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FOR RELEASE: 09:30 a.m. EST, January 8, 2002
Astronomers announced today that they have found the best evidence to date for cooling gas in the hot halo of a spiral galaxy. This cooling gas may indicate that the galaxy hosts a "galactic fountain" in which gas circulates between the disk and the halo of the galaxy. The result is significant because it may solve the long-standing mystery of the ultimate fate of energy from supernovae.
The report was presented to the American Astronomical Society Meeting in Washington, D.C. today by Dr. Edward M. Murphy of the University of Virginia in Charlottesville, VA, Dr. Birgit Otte and Dr. Christopher Howk of the Johns Hopkins University in Baltimore, MD, Dr. Daniel Wang of the University of Massachusetts in Amherst, MA, Dr. William Oegerle of the NASA Goddard Space Flight Center in Greenbelt, MD, and Dr. Kenneth Sembach of the Space Telescope Science Institute in Baltimore, MD.
The astronomers used the NASA Far Ultraviolet Spectroscopic Explorer (FUSE) to detect ultraviolet emission from O+5 ions in the halo of the spiral galaxy NGC 4631. This ion of oxygen is present in gas with temperatures of a few hundred thousand degrees Kelvin (about one-half million degrees Fahrenheit) at which temperature the collisions between atoms are energetic enough to strip 5 out of the 8 electrons from the oxygen atoms. They believe that the gas has cooled from the million- degree Kelvin (about 2 million degrees Fahrenheit) gas that surrounds the galaxy. Such cooling is predicted by models of the "galactic fountain."
In a landmark paper published in 1956, Lyman Spitzer of Princeton University predicted that the Milky Way is surrounded by a halo of hot gas with a temperature of a million degrees. In 1976, Paul Shapiro and George Field of the Harvard/Smithsonian Center for Astrophysics promoted the idea that this hot halo might be the result of the circulation of gas between the disk and the halo of the Milky Way. They coined the term "galactic fountain" to describe this cycle, which is similar to the water cycle on Earth. In the galactic fountain, the hot gas created by multiple, overlapping supernovae rises out of the galaxy like a hot air balloon. As it climbs out of the galaxy, the top of the bubble may burst, releasing the hot gas into the halo. The gas will remain in the halo for millions of years before cooling off and falling back into the disk where it will be incorporated into the next generation of stars.
The most direct way to study extended galactic halos and their galactic fountains is to observe nearby spiral galaxies seen edge-on. One of the best candidates for study is NGC 4631, a spiral galaxy in the constellation Canes Venatici at a distance of 24.5 million light years (7.5 million parsecs). Due to interaction with the neighboring galaxy NGC 4656, NGC 4631 has very active star formation occurring throughout the galaxy. In July 2001, Q. Daniel Wang at the University of Massachusetts and his collaborators published Chandra X-ray Observatory images that clearly demonstrated the presence of an extended hot halo around NGC 4631. They also presented Hubble Space Telescope images that show large loops and filaments that may be portions of superbubbles of hot gas.
"The FUSE observations are a crucial piece of the puzzle," said Edward Murphy, an assistant professor at the University of Virginia. "We know that NGC 4631 is actively forming stars, we see evidence for bubbles of hot gas, and we know that it is surrounded by a hot halo. The FUSE observation of the cooling gas makes it likely that we are seeing a galactic fountain in operation."
The FUSE observations may also answer another long-standing puzzle. Astronomers have estimated that supernovae occur in NGC 4631 every 20-60 years. It has been assumed that much of this energy will escape in the form of radiation from hot and cool gas. However, the X-ray observations account for only a small fraction of the total energy from supernovae. Models of galactic fountains show that most of the energy will be released by the cooling gas that is the subject of the FUSE study. "If the radiation seen by the FUSE observations represents the cooling of a galaxy-wide fountain, it could balance the supernova heating in the galaxy," said Dr. Q. Daniel Wang from the University of Massachusetts. "The X-ray observations revealed a reservoir of hot gas. Now the FUSE observations have showed us where the gas is going."
"The next step is a quantitative measurement of the amount of cooling material," said Dr. Birgit Otte, a postdoctoral researcher at Johns Hopkins University in Baltimore, MD. At this point, the team has only observed two positions in the halo of NGC 4631. "We have been granted additional FUSE observations to determine if the cooling gas is confined to a single bubble that burst into the halo, or if it is a galaxy-wide fountain," Otte added. In addition, they must account for significant absorption of the ultraviolet emission by dust in the halo of NGC 4631. Optical observations indicate that the dust may be absorbing 80% or more of the light. "We may only be seeing a small fraction of the total emission from O+5," added Dr. William Oegerle from the NASA Goddard Space Flight Center in Greenbelt, MD. In addition, the team will be analyzing similar data for NGC 891, another edge-on spiral galaxy. NGC 891 has a more modest star formation rate than NGC 4631 and is a near twin to the Milky Way.
The Far Ultraviolet Spectroscopic Explorer is a NASA/CNES/CSA mission and is operated for NASA by the Johns Hopkins University in Baltimore, MD. This work was supported by NASA through the FUSE Principal Investigator and Guest Investigator programs.
For more information: Dr. Edward M. Murphy (434) 924-4890 emurphy@virginia.edu Dr. Birgit Otte (410) 516-5147 otte@pha.jhu.edu
More information on the Far Ultraviolet Spectroscopic Explorer, including images, is available at: http://fuse.pha.jhu.edu
Images of NGC 4631 are available from the Chandra X-ray Observatory homepage.
A nice color image of NGC 4631 is available from the NOAO webpage.
This sketch of the galactic fountain is courtesy of Dr. Kenneth Sembach (Space Telescope Science Institute.
The discovery spectra of O+5 taken with the Far Ultraviolet Spectroscopic Explorer courtesy of Dr. Edward Murphy (U. of Virginia).
The locations of the two FUSE apertures on an image from the Astrophysical Journal (Wang et al. 1995, 439, 176).
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© 2007 by The Rectors and Visitors of The University of Virginia. Maintained by the webmaster |