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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