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Better view of the Milky Way

New evidence revises the known structure of our galaxy

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4:37pm, June 3, 2008
Magazine issue: Vol. 173 #19, June 21, 2008
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ST. LOUIS — Astronomers are in the midst of a Milky Way makeover. New maps, created using infrared and radio telescopes to peer through the galaxy’s dusty lanes, reveal some disarming information.

One study revises the standard view that four major star-forming arms spiral around the disk-shaped Milky Way. Presenting in St. Louis at a meeting of the American Astronomical Society, scientists report that two of the arms are merely faint, minor-league players in the galaxy’s structure. Another study discussed at the meeting reveals a new, short gaseous arm that appears to emanate from one end of a bar-shaped concentration of stars at the galaxy’s center. That gaseous arm appears to be the symmetric counterpart of a previously known arm that emanates from the opposite side of the bar.

It may seem strange that astronomers are still learning about — and debating — the structure of our own galaxy when they have clear images of so many other galaxies millions of light-years distant. But because Earth is embedded in our galaxy’s flat, spiral disk, which is thick with dust and gas, most of the Milky Way appears to us as a blurry band of light that stretches across the sky. Visible-light and ultraviolet telescopes are blind to many of the stars in the disk because clouds of dust and gas near the galaxy’s center block the starlight.

Observations at radio and infrared wavelengths can penetrate the dust and have for several decades been a mainstay for deciphering the galaxy’s structure. But newer, more sensitive telescopes, especially in the infrared, are now peering through the murk as never before, says Robert Benjamin of the University of Wisconsin–Whitewater.

Maps of the galaxy first made in the 1950s used radio telescopes, which detect emissions from neutral hydrogen and carbon monoxide gas, and suggested that the Milky Way has four major star-forming arms, called Norma, Scutum-Centaurus, Sagittarius and Perseus. The sun lies in a small partial arm, dubbed the Orion Spur, between the Sagittarius and Perseus arms.

Large infrared sky surveys in the 1990s added new information to this picture, confirming that a large bar-shaped concentration of stars lies at the Milky Way’s center.

Now, using a wealth of new images from NASA’s Spitzer Space Telescope — including 800,000 snapshots of the inner part of the Milky Way stretching 130 degrees across the sky — Benjamin’s team finds that two of the previously known arms do not play a starring role. By counting the number of stars thousands of light-years along different directions of the Milky Way, a feat not possible before Spitzer, the researchers found an expected jump in the density of stars in the direction of the Scutum-Centaurus arm. But the team found no such jump in star counts when they looked in the direction of the Sagittarius and Norma arms.

Those arms exist but not as the robust appendages previous visualizations had depicted. These two arms should now be demoted in importance because they do not contain high densities of young, bright stars and older stars, Benjamin says. Because the fourth arm, Perseus wraps around the outer portion of the Milky Way, it can’t be seen in the new Spitzer images, he adds.

“We propose that there are two major arms, consisting of a concentration of gas and star-forming regions, and two minor arms, consisting of regions where gas gets compressed and some stars form,” says Benjamin. The two major arms are places where gas in the rotating galaxy piles up like in a traffic jam, compressing and making a motherlode of new stars. “This sort of behavior is actually seen in other galaxies, so we think that this is plausible for the Milky Way.”

Leo Blitz of the University of California, Berkeley says he finds the work intriguing, but would like to see how the star counts match up with radio maps of the same regions.

Another team, using radio observations to track gas in the inner part of the galaxy, discovered the long-sought counterpart to a small, gaseous spiral arm, found 50 years ago, that lies about 10,000 light-years from the Milky Way’s center. Researchers had suspected the existence of a mirror-image counterpart, extending from the other side of the Milky Way’s center, since other spiral galaxies have them. The detection of the twin arm shows that the Milky Way indeed has “a beautiful symmetry,” says Tom Dame of the Harvard-SmithsonianCenter for Astrophysics in Cambridge, Mass. His team identified the arm using a millimeter-wave telescope at Cerro Tololo Inter-American Observatory in Chile. He suggests that the stirring movement of the bar may generate these arms.

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