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Science Forum Index » Astro Forum » Evidence of Ancient Galactic Collision Found, Confirmed by U
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| Andrew Yee |
 Posted: Tue Jan 23, 2007 7:47 pm |
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Office of News and Information
University of Massachusetts-Amherst
Contact:
Mark Fardal, 413/577-2108
Jan. 22, 2007
# 116-07
Evidence of Ancient Galactic Collision Found, Confirmed by UMass Amherst
Atronomer's Model
AMHERST, Mass. -- Astronomers surveying the nearby Andromeda galaxy have
discovered an association of stars in its outskirts, which they believe to
be part of a separate galaxy that merged with Andromeda about 700 million
years ago. The discovery corresponds to computer models created by
University of Massachusetts Amherst astronomer Mark Fardal, which simulate
a dwarf galaxy merging with Andromeda.
The findings suggest that Andromeda's outer swathes of stars are from the
same parent galaxy, and may help astronomers determine the Andromeda's
total mass. Both findings were presented Jan. 7 at the American
Astronomical Society meeting in Seattle.
Large galaxies are believed to be built partly by the merging of smaller
galaxies, an event that destroys the smaller galaxy, explains Karoline
Gilbert, a graduate student at the University of California, Santa Cruz,
who presented at the meeting. The strong gravitational forces of a large
galaxy can rip apart a small galaxy, producing loose streams of stars that
astronomers call tidal debris. Measuring the amount of the tidal debris
that exists in present-day galaxies allows astronomers to examine the role
mergers play in galaxy formation.
At a distance of 2.5 million light-years, Andromeda is the nearest large
spiral galaxy to Earth and is very similar to our own Milky Way Galaxy.
Spiral galaxies are made up of three main visible features: a flat disk of
stars and gas, a bright central bulge of densely concentrated stars, and
in some cases a faint, spherically distributed halo of stars that extends
to large distances. Most of the mass of the galaxy, though, is made up of
invisible or "dark'' matter, detectable only through its gravitational
force.
Astronomers have discovered several areas in Andromeda's outskirts that
stand out for their enhanced density of stars, stellar features that are
likely tidal debris from past galaxy mergers. The two brightest features
are a stream of stars to the south of the galaxy, called the "giant
southern stream," and a faint extension to one side of Andromeda's disk
called the "northeast shelf." Until recently, these stellar features were
not clearly associated.
UMass Amherst's Fardal developed computer simulations of the merger of a
dwarf galaxy with Andromeda, building them to match observations of the
giant southern stream and northeast shelf. The simulation also predicted
other observed features around Andromeda, including a tidal debris feature
in the same location and with the same range of velocities as that later
discovered by Gilbert and her collaborators.
"The tidal debris feature we have discovered is almost an exact match with
the feature predicted in the simulations. This implies that this new
stellar stream and the giant southern stream, as well as the other stellar
features reproduced in the Fardal simulations, are all from the same
parent galaxy,'' said Gilbert.
"In the model, the two galaxies first collided about 700 million years
ago," explains Fardal. The smaller galaxy had about 2 billion Sun's worth
of stars, but Andromeda itself has about fifty times that. "Eventually,
the tidal debris from the satellite galaxy will smear out and be
indistinguishable from the rest of the halo," says Fardal.
Because the stars in the tidal debris are moving together, astronomers can
use them to measure the strength of the gravity around the Andromeda
galaxy. Gilbert's discovery of a new tidal debris feature, combined with
velocity measurements of the other related tidal debris, will provide
observations necessary to measure how much dark matter is in Andromeda and
how it is distributed, explains Fardal.
Gilbert and her collaborators are conducting a study of the stellar halo
of Andromeda, using observations at the W.M. Keck Observatory and the
Canada-France-Hawaii Telescope in Hawaii, and the Kitt Peak National
Observatory in Arizona.
Tidal debris features are usually found by inspecting images of a galaxy
and identifying areas that have a greater density of stars than expected.
The feature discovered by Gilbert is too faint to be seen this way.
Instead, the group used the sensitive DEIMOS spectrograph at the Keck
Observatory to separate light of different colors from individual stars in
the halo of Andromeda. They could then measure the velocities of the stars
from faint shifts in the colors of their light. The tidal feature showed
up as a group of stars with similar velocities.
Fardal's collaborators include Arif Babul and Alan McConnachie of the
University of Victoria in Canada. Gilbert's collaborators include Jasonjot
Kalirai, Puragra Guhathakurta, and Jedidah Isler at UCSC; Mike Rich and
David Reitzel at UCLA; and Steven Majewski, James Ostheimer, and Richard
Patterson at the University of Virginia.
[NOTE: An image supporting this release is available at
http://www.astro.umass.edu/~fardal/m31_images.html ] |
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