In space, when you ignore all the stars and galaxies you end up with a lot of hot air. Multi-million-degree gas, to be precise. And you can learn something by doing this.
In a new study, the scheme was used to examine gas near the center of the Milky Way Galaxy that is so hot it emits X-rays.
Using images from NASAs Chandra X-ray Observatory, astronomers removed the X-ray output of 2,357 point-like sources, including neutron stars, black holes, other stars and background galaxies. Then they studied the remaining hot, X-ray emitting gas in a region about 100 light-years across at the heart of the galaxy.
They're left with a puzzle as to how the gas is being heated.
"The best explanation for the Chandra data is that the high-energy X-rays come from an extremely hot gas cloud," said study leader Michael Muno of the University of California, Los Angeles. "This would mean that there is a significant shortcoming in our understanding of heat sources in the center of our galaxy."
The combined gravity from the known objects in the center of the galaxy -- all the stars and a supermassive black hole to boot is not strong enough to prevent the escape of the 100 million degree gas from the region. If should have escaped within 10,000 years or so of the galaxy's formation, scientists figure, but after at least 10-billion-years -- the lifetime of the galaxy -- it's still there.
So the gas must be constantly regenerated and heated. Winds of material blown off massive stars might be the source, the researchers said. And perhaps magnetic turbulence produced by supernova shock waves -- the explosions of massive stars -- heats the gas. Alternatively, high-energy protons and electrons produced by the explosions could be the heat source.
Problem is, these ideas have three problems: Patterns in the X-ray emissions are not consistent with heating by high-energy particles; the observed magnetic field in the galactic center does not have the proper structure; and the rate of supernova explosions does not appear to be frequent enough to provide the necessary heating.
So maybe the gas is the combined glow of many as-yet unfound objects. But you'd need about 200,000 such sources. And while there are some 30 million stars in the study region, the type needed to produce X-rays this powerful number only about 20,000, the researchers estimate.
"There is no known class of objects that could account for such a large number of high-energy X-ray sources at the galactic center," said study member Fred Baganoff of the Massachusetts Institute of Technology (MIT) in Cambridge.
A paper will describe the study in the Sept. 20, 2004 issue of the Astrophysical Journal. Maybe by then somebody will figure out what it means.
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