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.
Survivors
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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