The Galactic Dark Matter Halo Is Shaped Like a Ball

A rather flattened one, astronomers say

By on January 7th, 2010 07:56 GMT
Astrophysicists determined some time ago that the Milky Way, and probably most other galaxies too, was surrounded by a halo of dark matter. The stuff cannot be detected by conventional means, and neither analyzed directly, so its existence can only be inferred from the effects it exerts on stars, galaxies, and other, “regular” matter around. In fact, experts also know, dark matter is the main contributor to the weight of galaxies, and also makes up about 23 percent of everything in the Universe. Dark energy accounts for 72 percent, while the rest of five percent is everything we can actually see.

Speaking at the 215th meeting of the American Astronomical Society (AAS) yesterday, in Washington DC, experts from the University of California in Los Angeles (UCLA) said that they had finally managed to determine the shape of the dark matter halo surrounding our galaxy. According to UCLA Professor David Law, who presented the team's work, this was made possible through the study of the debris field left behind by Sagittarius. This is a small, dwarf galaxy, which orbits the Milky Way at a relatively low speed, allowing astrophysicists insight into how dark matter influences debris fields.

“You can't actually see it [dark matter] directly, you can see it through its effects on stellar structure, star clusters, and dwarf galaxies orbiting around the Milky Way. So what you want to do is map where these star clusters and dwarf galaxies go and use that to reconstruct their orbits and where the mass is,” Law explained at the meeting. He also said that one of the main reasons why the team turned to the Sagittarius galaxy was the fact that the cosmic object had been a subject of exhaustive studies in the past. Therefore, the team set out in its investigation on a solid, scientific background of their target.

According to Law, the best possible way to describe the dark matter halo is to liken it to a “cosmic beach ball, squashed from the side.” In the gravity model the team created, the researchers allowed for the halo to have different sizes in all directions. This is what ultimately allowed for the new shape to be obtained. Other simulations only allowed for a single size of the halo in all directions, so the algorithms needed to combine all the data available in the best way possible. “It's a little weird in current dark matter models, but it'll be very useful in helping constrain future models, not only of dark matter itself but also how galaxies such as our own form in the Universe,” Law concluded, quoted by the BBC News.

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