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Andromeda's Stellar Halo Shows Galaxy's Origin to Be Similar to Thatof Milky Way (Forwarded)



 
 
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Default Andromeda's Stellar Halo Shows Galaxy's Origin to Be Similar to Thatof Milky Way (Forwarded)

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February 27, 2006

Andromeda's Stellar Halo Shows Galaxy's Origin to Be Similar to That of
Milky Way

PASADENA, Calif. -- For the last decade, astronomers have thought that
the Andromeda galaxy, our nearest galactic neighbor, was rather
different from the Milky Way. But a group of researchers have determined
that the two galaxies are probably quite similar in the way they
evolved, at least over their first several billion years.

In an upcoming issue of the Astrophysical Journal, Scott Chapman of the
California Institute of Technology, Rodrigo Ibata of the Observatoire de
Strasbourg, and their colleagues report that their detailed studies of
the motions and metals of nearly 10,000 stars in Andromeda show that the
galaxy's stellar halo is "metal-poor." In astronomical parlance, this
means that the stars lying in the outer bounds of the galaxy are pretty
much lacking in all the elements heavier than hydrogen.

This is surprising, says Chapman, because one of the key differences
thought to exist between Andromeda and the Milky Way was that the
former's stellar halo was metal-rich and the latter's was metal-poor. If
both galaxies are metal-poor, then they must have had very similar
evolutions.

"Probably, both galaxies got started within a half billion years of the
Big Bang, and over the next three to four billion years, both were
building up in the same way by protogalactic fragments containing
smaller groups of stars falling into the two dark-matter haloes,"
Chapman explains.

While no one yet knows what dark matter is made of, its existence is
well established because of the mass that must exist in galaxies for
their stars to orbit the galactic centers the way they do. Current
theories of galactic evolution, in fact, assume that dark-matter wells
acted as a sort of "seed" for today's galaxies, with the dark matter
pulling in smaller groups of stars as they passed nearby. What's more,
galaxies like Andromeda and the Milky Way have each probably gobbled up
about 200 smaller galaxies and protogalactic fragments over the last 12
billion years.

Chapman and his colleagues arrived at the conclusion about the
metal-poor Andromeda halo by obtaining careful measurements of the speed
at which individual stars are coming directly toward or moving directly
away from Earth. This measure is called the radial velocity, and can be
determined very accurately with the spectrographs of major instruments
such as the 10-meter Keck-II telescope, which was used in the study.

Of the approximately 10,000 Andromeda stars for which the researchers
have obtained radial velocities, about 1,000 turned out to be stars in
the giant stellar halo that extends outward by more than 500,000
light-years. These stars, because of their lack of metals, are thought
to have formed quite early, at a time when the massive dark-matter halo
had captured its first protogalactic fragments.

The stars that dominate closer to the center of the galaxy, by contrast,
are those that formed and merged later, and contain heavier elements due
to stellar evolution processes.

In addition to being metal-poor, the stars of the halo follow random
orbits and are not in rotation. By contrast, the stars of Andromeda's
visible disk are rotating at speeds upwards of 200 kilometers per second.

According to Ibata, the study could lead to new insights on the nature
of dark matter. "This is the first time we've been able to obtain a
panoramic view of the motions of stars in the halo of a galaxy," says
Ibata. "These stars allow us to weigh the dark matter, and determine how
it decreases with distance."

In addition to Chapman and Ibata, the other authors are Geraint Lewis of
the University of Sydney; Annette Ferguson of the University of
Edinburgh; Mike Irwin of the Institute of Astronomy in Cambridge,
England; Alan McConnachie of the University of Victoria; and Nial Tanvir
of the University of Hertfordshire.

Related Link:

* Scott Chapman's Page
http://www.astro.caltech.edu/~schapman/m31haloinfo.html
 




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