New Milky Way views: active center, gas-strewn edges

Scientists announce huge 'galactic corona' of gas, and first glimpse of the galaxy core, a hotbed of new stars.


Armed with new telescopes, high-speed computers, and data from ambitious all-sky surveys, researchers are gleaning new insights into the structure and inner workings of the Milky Way.

Discoveries announced this week range from the center of our home galaxy to its furthest reaches.

In the process, the galaxy is increasingly coming to be viewed as an astronomical ecosystem.

"What we really want to understand is how ... different components of the galaxy interact," says Daniel Wang, an X-ray astronomer at the University of Massachusetts at Amherst.

That understanding will, in turn, pay dividends as researchers try to plumb the secrets of the more-distant galaxies that populate the cosmos.

"Our galaxy really is a laboratory," says Vera Rubin, an astronomer at the Carnegie Institution of Washington.

The Milky Way has been sketched in increasing detail in the decades since Harlow Shapley first located its center in 1918 and gave the world the notion that it consisted of a central bulge surrounded by a disk. But surprises continue.

For example, astronomers knew of a halo of gas extending up to 33,000 light years from the galaxy and thought to be driven by stellar explosions in the disk.

A team of researchers this week announced its discovery of a more-extended, tenuous envelope of hot gas wrapping the galaxy, with a mass 10 million times greater than the sun's. This galactic corona extends at least 150,000 light years from the center of the Milky Way, according to the research team, led by Kenneth Stembach, an astronomer with the Space Telescope Science Institute in Baltimore, Md. The announcement came at the American Astronomical Society winter meeting here.

The team discovered the corona as it studied cold clouds of intergalactic gas that pelt the galaxy at high velocities. The researchers noted that the clouds' surfaces grew extremely hot as they approached. They concluded that the dense clouds were passing through a shroud of gas and being heated much like meteors when they plunge into Earth's atmosphere.

This discovery could help researchers pin down the mechanisms that gave rise to the galaxy. Dr. Stembach notes that this corona could represent a galactic "wind," the echo of an enormous burst of star formation earlier in the galaxy's history. Or it could represent fossil evidence of heating that took place as gas fell in toward a fledgling Milky Way and heated as it met more gas and was compressed by gravity.

"Any model for the formation of galaxies like our own will now have to include such information," he says.

Other discoveries are coming as the galaxy is viewed through fresh telescopic eyes. This week, a team led by Dr. Wang unveiled a mosaic image of the Milky Way's nucleus. The image, of a 400-by-900 light year patch of the galaxy, was taken with NASA's Chandra X-Ray Observatory.

Astronomers have long seen the center of the galaxy as a copious source of X-rays, but their origin was a mystery.

Some researchers had proposed that the emissions came from a huge reservoir of gas at a searing 100 million degrees Celsius. But if the gas were so hot, why didn't it expand and dissipate?

Optical observations didn't help, because the center is obscured by interstellar dust. Chandra, showing objects in finer detail than other x-ray instruments, allowed astronomers to distinguish between individual sources, such as neutron stars and black holes, and more diffuse sources, such as hot-gas clouds.

Chandra, revealed that individual sources account for much of the x-ray emissions. "This means the gas is hot, but not as hot as we scientists previously believed," Wang says. The new temperature guess: a frostier 10 million degrees.

Still other groups studying the Milky Way are taking advantage of data pouring in from sweeping surveys of the night sky, which are capturing hundreds of millions of objects.

A team led by Michael Skrutskie at the University of Virginia in Charlottesville has used data from the 2MASS infrared all-sky survey to develop the first "bird's eye" view of the galaxy. Taking the images in the infrared portion of the electromagnetic spectrum allows astronomers to see through intervening dust. Guided by "standard candles" - 30,000 stars known to have a common brightness - Dr. Skrutskie's galactic map is the first to be generated by directly measuring the distance to stars all over the galaxy.

Meanwhile, Columbia University's David Helfand is using data from radio, X-ray, and infrared telescopes to compile a census of star-forming regions as well as of dead and dying stars. This stellar life cycle "governs the chemical evolution of the Milky Way," he says.

Indeed, Dr. Wang notes that the Chandra observatory will help trace distribution routes from the galaxy's central chemical factory over galaxy-wide scales. The nucleus, a hotbed of star formation and destruction, generates fountains of chemically enriched gas that rise from the core, spread over the galactic disk, then cool and fall into the disk. One of the riddles astronomers hope to solve is the relative importance of this galactic fountain in enriching the disk, compared with more-local events.

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