The first unambiguous evidence for a giant halo of hot
gas around a nearby, spiral galaxy much like our own Milky
Way was found by astronomers using NASA’s Chandra X-ray
Observatory. This discovery may lead to a better
understanding of our own Galaxy, as well the structure and
evolution of galaxies in general.

A team of astronomers, led by Professor Daniel Wang of the
University of Massachusetts, Amherst, observed NGC 4631, a
spiral galaxy approximately 25 million light years from Earth
with both Chandra and NASA’s Hubble Space Telescope.

While previous X-ray satellites have detected extended X-ray
emission from this and other spiral galaxies, this is the
first time that astronomers were able to separate the
individual X-ray sources from the diffuse halo, thanks to
Chandra’s exceptional resolution. Chandra found the diffuse
halo of X-ray gas to be radiating at a temperature of almost
3 million degrees.

“Scientists have debated for over 40 years whether the Milky
Way has an extended corona, or halo, of hot gas,” said Wang,
lead author of the paper which appeared this month in The
Astrophysical Journal Letters. “Of course since we are within
the Milky Way, we can’t get outside and take a picture.
However, by studying similar galaxies like NGC 4631, we can
get an idea of what’s going on within our own Galaxy.”

The Chandra image reveals a halo of hot gas that extends for
approximately 25,000 light years above the disk of the
galaxy. One important feature of the X-ray emission from NGC
4631 is that it closely resembles the overall size and shape
seen in the radio emission from the galaxy. This indicates
that there may be a close connection between the outflows of
hot gas, seen in X-rays, and the galaxy’s magnetic field,
revealed by radio emission.

The Hubble image of NGC 4631 shows filamentary, loop-like
structures enclosing enhanced X-ray-emitting gas and
emanating from regions of recent star formation in the
galaxy’s disk. These data clearly show the hot gas is heated
by clusters of massive stars and is now expanding into the
halo of the galaxy.

“What we see in NGC 4631 can be thought of as the bursting
flames of a gigantic cosmic camp fire,” said Wang. “Using
Chandra and Hubble together, we really get a complete story
of what is happening in this galaxy.”

NGC 4631 is a galaxy that has high amounts of star formation,
possibly triggered by interaction with neighboring galaxies.
Such star formation might have created the conditions
necessary to heat the gas seen by Chandra, as vast amounts of
energy are released from supernovae and massive stars in
star-forming regions – enough to lift the gas out of the
plane of the galaxy.

These new results provide important clues about the cycling
of energy and mass in a galaxy like our own Milky Way and
about the evolutionary history of galaxies, which are thought
to be more active in star formation in the past than at the

Other members of the research team include: Stefan Immler,
University of Massachusetts; Rene Walterbos, New Mexico State
University; James Lauroesch, Northwestern University,
Evanston, IL, and Dieter Breitschwerdt, Max Plank Institute,

Chandra observed NGC 4631 with its Advanced CCD Imaging
Spectrometer instrument, developed for NASA by Pennsylvania
State University, University Park, and Massachusetts
Institute of Technology, Cambridge. NASA’s Marshall Space
Flight Center in Huntsville, AL, manages the Chandra program,
and TRW, Inc., Redondo Beach, CA, is the prime contractor for
the spacecraft. The Smithsonian’s Chandra X-ray Center
controls science and flight operations from Cambridge, MA.

Images associated with this release are available on the
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