Astronomers at NASA’s Chandra X-ray Observatory have found evidence that the Milky Way galaxy is surrounded by a halo of hot gas. Extending for hundreds of thousands of light years, the mass of the gas cloud is estimated to be comparable to the mass of all the stars in the galaxy.

“Our work shows that, for reasonable values of parameters and with reasonable assumptions, the Chandra observations imply a huge reservoir of hot gas around the Milky Way,” said Smita Mathur, astronomy professor at Ohio State University and co-author of the paper, which has been published in The Astrophysical Journal. “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.”

A team of 5 astronomers using data from the Chandra, the European Space Agency’s XMM-Newton space observatory, and Japan’s Suzaku satellite have determined that the temperature of the halo is between 1 million and 2.3 million kelvins. The surface of the Sun is around 5778 kelvins.

The published observations used X-ray sources located far outside the galaxy to measure the absorption of the X-rays by oxygen ions outside the Milky Way. Using this method, the astronomers were also able to estimate that the gas halo is as massive as 10 billion suns, and perhaps as massive as 60 billion suns. The density of the halo, however is so low that similar halos around other galaxies might have been missed.

NASA stated that the size and mass of the gas halo, if confirmed, could explain the “missing baryon” problem for the Milky Way. The “missing baryon” problem has to do with the measurement of baryonic matter from the beginning of the universe. While measurements indicate that baryonic matter (such as neutrons and protons) present when the universe was only a few billion years old was one-sixth the mass and density of the unobservable (dark) matter, current measurements of the Milky Way and nearby galaxies show around half the expected baryons are missing.

(Illustration courtesy NASA/CXC/M.Weiss; NASA/CXC/Ohio State/A Gupta)