Multiple galaxy mergers continue in the Milky Way (Forwarded)

Sloan Digital Sky Survey

Contacts:

Vasily Belokurov, Cambridge University
01223 337515

Daniel Zucker, Cambridge University
01223 737529

Wyn Evans, Cambridge University
01223 765847

David Weinberg, Scientific Spokesperson, Sloan Digital Sky Survey
614-292-6543

Gary S. Ruderman, Public Information Officer, Sloan Digital Sky Survey
312-320-4794

May 8, 2006

Multiple galaxy mergers continue in the Milky Way

A new map of stars in the Milky Way Galaxy, constructed with data from
the Sloan Digital Sky Survey (SDSS-II), reveals a night sky
criss-crossed with streams of stars, left behind by satellite galaxies
and star clusters spiraling to their deaths.

Analyzing five years of data spanning nearly one-quarter of the sky,
Cambridge University (UK) researchers Vasily Belokurov and Daniel Zucker
created a dramatic new image of the outer Milky Way, using stellar
colors eliminating the redder, nearby stars that would otherwise swamp
the view of background structures. They found so many trails of stars in
their high contrast image that they named the area the "Field of Streams."

Satellite galaxies orbiting the Milky Way are literally ripped apart by
the tidal forces of our galaxy. As these satellites sink in
gravitational quicksand, their stars are torn from them in giant streams
that trace their orbital paths -- just like meteor streams lie along the
paths of defunct comets in the Solar system.

Dominating the Field of Streams image is the enormous, arching stream of
the Sagittarius dwarf galaxy. The Sagittarius dwarf was discovered more
than a decade ago and other researchers have previously mapped its long
tidal stream in other regions of the sky.

Forked streams

But the new SDSS-II data had a remarkable surprise in store.

"The stream appears forked," said Belokurov. "We are seeing different
wraps superimposed on the sky, as the stream goes around the galaxy two
or three times."

Because of the multiple wraps, the observations provide strong new
constraints on the dark matter halo of the Milky Way, according to Mike
Fellhauer of Cambridge University. "The leading theories of dark matter
predict that the Galaxy's halo should be flattened, like a rugby
football. But our simulations only match the forked Sagittarius stream
if the inner halo is round, like a soccer ball."

In addition to the Sagittarius arches, the Field shows faint trails of
stars torn from globular clusters, and other rings, trails, and lumps
that appear to be the remains of disrupted dwarf galaxies. "There are
more streams here than in a river delta, commented Zucker.

Prominent among these is the Monoceros stream, discovered previously by
SDSS-II scientists Heidi Jo Newberg of Rensselaer Polytechnic Institute
and Brian Yanny of the Fermi National Accelerator Laboratory. The
multiple rings of stars are all that remain from a dwarf satellite that
was absorbed by the Milky Way long ago.

Crossing the Field is an enigmatic, new stream of stars extending over
70 degrees on the sky, whose original source remains unknown.

"Some of these 'murdered' galaxies have been named," explained SDSS-II
team member Wyn Evans of Cambridge, "but this galactic corpse hasn't
been identified yet. We're looking for it right now."

These new discoveries add weight to a picture in which galaxies like the
Milky Way are built up from the merging and accretion of smaller galaxies.

"We've known about merging for some time" said Yanny, "but the Field of
Streams gives us a striking demonstration of multiple merger events
going on the Milky Way galaxy right now. This is happening all over the
Universe, as big galaxies grow by tearing up smaller ones into streams."

These streams also provide new tests of the nature of dark matter
itself, according to theorist James Bullock of University of California
at Irvine; Bullock was not part of the SDSS team. "The fact that we can
see a Field of Streams like this suggests that dark matter particles are
very 'cold', or slow moving. If the dark matter was made up of warm,'
fast moving particles, we wouldn't expect these thin streams to hang
around long enough for us to find them."

Authors:

* V. Belokurov, University of Cambridge
* N.W. Evans, University of Cambridge
* G. Gilmore, University of Cambridge
* S. Vidrih, University of Cambridge
* D. M. Bramich, University of Cambridge
* H. J. Newberg, Rensselaer Polytechnic Institute
* R. F. G. Wyse, Johns Hopkins University
* M. J. Irwin, University of Cambridge
* M. Fellhauer, University of Cambridge
* P. C. Hewett, University of Cambridge
* N. A. Walton, University of Cambridge
* M. I. Wilkinson, University of Cambridge
* N. Cole, Rensselaer Polytechnic Institute
* B. Yanny, Fermi National Accelerator Laboratory
* C. M. Rockosi, University of California, Santa Cruz
* T. C. Beers, Michigan State University
* E. F. Bell, Max Planck Institute for Astronomy
* J. Brinkmann, Apache Point Observatory
* Z. Ivesic, University of Washington
* R. Lupton, Princeton University Observatory

IMAGE CAPTION:
[http://www.sdss.org/news/releases/20..._enlarge.html]
The "Field of Streams" -- this image is a map of stars in the outer
regions of the Milky Way covering about one-quarter of the night sky, as
observed by the Sloan Digital Sky Survey (SDSS-II). The trails and
streams that cross the image are stars torn from disrupted Milky Way
satellites. The color corresponds to distance, with red being the most
distant and blue being the closest. The large, forked feature is the
Sagittarius stream, further away from us (lower left) and closer to us
(middle right). Other features marked are the Monoceros ring and the as
yet unidentified stream.
(Credit: Vasily Belokurov, SDSS-II Collaboration)