They figured that the galaxy is embedded in superheated gas that is between 1 million degrees Kelvin to 2.5 million degrees Kelvin (about 1.8 million-4.5 million degrees Fahrenheit) - a few hundred times hotter than the surface of the sun. It also is massive, equivalent to more than 10 billion suns and perhaps as many as 60 billion suns.
"It may extend for a few hundred thousand light-years around the Milky Way or it may extend farther into the surrounding local group of galaxies. Either way, its mass appears to be very large," astronomer Smita Mathur, with Ohio State University in Columbus, said in a NASA press release.
The gas halo is in addition to a previously discovered pocket of warm gas enveloping the Milky Way. That gas is between 100,000 degrees Kelvin and 1 million degrees Kelvin, or 180,000-1.8 million degrees Fahrenheit.
The new findings show the outer, hotter gas halo is much larger than the previously discovered warm gas shroud. It also may help answer a long-standing cosmological mystery about baryons - particles that comprise more than 99.9 percent of the cosmos' atoms, most of which are unaccounted for.
The new research suggests the missing baryons are hiding in hot gas halos surrounding galaxies, including the Milky Way.
The research is published in the Sept. 1 issue of The Astrophysical Journal.
Image: Artist's illustration of the Milky Way surrounded by an enormous halo of hot gas (shown in blue). To the left are two small neighbor galaxies, the Small and Large Magellanic Clouds. Astronomers used NASA's Chandra, Europe's XMM-Newton, and Japan's Suzaku telescopes to get information about the halo's temperature, distance and mass. Credit: NASA/CXC/M.Weiss; NASA/CXC/Ohio State/A.Gupta et al.