Halo Around Milky Way Is Reported
A mysterious high-energy halo surrounds the Milky Way galaxy, a UC Riverside astronomer announced Tuesday.
If the findings are confirmed, this previously unseen gamma ray glow may shed light on one of the most perplexing puzzles of cosmology: the nature of the exotic “dark matter” thought to make up as much as 99% of the universe. “It’s the first light on dark matter,” said astrophysicist Lynn Cominsky, of Sonoma State University.
Because the glow appears to be emanating from virtually matter-free regions of the galaxy’s outskirts, some astronomers surmise that it must be produced by collisions involving previously unseen forms of matter. “There’s nothing out there that should be a source of gamma rays,” said UC Riverside astronomer David Dixon, before his presentation at a high-energy astrophysics meeting in Estes Park, Colo.
“We can say with confidence that it’s really there,” Dixon said. “But exactly what it is is uncertain. We’re kind of riding on the hairy edge of things.”
Although some astronomers are offering more mundane explanations, Cominsky and others were excited that at last, exotic particles of dark matter--long suspected but never seen before--had “shown” themselves.
Anywhere from 90%-99% of the matter in the universe cannot be seen directly, most astronomers agree. Instead, the presence of this so-called dark matter is inferred from its gravitational pull on visible objects. Like the puppeteer’s hands making dolls dance on a stage, so the gravity of these unseen objects causes stars to travel in apparently unnatural paths.
Exactly what dark matter might be, however, is hotly debated. Some astronomers believe that it is composed primarily of dark stars or big planet-like bodies--”big chunks of rock,” as Cominsky described them. Astronomers call these dark matter objects MACHOs, for Massive Compact Halo Objects.
However, other astronomers don’t believe that there is enough ordinary matter to account for all the dark matter, and instead propose the existence of exotic particles collectively known as WIMPs, or Weakly Interacting Massive Particles.
These WIMPs--if they exist--might be colliding with each other in the outskirts of the Milky Way, producing the gamma ray glow. (MACHOs have been detected indirectly, but they would not produce gamma rays.)
“If you believe in WIMPs, that’s where they would be,” Cominsky said.
Although faint halos have been detected before, this is the first time that the light was bright enough to suggest that they were seeing gamma rays produced by the collision of dark matter particles. Gamma rays are the same as visible light, but up to a billion times more energetic. They are produced by the most violent events in the universe, including exploding stars and black holes.
The glow was detected by one of the instruments aboard the Earth-orbiting Compton Gamma Ray Observatory, launched by NASA in 1991 specifically to view the universe in gamma ray light.
Some scientists skeptical of the findings cite uncertainties in the observatory’s ability to pick out a glow against an already bright background. “I certainly applaud Dr. Dixon,” said Hampton-Sidney astrophysicist Don Kniffen. “My concern is that the instrument has a lot of quirks in it that we don’t understand.”
The glow is exceedingly dim and therefore difficult to see next to the diffuse gamma ray background created by the steady stream of particles raining down from outer space. Detecting it is like looking for a firefly against the background of a roaring flame. “We have the same problem with the Milky Way,” Kniffen said. “It really lights up in gamma rays.”
Using some sophisticated mathematical filtering, Dixon and his colleagues were able to “subtract” the background gamma rays from their picture, in effect blotting them out like the moon blots out the sun during a solar eclipse.
What’s left is the newly discovered glow. Kniffen saw a similar glow in a study yet to be published, but didn’t feel confident enough to go public. “We didn’t make such a big deal out of it,” said Kniffen, who admitted that the gamma rays he saw are only one-tenth the energy of those reported by Dixon.
“If he’s right, this may open up a whole new channel for study,” Kniffen said.
Other researchers who looked at the images felt that although the gamma ray halo was real enough, it didn’t necessarily indicate the presence of dark matter. Caltech astrophysicist Shri Kulkarni agreed with those who thought that the gamma rays could be created by high-energy electrons rather than dark matter.
However, for electrons to produce such high-energy light would require “physics we don’t understand,” Cominsky said.
Other theories explain the gamma ray halo as the remnant glow of an ancient era of rapid star formation in the Milky Way, when the galaxy was a so-called starburst galaxy, setting off one stellar explosion after another like a string of firecrackers. Still another explanation attributes the gamma rays to pulsars, rapidly spinning stellar cores that send out beams of gamma rays.
Pulsars and exploding stars should produce clear evidence of their existence, however, Cominsky said. “You should see hundreds of pulsars in this halo, and we don’t see any of them.”
That leaves WIMPs.
Researchers will need new data from the next generation of gamma ray telescopes to find “the smoking gun” that will determine the true source of the gamma ray halo, said Dixon.
“It’s the signature of something going on, and it’s telling us there’s more going on out there than we expected,” he said.
But as yet, they don’t know exactly what the signature spells out. In the meantime, even skeptics find the new images intriguing.
“The halo is very difficult to study,” Kulkarni said. “It’s the outer reaches of the Milky Way. This would be a new way to understand what’s going on out there [even if] it doesn’t necessarily mean there’s this whole new thing going on.”
(BEGIN TEXT OF INFOBOX / INFOGRAPHIC)
In a Cloud
A cloud of gamma rays forms a halo many thousands of light years thick and possibly surrounding the entire Milky Way, the galaxy containing Earth, researchers said Tuesday. The huge aurora of photons, which are high-energy particles of light, mapped by NASA’ Compton Gamma Ray Observatory exists in an area empty of celestial objects known to generate gamma rays. The cloud may be evidence of dark matter, the missing mass of the universe that scientist have not been able to observe directly.
Source: UC Riverside, Clemson University and University of Chicago
