Astronomers using the Far Ultraviolet Spectroscopic Explorer have shown that the Milky Way Galaxy is surrounded by a huge, extended "corona" of hot gas probably left over from its initial formation. Ken Sembach, an astronomer at the Hubble Space Telescope Science Institute and a former research associate in the Krieger School of Arts and Sciences, presented the new findings at the semiannual meeting of the American Astronomical Society earlier this month.

Astronomers call the new structure the corona to differentiate it from the halo, a smaller area of hot gas close to the disc of the Milky Way.

"The amount of mass in this corona could be as much as 100 million suns, and the temperature of the gas in the corona probably exceeds 1 million degrees Kelvin, or nearly 2 million degrees Fahrenheit," says Sembach. "This is the first observational evidence we've had for this part of the Milky Way, and it could prove a great place to look for new insights into how our galaxy formed."

FUSE is operated by Johns Hopkins for NASA.

The new results sprang unexpectedly from a FUSE survey of distant, extragalactic objects like quasars. While studying data from the survey, astronomers realized that they'd also detected oxygen atoms falling into the Milky Way Galaxy. The oxygen was absorbing characteristic wavelengths of ultraviolet radiation that revealed it was an electrically charged form of the element known as oxygen VI. Scientists use oxygen VI to identify hot gases because only high-energy, high-temperature environments can produce oxygen VI by stripping five electrons off each oxygen atom.

In some cases, the FUSE readings of oxygen VI falling into the galaxy aligned with prior observations by the Hubble Space Telescope and radio telescopes of clouds of cold, neutral hydrogen falling into the galaxy. This suggested to the astronomers that what they and other scientists were observing were different aspects of the same clouds falling into the galaxy.

How to account for the temperature contrast between the cold hydrogen and hot oxygen VI? Sembach and his colleagues reasoned that the clouds of gas falling into the galaxy had to be hitting something they couldn't see: a cloud of hot gas surrounding the galaxy that created a shockwave on the leading edge of the falling clouds.

"It's a bit like what happens when a meteor hits Earth's atmosphere," says Bill Blair, chief of observatory operations for FUSE. "Like a meteor, the infalling clouds develop a hot boundary layer on their leading edge as they pass through the corona." This heating makes the leading edge of a meteor flare with light. In the clouds, though, it creates oxygen VI, which can absorb the light of background objects like quasars.

Previous observations, including data acquired by FUSE, already had revealed that a halo of hot gas exists around the disk of the Milky Way. The corona, however, extends much farther than the halo does. It reaches out approximately 150,000 light-years above and below the plane of the galaxy, extending almost to the Large and Small Magellanic Clouds, two small satellite galaxies that orbit the Milky Way at about 170,000 light-years out.

The nearby halo is believed to be composed of hot gases ejected from the plane of the galaxy by exploding stars known as supernovae and is rich in elements produced inside stars, like oxygen and nitrogen. However, the extended corona and infalling clouds may be much older and could be remnant material from the Milky Way's formation or from its continuing accretion of smaller galaxies.

Co-authors with Sembach on the presentation at AAS were Blair Savage, Bart Wakker, Philipp Richter and Marilyn Meade, all of the University of Wisconsin-Madison.