Streams of stars that are being ripped to shreds as they spiral into the Milky Way have been imaged in unprecedented detail using data from the Sloan Digital Sky Survey. The so-called Field of Streams suggests the halo of dark matter that cocoons our galaxy is spherical 鈥� not squashed like an American football as previously thought.
Galaxies are thought to have massive dark matter halos that extend many times farther than the galaxies themselves. Theories predict that the halos are flattened 鈥� ellipsoid 鈥� in shape, but different observations of gas and stars in the Milky Way have suggested different shapes for its halo.
Then in 1997 astronomers discovered a powerful new tool to probe the structure of the halo 鈥� a stream of stars extending from a small galaxy called Sagittarius that lies about 82,000 light years from Earth. The dwarf galaxy is being stretched into a stream by the gravitational tug of the Milky Way, which will one day consume it entirely.
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鈥淚t鈥檚 spiralling in and losing its stars all the way,鈥� says Vasily Belokurov at the University of Cambridge, UK.
Several smaller streams have since been observed trailing from clusters of stars, and potentially another dwarf galaxy, as they fall prey to the Milky Way鈥檚 pull. Now, Belokurov and colleagues have used the Sloan Digital Sky Survey 鈥� which is mapping a quarter of the sky 鈥� to study this 鈥淔ield of Streams鈥�.
The team found a new stream of stars, dubbed Orphan (on the right of the image), whose source is not yet known. 鈥淭his stream is like a blood trail that should bring us to the murdered Milky Way satellite,鈥� Belokurov told New 杏吧原创.
Forked streams
Because the streams trace the doomed objects鈥� orbits, they can illuminate how dark matter is distributed in the halo. But previous studies of the Sagittarius stream, which circles the Milky Way and is by far the most prominent stream, left the shape of the Milky Way鈥檚 halo ambiguous. That is because these surveys did not cover part of the leading arm of the stream.
Now Belokurov鈥檚 team has imaged that missing section and found that the stream actually forks into two streams at that point. This suggests that the dwarf galaxy鈥檚 orbital plane shifts over time and the streams represent two different orbits around the Milky Way, they say.
They conclude that such an orbital shift implies the halo must be spherical 鈥� not American football-shaped as most halos are thought to be. 鈥淚t means that, at least in the case of the Milky Way, the prediction of the current cosmological theory is not exactly true,鈥� Belokurov told New 杏吧原创. 鈥淲e definitely need to do a lot of thinking to understand what is happening.鈥�
Scott Chapman of the California Institute of Technology in Pasadena, US, agrees with their conclusion. 鈥淭hese observations are at the 鈥榮weet spot鈥� for trying to constrain the shape of the halo, and I think they鈥檝e shown very robustly that the halo is spherical,鈥� Chapman told New 杏吧原创.
That is in stark contrast to what Chapman and colleagues find in the Milky Way鈥檚 nearest large neighbour, the Andromeda galaxy. Observations of a stellar stream in that galaxy suggest its halo is flattened, he says.
Plumped-up halos
鈥淲e find exactly the opposite situation in Andromeda,鈥� he says. 鈥淚 think it tells us that the conditions in the Milky Way were quite rare in the overall formation of galaxies.鈥�
But James Bullock, an astrophysicist at the University of California in Irvine, US, says spherical halos may be more common than first thought. He says recent models have shown that flattened halos can plump up over time because the discs of galaxies within them attract dark matter towards them. 鈥淲hen the disc forms, it tends to drag matter in and the halo becomes more round,鈥� Bullock told New 杏吧原创.
Bullock adds that the high number of streams observed so far bolsters standard theories that dark matter is made of cold, slow-moving particles. 鈥淚f the dark matter was made up of warm, fast-moving particles, we wouldn鈥檛 expect these thin streams to hang around long enough for us to find them,鈥� he says.
The new work has been submitted to the Astrophysical Journal.
