You could think of our galaxy as a very, very radiant and lumpy pancake.
The Milky Way pancake, which is mostly thin with a huge bulge at its center, emits particles and energies of various kinds. And strangely, it is shot through with a layer of short-wavelength, extremely penetrating "hard" X-rays. This powerful radiation runs through the plane of the galaxy. Scientists have known this for more than 20 years.
The question is: where does that radiation come from?
There have been two theories -- either it's coming from powerful stars within the Milky Way or it's coming from plasma, a type of supercharged, superhot gas, swimming around in our galaxy.
Until recently, there has been no way of telling the difference because there were no observatories that could see through galactic dust to detect a broad range of X-rays at various energy levels in space and resolve them into sharp images.
Now that the Chandra X-ray Observatory is orbiting Earth (it was launched in 1999), it can see through the dust to get the data to answer the question about the radiation in the galactic plane.
Diffuse plasma wins.
"We clearly detected 36 new hard X-ray sources, as well as strong diffuse emission," said Ken Ebisawa, a researcher at NASA's Goddard Spaceflight Center.
The X-ray sources may sound like support for the theory that the X-rays come from galactic stars but the sources are a red herring -- they account for only 10 percent of the radiation's intensity in the patch of sky studied, Ebisawa said.
And besides, the team deduced that the sources are located beyond the Milky Way as the sources found in this patch of the galactic plane are sprinkled around just as uniformly as scientists would expect in the universe beyond the Milky Way, based on previous findings. That suggests none of the sources are within our galaxy and they contribute nothing to the X-rays in the galactic plane.
Given that the sources are outside our galaxy, the team thinks they are active galactic nuclei (AGNs) -- galaxies with supermassive black holes at their center sucking down matter. AGNs emit X-rays as mass spirals and nears the event horizon of the black hole, and then speeds and heats up before it disappears into the hole.
Problem with plasma
The trouble is that no one yet understands why the plasma hangs out in the galactic plane, let alone where it came from -- although it could be residue from exploded stars called supernovas.
"The plasma temperature is so high that it may not be bound by the gravity of the galaxy," Ebisawa said. "There are no accepted models to explain such hot galactic plasmas."
If scientists can figure that out, they'll know more about how the Milky Way was created and how it has evolved to become attractive to X-ray spewing plasma.
The findings were published in this week's issue of the journal Science.
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