Galaxies are born inside dark matter clumps (Forwarded)

News Service
Cornell University
Ithaca, New York

Media Contact:
Susan S. Lang, Cornell News Service
(607) 255-3613

April 17, 2006

Galaxies are born inside dark matter clumps, Cornell study of Spitzer
Space Telescope data shows

By Alex Kwan

Try mixing caramel into vanilla ice cream -- you will always end up with
globs and swirls of caramel. Scientists are finding that galaxies may
distribute themselves in similar ways throughout the universe and in
places where there is lots of so-called dark matter.

"Our findings suggest that unseen dark matter -- which emits no light
but has mass -- has had a major effect on the formation and evolution of
galaxies, and that bright active galaxies are only born within dark
matter clumps of a certain size in the young universe," said Cornell
University research associate Duncan Farrah, the lead author of a paper
on spatial clustering that appeared in the April 10 issue of
Astrophysical Journal Letters.

To investigate the spatial distribution of galaxies, Farrah used data
that recently became available from the Spitzer Wide-area InfraRed
Extragalactic (SWIRE) survey, one of the largest such surveys performed
by the Spitzer Space Telescope, which was launched in 2003.

A galaxy is typically made up of hundreds of billions of stars grouped
tightly together. But galaxies themselves often group together into what
astronomers call "large-scale structures." And, just as galaxies
themselves can take on such shapes as ellipticals and spirals, so, too,
can the large-scale structures, ranging from galaxy clusters to long
filaments of galaxies to large, empty voids.

"You might think that galaxies are just distributed randomly across the
sky, like throwing a handful of sand onto the floor," said Farrar. "But
the problem is they are not, and this has been a great puzzle."

Farrah is interested in how large-scale structures form. To measure the
amount of clustering in the early universe, he looked at light that had
traveled for several billion years from extremely distant galaxies. From
this he was able to calculate the amount of bunching in candidate galaxy
clusters in the early universe.

"We wanted to find the beacons of the first stages of the formation of a
galaxy cluster because, at that time, the clusters themselves had not
formed yet," said Farrah.

In particular, he was interested in objects that emit strongly in the
infrared and are surrounded by dense gas and dust. These objects, known
as ultraluminous infrared galaxies (ULIRGs), were thought to be
precursors of galaxy clusters. Farrah confirmed this by showing that
ULIRGs do, indeed, tend to cluster in their early phases. The ability to
pinpoint the locations of nascent galaxy clusters will enable
researchers to investigate early cluster formations and when they occurred.

Farrah's finding that distant ULIRGs are linked with large clumps of
dark matter was surprising for another reason. As its name suggests,
dark matter doesn't emit light so no conventional telescope can see it.
However, because dark matter has mass, its existence can be inferred by
the way stars are drawn to regions where this mysterious mass is
concentrated.

Unexpectedly, Farrah found that ULIRGs at different points in the
history of the universe coincide with clumps of dark matter haloes of
very similar masses. This observation suggests that a minimum amount of
dark matter is necessary for galaxies to form and to coalesce into
clusters. Farrah believes his study also provides valuable insights into
understanding how dark matter helped mold the evolution of the universe.

Carol Lonsdale of NASA's Jet Propulsion Laboratory, which manages the
Spitzer Space Telescope, is the principal investigator for the SWIRE
project.

[Graduate student Alex Kwan is a Cornell News Service writer intern.]

Related Information:

* SWIRE survey
http://swire.ipac.caltech.edu/swire/swire.html

IMAGE CAPTION:
[http://www.news.cornell.edu/stories/...l_clusters.jpg (65KB)]
A computer simulation of the distribution of "dark" matter at an early
point in the history of the universe. The observations by Cornell's
Duncan Farrah and colleagues provide solid evidence that galaxies in the
distant past trace this matter distribution very well and that these
galaxies will eventually reside in extremely rich clusters of galaxies
at the current epoch. Courtesy Volker Springel and the Millennium
Simulation group