September 25, 2012 - Astronomers have used NASA's Chandra X-ray Observatory
to find evidence our Milky Way Galaxy
is embedded in an enormous halo of hot gas that extends for hundreds of thousands of light years. The estimated mass
of the halo is comparable to the mass of all the stars in the galaxy.
If the size and mass of this gas halo is confirmed, it also could be an explanation for what is known as the "missing baryon" problem for the galaxy.
Baryons are particles, such as protons and neutrons, that make up more than 99.9 percent of the mass of atoms found in the cosmos. Measurements of extremely distant gas halos and galaxies indicate the baryonic matter present when the universe was only a few billion years old represented about one-sixth the mass and density of the existing unobservable, or dark, matter.
In the current epoch, about 10 billion years later, a census of the baryons present in stars and gas in our galaxy and nearby galaxies shows at least half the baryons are unaccounted for.
In a recent study, a team of five astronomers used data from Chandra, the European Space Agency's XMM-Newton
space observatory and Japan's Suzaku satellite to set limits on the temperature, extent and mass of the hot gas halo.
Chandra observed eight bright X-ray sources located far beyond the galaxy at distances of hundreds of millions of light-years. The data revealed X-rays from these distant sources are absorbed selectively by oxygen ions in the vicinity of the galaxy.
The scientists determined the temperature of the absorbing halo is between 1 million and 2.5 million kelvins, or a few hundred times hotter than the surface of the sun.
Other studies have shown that the Milky Way
and other galaxies are embedded in warm gas with temperatures between 100,000 and 1 million kelvins. Studies have indicated the presence of a hotter gas with a temperature greater than 1 million kelvins. This new research provides evidence the hot gas halo enveloping the Milky Way
is much more massive than the warm gas halo.