Dark matter haloes favour frisbee over rugby (Forwarded)

Royal Astronomical Society
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CONTACTS:

Aaron Robotham
H. H. Wills Physics Laboratory
University of Bristol
Tyndall Avenue
Bristol, BS8 1TL
Tel: +44 (0)117 928 7561

Professor Steven Phillipps
H. H. Wills Physics Laboratory
University of Bristol
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Bristol, BS8 1TL
Tel: +44 (0)117 9546881

PRESS INFORMATION NOTE: RAS PN 07/29 (NAM25)

EMBARGOED FOR 00:01 BST, THURSDAY 19 APRIL 2007

DARK MATTER HALOES FAVOUR FRISBEE OVER RUGBY

A new study of dark matter haloes indicates that they are shaped like
Frisbees, not Rugby-balls as has preferentially been suggested. Aaron
Robotham, of the University of Bristol, will be presenting the results at
the Royal Astronomical Society's National Astronomy Meeting in Preston on
Friday 20th April.

A group from the University of Bristol and the Cerro-Tololo Inter-American
Observatory in Chile has developed a sophisticated computer model to work
out the three-dimensional shape of the dark matter using the positions of
groups of galaxies which are embedded the haloes. Dark matter haloes are
studded with galaxies but being discrete objects the true halo shape may not
be apparent by just simply measuring their distribution. The new model is
able to fill in the gaps with an unprecedented degree of accuracy,
presenting a truer picture of the shape of the ellipsoids. The results for
the corrected data show that the dark matter haloes are non-spherical but
flattened out like a Frisbee, most preferentially in the smallest groups of
galaxies. When the group had analysed the raw data the ellipsoids had
appeared to be shaped like a Rugby ball or American Football, which was also
the shape preferred by previous studies.

"Our findings are that dark matter haloes are Frisbee shaped, that means
that dark matter is not simply spherically distributed, and indicates that
filamentary structure is not a strong influence on the shape of the group
halo -- prolate shapes would be strongly favoured in this case," said
Robotham. "The apparent oblate shapes that are allowed after correction
mirrors that seen for the Local Group and has often been suggested in
computer models."

The shape of dark matter haloes gives us information about how the early
universe formed and how the haloes have evolved. According to cosmological
theory, soon after the Big Bang cold dark matter formed the universe's first
large-scale structures, which then collapsed under their own weight to form
vast halos. The gravitational pull of these haloes sucked in normal matter
and provided a focus for the formation of galaxies. How the shape of these
halos have evolved over time is a subject of much debate, complicated by a
vast number of factors that mean extremely large samples are required in
order to extract meaningful statistics.

The scientists used the 2-degree Field Percolation Inferred Galaxy Groups
(2PIGG) catalogue, the largest survey of galaxy groups that is publicly
available. The group's findings have been submitted for publication in the
Astrophysical Journal.

NOTES FOR EDITORS

Royal Astronomical Society's National Astronomy Meeting

The RAS National Astronomy Meeting is the UK's premier meeting for the
astronomy, solar system and space science communities. The RAS-NAM 2007 is
hosted by the University of Central Lancashire and is joined by the UK Solar
Physics and Spring MIST meetings. It is sponsored by the Royal Astronomical
Society, the UK Science and Technology Facilities Council (STFC) and the
University of Central Lancashire.

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