Thursday, October 02, 2014

Fermi Bubbles: Remodelling the Milky Way

Image courtesy NASA.
If someone asked you what the galaxy looks like from afar, you might tell them it’s a spiral. You’d be half-right. If you thought to include dark matter in your answer, you might say that it’s a disk or a cloud, but you still wouldn’t get full credit. Don’t feel bad, though—there’s more to the Milky Way’s structure than meets the eye, and much of it was completely impossible for us to see up until very recently. When data from the Fermi Gamma-ray Space Telescope, launched in 2008, was analyzed, researchers were startled by an unexpected feature: two tremendous, bubble-like clouds of plasma stretching perpendicular to the galactic plane, which seem to originate in the vicinity of the supermassive black hole at the galaxy’s nucleus. The gamma rays shimmering from these bubbles tell us that the particles in them are extremely energetic, and explain why we hadn't seen them until so recently; the earth’s atmosphere filters out most of the light they cast. Their origin, however, remains mysterious.

Numerous explanations have been proposed, attributing these gamma rays to everything from galactic super-winds to magneto-sonic shock waves, but all have some issues. Among the more exotic ideas is the suggestion that dark matter particles are meeting their anti-particles in this region, and annihilating to produce the energy we observe. Since dark matter is currently hypothesized to make up roughly 85% of the universe, this seems like it should be a strong possibility. Very few theoretical objections to this view have been raised, but the lack of contradiction is partly due to the fact that so little is known about dark matter and its potential interactions: while the idea is hard to rule out, it is correspondingly hard to prove via experiment.
The fast-moving gas, pictured in red, is thought to be the result of
rapid star formation near the center of the galaxy.
Image courtesy NASA, ESA, and the Hubble Heritage Team.

One hypothesis which offers more testable predictions is that of galactic particle winds. The term “galactic wind” refers to the immensely energetic outflows of charged particles observed near the centers of galaxies such as Messier 82.  While many disagree with the idea that such processes could have produced structures as massive and uniformly luminous as the Fermi bubbles, a 2011 analysis from the Max Planck institute concluded that they could be explained as protons ejected during star formation over a period of billions of years, surfing a galactic wind out to their incredible distance and being confined there by strong magnetic effects.

Regardless of which stance you prefer, the discovery of the Fermi bubbles leaves no question that the galaxy is a wildly dynamic place, with surprising levels of activity driving electromagnetic processes on an unfathomable scale. Perhaps it’s ironic, then, that the discovery of such tremendous structures paints a picture of our galaxy which bears a striking resemblance to some of the smallest objects ever observed.

Left, Artist’s rendition of the galaxy with Fermi bubbles. Right, a computer model of the 3Dz^2 Hydrogenic atomic orbital.
Images courtesy NASA and Dr. Mark Winter, respectively.

But while it may be intuitively pleasing to imagine our galaxy as a single macroscopic molecule in a universal “gas”, if the question ever comes up while you're on Jeopardy!, you might do best to stick with “What is a spiral?”


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