A theoretical model of a galaxy like the Milky Way, showing trails of
stars torn from disrupted satellite galaxies that have merged with the
central galaxy. The structures seen in the SDSS-II star maps support
this prediction of a complicated outer Milky Way. The region shown is
about one million light years on a side; the sun is just 25,000 light
years from the center of the Milky Way and would appear close to the
center of this picture.
Credit: K. Johnston, J. Bullock
A new map of the halo of stars that surrounds our Milky Way Galaxy has revealed a complicated structure of crisscrossing stellar streams, many of which have never been detected before.
While the bulk of our galaxy's stars are concentrated in a fairly flat disk and a bulbous central region, the halo is the first thing an intergalactic traveler would encounter upon approaching our home galaxy. The halo begins at the edge of the disk around 65,000 light years from the galactic center and may extend out as far as 300,000 light years from the center of the galaxy. The halo comprises star clusters, clouds of gas, dark matter, and a few lone stars. Some of these pieces were grabbed up by the Milky Way from dwarf galaxies as they passed by.
The largest stellar streams in the halo have been mapped out over the last decade, but new data from the Sloan Digital Sky Survey (SDSS-II) has found many previously unknown smaller streams, remnants of dwarf galaxies that strayed too close and a few surviving companions.
The streams are remnants of smaller galaxies that have been consumed.
The new findings are being presented today at an international symposium in Chicago.
Small streams, small fraction
The survey measured the motions of nearly a quarter million stars in selected areas of the sky, looking for groups traveling at the same velocity. The search turned up 14 distinct structures, 11 of which had never before been seen.
Because the survey has only looked at a small fraction of the Milky Way, the 14 streams found "implies a huge number when we extrapolate out to the rest of the Milky Way," said Kevin Schlaufman, a graduate student at the University of California at Santa Cruz.
There could be close to 1,000 streams in the inner 75,000 light years of the Milky Way, Schlaufman said, assuming each of the 14 structures they observed is a separate stream. There is the possibility that there are actually fewer stream that are simply seen many times in different places.
Strands of pasta
Columbia University researcher Kathryn Johnston describes the halo as "a jumble of pasta."
"In the center of the galaxy, these stellar strands crowd together and you just see a smooth mix of stars," she said. "But as you look further away you can start to pick out individual strands, as well as features more akin to pasta shells that come from dwarfs that were on more elongated orbits."
Dwarf galaxies that pass close to the Milky Way can be stretched by gravitational tides into spaghetti-like strands, which wind around the galaxy as stars trace out the same orbital paths at different rates, Johnston said.
Heidi Newberg, of Rensselaer Polytechnic Institute, and her graduate student Nathan Cole have been trying to follow some of these strands as they wind their way around the galaxy.
"It's a big challenge to piece things together," said Cole, "because the stream from one dwarf galaxy can wrap around the [Milky Way] and pass through streams of stars ripped from other dwarf galaxies."
Newberg and Cole found at least two superposed structures, possibly three or more, toward the constellation Virgo where SDSS images revealed an excess of stars covering a huge area of sky. Velocity measurements can be used to separate the overlapping systems, some of which come from a tidal arm of the Sagittarius dwarf galaxy.
The SDSS data also revealed 14 surviving dwarf companions to the Milky Way, including two new discoveries announced at the symposium. These satellite galaxies are oribiting within the halo of invisible dark matter whose gravity holds the Milky Way together.
The newly discovered dwarfs are much fainter than those known before the survey. Though SDSS can detect ultra-faint dwarfs, it can only do so if they are nearby, so there could be several hundred or more further out in the Milky Way's dark halo.
"The SDSS has taught us a huge amount about the Milky Way and its neighbors," said Johnston. "But we're still just beginning to map the galaxy in a comprehensive way, and there's a trove of discoveries out there for the next generation of surveys, including the two new Milky Way surveys that will be carried out in SDSS-III."
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