SPACE.com -- X-Ray Halo Found Around Galaxy Like Ours

The central region of galaxy NGC 4631 as seen edge-on from NASAs Chandra X-ray Observatory and Hubble Space Telescope. The Chandra data, shown in blue and purple, is the first conclusive evidence that a halo of hot gas surrounds a galaxy very similar to our Milky Way. The bright pink middle is from Hubble observations. Click to enlarge.

This composite of the black-and-white optical, the Ultraviolet Imaging Telescope, and Chandra X-ray images, showed astronomers at the University of Massachusetts, Amhurst that an X-ray corona surrounds this spiral galaxy by separating individual X-ray sources from the diffuse corona. The corona is shown in the smallest box. Click to enlarge.

This three-panel image puts the black-and-white optical view, the X-ray, ultraviolet composite, and the X-ray and H-alpha emission composite into context with one another.Click to enlarge.

This is an optical image of our neighboring spiral galaxy, NGC 4631. The larger of the two boxes represents the area covered by the X-ray/ultraviolet image and the smaller represents the X-ray/optical composite. Click to enlarge.

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	X-Ray Halo Found Around Galaxy Like Ours By SPACE.com Staff posted: 02:13 pm ET 20 July 2001

The first conclusive evidence that there is a giant halo of 3 million degree gas surrounding a nearby spiral galaxy was recently found by astronomers using NASA's Chandra X-ray Observatory

A team of astronomers has discovered a giant halo of 3 million-degree gas surrounding a spiral galaxy that resembles our Milky Way.

The discovery could lead to a better understanding of the structure and evolution of galaxies in general, most particularly spiral galaxies and our own galaxy.

The researchers at the University of Massachusetts, Amherst used the Chandra X-ray Observatory and Hubble Space Telescope to observe our neighboring galaxy NGC 4631. The galaxy is about 25 million light years from Earth, and its spiral structure is very similar to the Milky Way.

Although previous images have suggested the presence of X-ray emission halos around NGC 4631 and other spiral galaxies, this is the first time that astronomers could separate individual X-ray sources in the galaxy and prove the halo's existence.

"Scientists have debated for over 40 years whether the Milky Way has an extended corona, or halo, of hot gas," said Daniel 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."

Luminous structures

Curiously, the X-ray halo from NGC 4631 closely mirrors the size and shape of the galaxy's radio emission.

This indicates there may be a close connection between the outflows of hot gas, seen in X-rays, and the galaxy's magnetic field, as revealed in the radio emissions, Wang said. The Chandra image reveals the halo extends for about 25,000 light years beyond the galaxy.

The Hubble image of NGC 4631 shows filamentary, looping X-ray structures that enclose enhanced X-ray-emitting gases. These "packages" emanate from regions of recent star formation in the disk. The image clearly shows hot gases being heated by clusters of massive stars and expanding out to 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," Wang said. "Using Chandra and Hubble together, we really get a complete story of what is happening in this galaxy."

Important clues

NGC 4631 has a high amount of star formation, possibly because of interactions with nearby galaxies. Such active star formation might have caused the intense heating of the gases seen by Chandra, as vast amounts of energy are released from supernovae and massive stars in star-forming regions.

These new results provide important clues about the evolutionary history of galaxies and the recycling of energy and mass in a galaxy like our own Milky Way, which is thought to have been more active in star formation in the past than at the present.