AMHERST, Mass. - A team of astronomers led by Daniel Wang of the University of Massachusetts has taken the sharpest-ever image of the heart of our Milky Way galaxy. The image, a panorama of the galaxy''s center taken with NASA''s Chandra X-ray Observatory, reveals hundreds of white dwarf stars, neutron stars, and black holes bathed in an incandescent fog of multimillion-degree gas around a supermassive black hole.
Wang presented the panorama - a mosaic of 30 separate images which covers a 400- by 900-light-year swath of the galaxy - today at this week''s meeting of the American Astronomical Society in Washington, D.C. The findings will be detailed in the Jan. 10 issue of the journal Nature. The co-authors are Eric Gotthelf of Columbia University, and UMass postdoctoral researcher Cornelia Lang.
Astronomers have detected X-ray radiation from the Milky Way''s center for more than two decades, Wang said, "but the origin of this X-ray radiation has been a mystery. The reasons are simple: we simply didn''t have good enough resolution." Pictures produced with previous X-ray telescopes would show clouds and bulges of the radiation, Wang said, but none of these pictures gave astronomers the details they sought. This new image allows scientists to see individual X-ray sources separately from the diffuse glow produced by hot gases.
The finding appears to answer at least one question that astronomers and physicists have been puzzling over for decades: why did the hot gas in the galaxy''s center, which scientists believed was as hot as 100 million degrees Celsius, stay relatively clustered rather than expand? The answer: "Much of the high-energy X-ray radiation is due to discrete sources that most likely represent white dwarf stars, neutron stars, and black holes. The remaining less-energetic radiation is due to hot gas. This means that the gas is hot, but not as hot as we scientists previously believed," said Wang. Rather than 100 million degrees, the gas at the galaxy''s center climbs to a relatively mild 10 million degrees.
The X-ray-emitting discrete sources and diffuse gas are the remnants of stars, which are forming in the center at a much more rapid pace than at the outer reaches of the galaxy. Many of the most massive stars are located in the galaxy''s center, and are boiling off their outer layers in searing stellar winds. Supernova explosions are far more common in the region, and send shock waves booming through the inner galaxy. "It''s a very high-pressure environment," said Lang. "It''s a nice place to visit - with a telescope - but I wouldn''t want to live there."
Also, the image shows that the high-pressure and high-temperature gas is apparently escaping from the center into the halo of the galaxy. The outflow of gas, chemically enriched from the frequent destruction of stars, will enrich the farther reaches of the galaxy, Wang said.
The center of the galaxy is 20,000 light-years away. "It''s far away, but still much closer than anything else we can look at," said Wang. "The nearest galaxy that is similar to our own, Andromeda, is a hundred times farther away. Thus, our own galaxy becomes a galaxy laboratory for us to learn about the activities and processes which govern other galaxies." Wang studies the "hot universe;" that is, objects and gas with temperatures of at least a million degrees.
Daniel Q. Wang can be reached at firstname.lastname@example.org, or, after Jan. 10, at 413/545-2131.