Milky Way Has a “Squashed Beachball”-Shaped Dark Matter Halo

by Ryan Anderson on January 6, 2010

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This illustration shows the visible Milky Way galaxy surrounded by a "squashed beachball"-shaped dark matter halo. Click the image for a 3D flyaround view. Source: UCLA

This illustration shows the visible Milky Way galaxy surrounded by a “squashed beachball”-shaped dark matter halo. Source: UCLA

Our galaxy is shaped like a flat spiral right? Not if you’re talking about dark matter. Astronomers announced today that the Milky Way’s dark matter halo, which represents about 70% of the galaxy’s mass, is actually shaped like a squashed beachball.

Dark matter is completely invisible, but it still obeys the law of gravity, so the existence of dark matter haloes, and their shape, can be inferred by monitoring the orbits of dwarf galaxies orbiting the much larger Milky Way.

Unfortunately, to determine the orbit of an object, you have to measure its position at several points in that orbit, and dwarf galaxies take about a billion years to go around the Milky Way. Astronomers just haven’t been around long enough to watch even a fraction of a complete orbit. Luckily, they don’t have to.

Dwarf galaxies, just like their full-sized counterparts, and made of billions of stars. When the tidal forces from a big galaxy like the Milky Way act on a dwarf galaxy, the result is a streamer of stars that trace out the dwarf galaxy’s orbit. By using data from huge all-sky surveys, a group of astronomers led by David Law at UCLA were able to reconstruct the orbit of the Sagittarius Dwarf Galaxy. There was just one problem: different parts of the dwarf galaxy had different orbits, which led to wildly different dark matter halo shapes.

Law and his colleagues Steven Majewski (University of Virginia) and Kathryn Johnston (Columbia University) solved this problem by allowing models of the dark matter halo to be “triaxial” – in other words, have different lengths in all three dimensions. The best model solution results in a halo shaped like a beach ball that has been squashed sideways.

“We expected some amount of flattening based on the predictions of the best dark-matter theories,” said Law, “but the extent, and particularly the orientation, of the flattening was quite unexpected. We’re pretty excited about this, because it begs the question of how our galaxy formed in its present orientation.”

Sagittarius is not the only dwarf galaxy orbiting the Milky Way, and Law and his colleagues plan to study the orbits of other dwarf galaxies to refine their model. “It will be important to see if these results hold up as precise orbits are measured for more of these galaxies. In the meantime, such a squashed dark-matter halo is one of the best explanations for the observed data.”

This illustration shows the visible Milky Way galaxy (blue spiral) and the streams of stars represent the tidally shredded Sagittarius dwarf galaxy. Click the image for a flyaround view. Source: UCLA

This illustration shows the visible Milky Way galaxy (blue spiral) and the streams of stars represent the tidally shredded Sagittarius dwarf galaxy. Source: UCLA

clatonium January 6, 2010 at 9:25 AM

Yes that’s nice but you need a username and password to view the movies.

SuperKevin January 6, 2010 at 9:50 AM

Amazing find, and I’m sure it’s just the tip of the iceberg.

Torbjorn Larsson OM January 6, 2010 at 10:04 AM

Well, squash me sideways! Unexpected indeed.

Sili January 6, 2010 at 10:20 AM

Unexpected, yes. But that’ll certainly make it all the more awesome if other dwarven tails confirm it. And if they don’t we’ll finally have some solid evidence to make us rethink the halo idea.

ioresult January 6, 2010 at 11:45 AM

I want to see the movie, but it asks for a login/password!

agmartin January 6, 2010 at 12:45 PM

You can see the second one here:

http://www.astro.virginia.edu/~srm4n/Sgr/movie.html

JocaBoboca January 6, 2010 at 6:03 PM

WTF, is there a Puzzle to solve to get the user/password? Please give me a hint, I can’t think of anything.

Lawrence B. Crowell January 7, 2010 at 4:50 AM

Inside a uniform distribution of matter the gravitational potential can be calculated from Gauss’ law, and it is a harmonic oscillator. So the further away from the center, d, the force is inward as F ~ -d^2. So the two trails of stars and gas from dwarf galaxies are on nearly perpendicular planes, and their size is a signature of the potential along those planes.

This is a bit odd, for one would think that the DM halo would be rotating and bulging outwards. Strange indeed.

LC

Lawrence B. Crowell January 7, 2010 at 4:55 AM

I didn’t say this quite right, having looked at the movie. Yet the orbit is as the article indicates capable of “probing” the DM distribution across a spatial extent. The plane of the orbit of the material is rotated relative to the plane of the Milky Way.

LC

Dark Gnat January 7, 2010 at 7:17 AM

Strange, it is squashed in just the opposite way that we would expect. I’m wondering if this has any connection to the tendency for galaxies to be lines up along their axis’.

It makes me wonder of there really are “strings” of dark matter which galaxies form along.

Exciting!

Olaf January 7, 2010 at 10:04 AM

I agree it is not squashed the way I expected. I would expect the gravitational pull of the dark matter to pull the stars up.

Buxtehude January 7, 2010 at 11:19 AM

I found it impressive to hear that the team was able to reconstruct the orbit of the Sagittarius Dwarf Galaxy around our own Milky Way G.

However, in another post of Jan 6th., “Spitzer Peers into the Small Magellanic Cloud”, we read there, almost as a footnote:

“The team of researchers also indicated that their work may indicate that the Magellanic Clouds are not gravitationally bound to the Milky Way and may just be passing.”!

Makes me wonder if that observation may hamper future aspirations to further test/refine the theory set out in the present article or indeed may undermine it.

Lawrence B. Crowell January 7, 2010 at 11:23 AM

A spherical symmetrically distributed mass will gravitationally act on the motion of a particle inside like a harmonic oscillator. Examples of a harmonic oscillator are a mass on a spring or the motion of a pendulum with small oscillations. There is a curious property of the motion of a mass on a spring. The frequency of oscillations does not depend on the length the spring is extended. For this reason galaxies do not obey Kepler law dynamics, but rather rotates approximately as a disk does. So an outer star is like a mass oscillating on a spring with a large amplitude, while a star near the galaxy center is like a mass on a spring with a small amplitude. But both orbit with approximately the same frequency.

This was the initial motivation for dark matter, for it was observed that galaxies were not like solar systems, where inner planets have high frequencies and the outer planets plod along with low frequencies. Galaxies rotated more as a solid disk does. So there had to be this halo of invisible matter that enclosed the galaxy.

The squashing of the spherical distribution of dark matter adjusts things a little bit. Now the pointy end at the top and bottom do pull on matter in the galaxy, but in more or less equal and opposite directions. This is why stars will not leave the Milky Way galaxy decause of the prolate distribution of matter and DM above and below.

LC

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