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Astronomers pin
down dark matter distribution
11-Dec-2001
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).
CONTACTS
Dr Alan Heavens, Institute
for Astronomy, University of Edinburgh
Tel. (+44) (0) 131 668 8352.
Fax (+44) (0) 131 668 8416. afh@roe.ac.uk
Dr Licia Verde, Department
of Physics and Astronomy, Rutgers
University
Tel. (+1) 609 258 7323. Fax
(+1) 609 258 1020.
verde@physics.rutgers.edu
Dr Ofer Lahav, Institute of
Astronomy, University of Cambridge
Tel. (+44) (0) 1223 337540.
lahav@ast.cam.ac.uk
Dr Sarah Bridle, Institute
of Astronomy, University of Cambridge
Tel. (+44) (0)1223 339071.
sarah@ast.cam.ac.uk
Dr Matthew Colless, Research
School of Astronomy and Astrophysics
Australian National University.
Tel. (+61) 2 6125 8030.
colless@mso.anu.edu.au
For additional comment on the
2dF survey:
Dr Russell Cannon, Anglo-Australian
Observatory
Tel (+61) 2 68 426 291 (day)
or (+61) 2 68 426 279 (night).
rdc@aaocbn.aao.gov.au
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
_______________________________________________
Helen Sim - Public Relations
and Media Liaison
Anglo-Australian Observatory
PO Box 296
Epping NSW 1710
Australia
hsim@aaoepp.aao.gov.au
tel: +61 2 9372 4251 (bh),
0419-635-905 (mob)
fax: +61 2 9372 4444
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