Astronomers Pin Down Dark Matter Distribution

The Universe’s mysterious invisible Dark Matter is distributed on
large scales in exactly the same way the galaxies are, according to
scientists analysing data from the giant 2dF Galaxy Redshift Survey
done with the 3.9-m Anglo-Australian Telescope in eastern Australia.

The finding means the Universe is surprisingly simple. The Dark Matter
could have been clumpier than normal matter, or vice versa. Instead,
they’re the same.

Astronomers believe that slight clumping in the Dark Matter in the very
early Universe ‘seeded’ the growth of galaxies. “This result will place
strong constraints on theories of where and how galaxies form,” said Dr
Alan Heavens of the University of Edinburgh, UK, one of the lead
authors on a paper posted today on the online preprint service
astro-ph.

Galaxies are pulled around by the gravity of the Dark Matter, forming
into large-scale ‘sheets’ and ‘filaments’. In their paper Dr Heavens
and co-author Dr Licia Verde (Rutgers and Princeton Universities, USA)
and their colleagues show that on large scales the sheets and filaments
in the galaxy distribution revealed by the 2dF survey are just what is
expected if the galaxies and Dark Matter cluster in the same way.

“Imagine a mountain range at night, dotted with campfires,” said Dr
Matthew Colless of the Australian National University, a co-leader of
the 2dF Galaxy Redshift Survey team. “You can’t see the mountains, only
the fires. Where are the mountain peaks? We now know that everywhere
you see a fire – a galaxy – it marks the peak of a mountain – a
concentration of Dark Matter. One campfire, one peak.”

The result also confirms previous findings that show there is not
enough Dark Matter to stop the Universe expanding forever.

“Knowing how clustered the Dark Matter is, also reveals how much of it
there is,” said Dr Verde – about seven times as much as ordinary
matter, but only a quarter of what is needed to halt the expansion of
the Universe.

In a second study, also posted on astro-ph, Dr Ofer Lahav and Dr Sarah
Bridle (both from the Institute of Astronomy, Cambridge University, UK)
and their co-authors have compared the fluctuations in the 2dF galaxy
distribution with those in the Cosmic Microwave Background (CMB) –
radiation left over from the Big Bang. They found remarkable agreement
between the distribution of luminous galaxies and the distribution of
mass on scales larger than 30 million light-years. This gives
independent support to the finding of Verde and Heavens, which is based
on an entirely different method.

A second important result in both studies is that ripples in the mass
distribution are not as strong as previously thought. ‘”The ripples are
about 20 per cent smaller in amplitude, suggesting that the growth of
structure in the Universe is more gentle, and for example would produce
fewer galaxy clusters,” said Ofer Lahav.

This result tells astronomers how efficiently gas can turn into
observable galaxies such as our own Milky Way.

The 2dF (two-degree field) survey has compiled the world’s largest
database of more than 210 000 galaxies, using the Anglo-Australian
telescope in New South Wales, Australia.

Designed and built by the Anglo-Australian Observatory, the 2dF
instrument is one of the world’s most complex astronomical instruments,
able to capture 400 spectra simultaneously. A robot arm positions up to
400 optical fibres on a field plate, each to within an accuracy of 20
micrometres. Light from up to 400 objects is collected and fed into two
spectrographs for analysis. The expansion of the Universe shifts galaxy
spectra to longer wavelengths. By measuring this ‘redshift’ in a
galaxy’s spectrum, the galaxy’s distance can be determined.

The 2dF survey covers a total area of about 2 000 square degrees,
selected from both northern and southern skies.

2DF GALAXY REDSHIFT SURVEY TEAM MEMBERS:

Anglo-Australian Observatory – Joss Bland-Hawthorn, Terry Bridges,
Russell Cannon, Ian Lewis; Australian National University – Matthew
Colless*, Carole Jackson, Bruce Peterson; California Institute of
Technology – Richard Ellis, Keith Taylor; Johns Hopkins University –
Ivan Baldry, Karl Glazebrook; Liverpool John Moores University – Chris
Collins; University of Cambridge – George Efstathiou, Ofer Lahav,
Darren Madgwick; University of Durham – Carlton Baugh, Shaun Cole,
Carlos Frenk, Peder Norberg; University of Edinburgh – John Peacock*,
Will Percival, Will Sutherland; University of Leeds – Stuart Lumsden;
University of New South Wales – Warrick Couch, Kathryn Deeley, Roberto
de Propris; University of Nottingham – Edward Hawkins, Steve Maddox*;
University of Oxford – Gavin Dalton, Mark Seaborne; University of St
Andrews – Nicholas Cross, Simon Driver

* Team leaders

ACKNOWLEDGEMENT

The 2dF Galaxy Redshift Survey has been made possible by the dedicated
efforts of the staff of the Anglo-Australian Observatory, both in
creating the 2dF instrument and in supporting it on the telescope. The
Anglo-Australian Observatory is funded by the Australian government
(through DETYA) and the UK government (through PPARC).

PAPERS

The papers have been submitted to Monthly Notices of the Royal
Astronomical Society and can be downloaded from:

(Verde et al.) http://xxx.lanl.gov/abs/astro-ph/0112161

(Lahav et al.) http://xxx.lanl.gov/abs/astro-ph/0112162

OTHER RESOURCES

Graphics and papers
http://www.roe.ac.uk/~afh/seeing_the_invisible.htm

The 2dF galaxy redshift survey website, including a fly-through
movie of the survey
http://www.mso.anu.edu.au/2dFGRS

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