NASA'S Chandra Shows Milky Way is Surrounded by Halo of Hot Gas

Sept. 24, 2012

J.D. Harrington
Headquarters, Washington
202-358-5241
j.d.harrington@nasa.gov

Peter Edmonds
Chandra X-ray Center, Cambridge, Mass.
617-571-7279
pedmonds@cfa.harvard.edu

RELEASE: 12-331

NASA'S CHANDRA SHOWS MILKY WAY IS SURROUNDED BY HALO OF HOT GAS

WASHINGTON -- 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.

"We know the gas is around the galaxy, and we know how hot it is,"
said Anjali Gupta, lead author of The Astrophysical Journal paper
describing the research. "The big question is, how large is the halo,
and how massive is it?"

To begin to answer this question, the authors supplemented Chandra
data on the amount of absorption produced by the oxygen ions with
XMM-Newton and Suzaku data on the X-rays emitted by the gas halo.
They concluded that the mass of the gas is equivalent to the mass in
more than 10 billion suns, perhaps as large as 60 billion suns.

"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 co-author Smita
Mathur of Ohio State University in Columbus. "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."

The estimated mass depends on factors such as the amount of oxygen
relative to hydrogen, which is the dominant element in the gas.
Nevertheless, the estimation represents an important step in solving
the case of the missing baryons, a mystery that has puzzled
astronomers for more than a decade.

Although there are uncertainties, the work by Gupta and colleagues
provides the best evidence yet that the galaxy's missing baryons have
been hiding in a halo of million-kelvin gas that envelopes the
galaxy. The estimated density of this halo is so low that similar
halos around other galaxies would have escaped detection.

The paper describing these results was published in the Sept. 1 issue
of The Astrophysical Journal. Other co-authors were Yair Krongold of
Universidad Nacional Autonoma de Mexico in Mexico City; Fabrizio
Nicastro of Harvard-Smithsonian Center for Astrophysics in Cambridge,
Mass.; and Massimiliano Galeazzi of University of Miami in Coral
Gables, Fla.

NASA's Marshall Space Flight Center in Huntsville, Ala., manages the
Chandra program for NASA's Science Mission Directorate in Washington.
The Smithsonian Astrophysical Observatory controls Chandra's science
and flight operations from Cambridge.

For Chandra images, multimedia and related materials, visit:

http://www.nasa.gov/chandra

For an additional interactive image, podcast and video on the finding,
visit:

http://chandra.si.edu


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