the astronomers were able to infer the presence in the halo of oxygen VI and obtain its diagnostic spectral signature.

"Oxygen VI is a very sensitive probe and almost certainly indicates the existence of gas with temperatures approaching half a million degrees Fahrenheit," said Savage of the ionized oxygen atoms.

Oxygen is ubiquitous in the galaxy, and oxygen VI, said Savage, is an extremely important diagnostic because it can shed light on the violent processes of star death. The atoms observed by FUSE are believed to have been heated by supernova explosions to extremely high temperatures and blasted from the star-rich plane of the galaxy to its halo in a fountain of gas.

"One can gain many clues to how the galaxy works by studying gas like this," said Savage, who noted that atmospheric scientists gain important insights into Earth's climate and weather by studying aspects of the planet's upper atmosphere. Similar insights into the intricate workings of the galaxy can be derived from a better understanding of the nature and composition of the galactic halo.

The clouds of ionized oxygen appear in almost all directions and extend as much as 5,000 to 10,000 light years away from the plane of the Milky Way, the region of the galaxy where most stars and other objects reside. The astronomers used the ultraviolet light from a dozen distant but very energetic objects - quasars and active galactic nuclei - like flashlights to shine through the gas clouds. Because light is absorbed at certain known wavelengths by elements such as oxygen along a line of sight to a distant light source, astronomers can deduce features that would otherwise be hidden from view.

"We can see the absorption of light by highly ionized oxygen, and what we are seeing tells us there is a lot of it out there, that it is irregular in its distribution," said Savage. "There is more in some directions than others."

Some astronomers had believed that the gas in the halo might be far colder. The fact that the FUSE results confirm that the gas is hot and extends great distances away from the disk, tells astronomers something about how it got there.

"When a star explodes, it heats the gas around it to a high temperature, creating pressure which pushes the gas up beyond the plane of the galaxy," explained Savage.

The gas bursts out of the plane of the galaxy in what astronomers call a "galactic fountain." Over time, the gas cools and flows back toward the plane of the galaxy where it becomes the raw material for new stars.

FUSE, launched into Earth orbit in June of 1999, is designed to sample light in the ultraviolet region of the electromagnetic spectrum. Such light is impossible to detect from Earth and must be captured from telescopes launched above the Earth's atmosphere. Instead of taking pictures like conventional telescopes, the satellite looks at spectra, the constituent wavelengths of light.

FUSE was built and is operated by The Johns Hopkins University for NASA in collaboration with the space agencies of Canada and France, the University of Colorado and the University of California at Berkeley.

Savage, a University of Wisconsin-Madison professor of astronomy, is a FUSE co-investigator and a recognized authority on the halo of the Milky Way. He directed the FUSE observations that led to the discovery of the hot ionized gas in the galaxy's corona.