Astronomers have long puzzled over the origin of the x-ray glow in the middle of our Milky Way galaxy. Previous spectral measurements have hinted at the source of the diffuse light, but the data were inconclusive. Recently, observers spent 170 hours pointing NASA's Chandra x-ray telescope at a 100-light-year-wide region around the galactic center in hopes of elucidating matters. The results raise as many questions as they answer.

To study the diffuse x-ray emission, Michael Muno of UCLA and his collaborators removed from the Chandra image 2,357 bright points that came from obvious x-ray sources, like white dwarfs, neutron stars and black holes, as well as distant galaxies lying behind the Milky Way. The remaining sources in the image were too faint to identify. But the researchers determined that these unresolved sources could not account for all of the remaining x-rays. In fact, they estimated that the galactic center would have to contain 200,000 of the known x-ray objects--10 times more than are predicted to exist--in order to generate the leftover light in the image.

Instead the majority of the galactic center's ghostlike x-ray emission appears to be coming from two bodies of hot ionized gas, or plasma. The two plasmas occupy essentially the same volume, but one has a temperature of 10 million degrees Celsius and the other about 100 million degrees C. The cooler gas is most likely gas blown off during the violent deaths of massive stars.

But the origin of the hotter gas remains an enigma. It is so hot that the galaxy's gravity cannot keep it from boiling off into intergalactic space. Supernovae and stellar winds could replenish the gas, but their typical energy is not enough to produce a 100-million-degree plasma. As an explanation, the researchers posit that cosmic rays, magnetic fields and excess supernovae could provide additional heat, but they note that none of these theories are entirely successful.

The heating problem may call into question some of the basic understanding of the composition of the interstellar medium. "I think it deepens the mystery more than anything," Muno remarks. The answer, he thinks, may come when the Japanese satellite Astro-E2 is launched. Its ability to distinguish features in the x-ray spectrum will complement Chandra's ability to distinguish objects in the sky, Muno explains: "I think that should really nail it down." The team's findings are slated for the September 20 Astrophysical Journal.