ATLANTA — A international team of astronomers from the Sloan Digital Sky Survey
collaboration has discovered a giant clump of stars near the Andromeda Galaxy
that could be a previously unknown satellite galaxy of Andromeda or could be the
last remnants of a galaxy torn apart by Andromeda’s tidal forces.

The clump of stars, named Andromeda NE by the researchers for its location to
the northeast of the Andromeda Galaxy, is enormous. It covers a larger area on
the sky than the full Moon. If added together the total light from the stars in
Andromeda NE would rival many nearby galaxies in brightness. Yet because these
stars are so spread out, Andromeda NE appears 10 times dimmer than the faintest
known galaxy.

"This helps to explain why it remained undiscovered," said Daniel Zucker, one of
the Sloan Digital Sky Survey (SDSS) researchers. "The quality of the SDSS data
allowed us to find this elusive object close to Andromeda, one of the
best-studied galaxies in the sky," added team member Eva Grebel of the
University of Basel. More than 2 million light years away, the Andromeda Galaxy
or M31 is the nearest spiral galaxy, and is one of a handful of galaxies
(besides our own Milky Way) that can be seen with the naked eye.

Zucker and Eric Bell from the Max Planck Institute for Astronomy (MPIA) in
Heidelberg, Germany presented these findings today (January 5, 2004) at the
American Astronomical Society meeting in Atlanta.

While analyzing data from an SDSS scan of Andromeda, Zucker and his team used
special filtering techniques to select objects with specific colors and
brightness typical of Andromeda’s stars. When the SDSS team mapped the
distribution of these stars they detected a number of features previously noted
by other astronomers. But SDSS researchers didn’t know what to make of a large
concentration of stars — what they described at first as a giant, ghostly shape
— that appeared to the northeast of Andromeda.

"At first we thought it was some sort of artifact, some kind of problem with our
filtering technique," says Zucker. "But test after test showed that it was
really there."

Having determined that the stellar structure was real, the SDSS team members
from MPIA, the University of Washington in Seattle, the University of Basel in
Switzerland and New Mexico State University in Las Cruces worked to find out
what it was.

"One of the most important questions about Andromeda NE is its distance.
Although our findings don’t allow us to measure this precisely, our data
indicate that it is roughly at the same distance as the Andromeda Galaxy," said

That means that the clump of stars is gravitationally bound to the Andromeda
Galaxy, in orbit around its larger neighbor and possibly is in the process of
being torn apart by Andromeda’s tidal forces. These forces arise because
Andromeda’s gravitational pull is stronger on the near side of Andromeda NE than
on the far side, pulling the stars away from each other. "For most small
companion galaxies it is ultimately only a matter of time until they are
shredded by the tides of the parent galaxy," explained Hans-Walter Rix, director
of MPIA.

Over the past decade astronomers have found increasing evidence that the
distant, outer reaches of normal spiral galaxies like Andromeda and the Milky
Way are not quiet backwaters, but rather arenas of ongoing galaxy disruption. As
satellite galaxies are ripped apart by tidal forces, they lose stars in great
streams along their orbital paths. Researchers have detected such stellar
streams around both the Milky Way and Andromeda, suggesting that this kind of
galactic cannibalism is commonplace.

"SDSS data have been used to study the destruction of satellites by our galaxy,
and now they are adding to our knowledge of Andromeda’s eating habits," observed
Alexei Kniazev, an SDSS collaboration member and a researcher at MPIA.

In 2003, a team of SDSS scientists found what is believed to be the remnants of
a galaxy pulled apart by the Milky Way. The SDSS’s ability to do
three-dimensional mapping revealed that the concentration of stars of a certain
color and brightness were actually parts of a separate structure outside of the
Milky Way.

So far the exact nature of Andromeda NE remains a mystery. It could be a
satellite galaxy of Andromeda, perhaps one so stretched out by Andromeda’s tides
that it has become the enormous, remarkably diffuse structure seen today. Or it
could be part of a giant stream of stars, all that remains of a satellite that
was completely disrupted by Andromeda’s tidal forces in the distant past. "But
we’ll need more data to determine which it is," said Zucker.

[ (35KB)]
This composite illustration shows the location of Andromeda NE (arrow), the
complicated stellar structures in the halo of the Andromeda Galaxy (inset image
to scale) and a scaled photo of the moon as a size reference (to the right). The
colors in the image reflect the relative colors of stars that are predominantly
red giants at the distance of the Andromeda Galaxy. Bluer and whiter colors
generally indicate lower metallicity (elemental abundances) and yellow and red
stars indicate increasing elemental abundances. (Except for Andromeda NE, most
of the major structures shown here were also seen by Ferguson et al as noted in
Astronomical Journal, vol. 124, p. 1452, Sept. 2002, although the representation
of SDSS data is more informative.)

SOURCES: Sloan Digital Sky Survey; Inset of M31 from Bill Schoening, Vanessa
Harvey/REU program/NOAO/AURA/NSF; Photo of full moon from Lick Observatory.