Does Your Halo Ever Give You a Headache?

Apparently there is a halo of dark matter surrounding the Milky Way galaxy -- as there is, indeed, surrounding many other galaxies -- and it bears the improbable shape of a giant beach ball that's been partially deflated and flattened out, according to new results presented at last week's annual meeting of the American Astronomical Society. Scientists from UCLA, University of Virginia and Columbia University announced the finding after making the first ever measurement of the 3D shape of a single dark matter halo.

As massive as galaxies are, ordinary matter only accounts for roughly 30 percent of their mass; dark matter halos constitute the rest. We just can't see them through our usual methods of probing the cosmos (most of which involve electromagnetism); we can only see them via their gravitational effects on smaller dwarf galaxies orbiting the Milky Way. These poor little things are essentially "shredded by tidal forces... and leave stars like breadcrumbs along their path."

So astronomers made like Hansel and Gretel and tracked those breadcrumbs -- or "tidal debris" -- left by the Sagittarius Dwarf Galaxy, using data collected by the Two-Micron All Sky Survey and the Sloan Digital Sky Survey. They came up with some decent models for the our haloed Milky Way, but no one model applied to all parts of the dwarf galaxy's orbits. Which was weird, to say the least. The stellar breadcrumbs were leading astronomers down more than one path, and they couldn't figure out which one was correct.

Enter David Law, Steven Majewski, and Kathryn Johnson, who published a paper in the Sept. 2009 issue of the Astrophysical Journal, followed by a second follow-up paper last week. They figured out that in order to fit the entire orbit of Sagittarius into one model at the same time, it was necessary to allow the the dark matter halo to have different axis lengths in all three dimensions -- in other words, the halo must be triaxial, from which the team inferred that its shape looks for all the world like a beach ball squashed from the side.

That, too, was a bit of a surprise, the three astronomers reported at the meeting. It means that the halo of dark matter and the stars in our galaxy are perpendicular to each other -- an exciting result because it answers one question only to raise a new one, namely, "how our galaxy formed in its present orientation."

The next step is to look at other dwarf galaxies surrounding the Milky Way to see if they exhibit similar behavior. In the meantime, there's a nifty online mini-movie showcasing a flyaround of this fascinating system for your viewing pleasure. Check it out!