Milky Way's Hot Gas Halo

Melissa Weiss; Gupta et al, 2012; OSU; CXC; NASA

Larger and jumbo illustrations.

An enormous halo of extremely hot gas
has been found around the Milky Way
and some of its satellite galaxies,
such as the Small and Large Magellanic
Clouds (more).

Dark Matter around the Milky Way

On September 24, 2012, astronomers using NASA's Chandra X-Ray Observatory announced that our Milky Way Galaxy and some of its satellite galaxies are surrounded by an enormous halo of extremely hot gas at a temperature of one to 2.5 million kelvins. This halo is estimated to encompass at least 600,000 light-years in diameter around the Milky Way, but it may extend significantly further. It may mass between 10 and 60 billion Solar-masses. The estimated density of this halo is so low, however, that similar halos around other galaxies have been unobserved thus far (NASA press release).

The team of five astronomers used data from Chandra, the European Space Agency's XMM-Newton space observatory and Japan's X-Ray Suzaku satellite to set limits on the temperature, extent, and mass of the hot gas halo. Eight bright X-ray sources located far beyond the galaxy at distances of hundreds of millions of light-years were observed with Chandra, which revealed that the X-rays from these distant sources are absorbed selectively by oxygen ions in the vicinity of our galaxy. The astronomers then determined that the absorbing halo is a few hundred times hotter than the surface of our Sun, Sol.

Previous studies had shown that the Milky Way and other galaxies are embedded in warm gas with temperatures between 100,000 and 1 million kelvins. These studies also indicated the presence of a hotter gas with a temperature greater than 1 million kelvins. This new study provides evidence the hot gas halo enveloping the Milky Way is much more massive than the warm gas halo.

WMAP, GSFC, NASA

Larger illustration.

The cosmos appears to be
comprised of very little
ordinary matter made of
atoms (around four percent),
that form the stars, planets,
and clouds of gas and dust
(latest WMAP results).

If the size and mass of this gas halo is confirmed, it suggests a source for some of the "baryons" so far unobserved. Baryons are particles of normal or "ordinary" matter (e.g., 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. About 10 billion years later in our modern universe, a census of the baryons present in stars and gas in Milky Way and nearby galaxies suggests that at least half the baryons are unaccounted for.

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