Ray Villard Space Telescope Science Institute 410 338 4514 villard@stsci.edu Michael Purdy The Johns Hopkins University News Office 410 516 7906 mcp@jhu.edu EMBARGOED UNTIL 9:30 A.M. EST Tuesday 8 January 2002 FUSE SATELLITE REVEALS VAST EXTENT OF GALACTIC CORONA NASA's Far Ultraviolet Spectroscopic Explorer (FUSE) satellite has provided evidence for a vast invisible corona of hot gas enveloping our Milky Way galaxy. This corona is far larger than previously envisioned. The surprising finding comes from observations of clouds of hydrogen gas raining onto our galaxy from outside. Though these clouds had been suspected before of falling into our galaxy like intergalactic comets, the FUSE observatory was needed to detect the previously unseen 100,000 to one million degree Fahrenheit surfaces of the clouds. This is a clear signature the clouds are barreling through the extensive, hot tenuous medium of the galactic corona at nearly one million miles per hour. The FUSE observations are akin to knowing the Earth has a highly extended atmosphere by seeing the trails of meteors as they burn up from friction with the atmospheric gases. The infalling clouds may be fragments of smaller galaxies gravitationally torn apart by the Milky Way, or may be gas left over from the formation of our galaxy. This second possibility is supported by data from FUSE and the Hubble Space Telescope, which show that some of these clouds have lower chemical abundances than clouds in the disk of the Milky Way. The hot glowing outer surfaces of the clouds were discovered during a FUSE survey of quasars and other distant objects far from the Milky Way. Dr. Kenneth Sembach of the Space Telescope Science Institute in Baltimore, Maryland and his collaborators detected the absorption of ultraviolet light produced by oxygen atoms that have had five of their eight electrons stripped off as the infalling clouds of gas pass through the hot galactic corona. Some of these clouds were detected previously by radio telescopes on Earth and by the Hubble Space Telescope, but FUSE for the first time picks up the hot outer edges of the clouds produced by interactions with the galactic corona. The presence of a very hot corona surrounding the Milky Way was first postulated by the late Lyman Spitzer of Princeton University in the 1950s. Astronomers have known about a much smaller halo of hot gas around the galactic disk for some time but were previously unaware of the extended corona just discovered. "This is an astonishing discovery from FUSE" says Sembach. "The fact that we see the ionized clouds in so many directions means the galactic corona must be extensive. It may extend as far as the Milky Way's nearest neighboring galaxies, the Magellanic Clouds." The vast region of hot gas completely encapsulates the Milky Way and may be 100,000 light years or more in size. The corona could be left over from the formation of the Milky Way, or it may have been created by early episodes of star formation in which the hot gas was heated by supernovae and expelled from the galactic disk. The FUSE observations of clouds falling into the corona are important because they indicate that the Milky Way continues to accrete material even though the galaxy is billions of years old. Studying the accretion of the clouds and their interaction with the hot corona around the Milky Way will help astronomers understand better how galaxies formed and have evolved over time. Sembach is presenting the results at the meeting of the American Astronomical Society in Washington, D.C. along with co-investigators Drs. Blair Savage, Bart Wakker, Philipp Richter, and Ms. Marilyn Meade of the University of Wisconsin-Madison. The Far Ultraviolet Spectroscopic Explorer is a NASA/CNES/CSA mission operated for NASA by the Johns Hopkins University in Baltimore, Maryland. This work was supported through the FUSE Principal Investigator program. For more information about FUSE, refer to: http://fuse.pha.jhu.edu