Date: 10 Nov 2006 23:59:53
From: Sam Wormley
Subject: Galactic Building Block Busters
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Galactic Building Block Busters
http://sciencenow.sciencemag.org/cgi/content/full/2006/1110/2
By Govert Schilling
ScienceNOW Daily News
10 November 2006
If a castle is built with nearby rocks, you'd expect the stone walls
to be made of the same stuff as any remaining boulders in the area.
Likewise, if our Milky Way galaxy grew through the merging of smaller
dwarf galaxies, you'd expect it to contain the same types of stars as
the remaining dwarfs that have not been incorporated. But detailed
observations of four of these dwarf galaxies show that this is not
the case, indicating that theories about how galaxies form are
incomplete.
According to current cosmological wisdom, small primordial
clouds--mainly consisting of mysterious cold dark matter--first
coalesced into dwarf galaxies. Subsequently, the dwarfs merged to
build up large galaxies. When dwarf galaxies are swallowed by the
Milky Way, their constituent stars end up in our galaxy's halo--a
large, nearly spherical distribution of mostly old stars surrounding
the Milky Way on all sides. As a result, the stellar content of the
halo should reflect the properties of dwarf galaxies.
But that doesn't seem to be true, according to work by an
international team of astronomers led by Amina Helmi of the
University of Groningen in the Netherlands. Observations made by the
team with the European Very Large Telescope in Chile indicate the
stars in four neighboring dwarf galaxies, known as Sculptor, Sextans,
Fornax and Carina, have a markedly different chemical makeup than the
stars in the halo. In particular, stars with an extremely small
amount of heavy elements (just a thousandth of the sun's) occur in
the Milky Way's halo but are not found in the dwarf galaxies. "The
progenitors of the Milky Way and the [dwarf galaxies] must have been
different," write Helmi and her colleagues in a paper in the 10
November Astrophysical Journal Letters
See: http://sciencenow.sciencemag.org/cgi/content/full/2006/1110/2
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