'Dark Matter' can't be all neutrinos, Galaxy survey shows

Neutrinos, the lightest of the known elementary particles, can make up no more than a fifth of the Universe's 'dark matter', according to a new finding made with the Anglo-Australian Telescope near Coonabarabran, NSW.

Dr Ofer Lahav of Cambridge University will present the result to the UK National Astronomy Meeting in Bristol, UK, on Wednesday 10 April (UK time).

It's the latest in a string of cosmology findings flowing from the world's largest galaxy survey, the 2dF (two-degree field) Galaxy Redshift Survey, which has mapped the 3-D positions in space of more than 220 000 galaxies.

The survey, which started in 1997, winds up on the night of Thursday 11 April (AEST) when astronomers Dr Scott Croom and Dr Simon Driver make the last observations.

"The observing may be done, but there's a lot of science still to come," says Dr Matthew Colless of the Australian National University, one of the coordinators of the 30-strong survey team.

The sheer size of the survey has allowed astronomers to pit theory against observation in ways not possible before.

The neutrino finding is an example.

Dr Oystein Elgaroy and Dr Ofer Lahav, both of Cambridge University in the UK, used the galaxy survey data to show that neutrinos must weigh less than a billionth (one part in a thousand million) of a hydrogen atom.

Neutrinos come in three different 'flavours'. It was long thought that they had no mass at all, but in recent years that idea was overturned.

The Universe is awash with neutrinos, most left over from the Big Bang. So with even a tiny mass they could make up the Universe's unseen 'dark matter' — the dominant form of matter in the Universe.

The value of the mass of the neutrino affects how tightly clustered galaxies are. So Drs Elgaroy and Lahav calculated how galaxies would be clustered for different values of the neutrino mass, then compared their predictions with what the galaxy survey actually mapped.

But their finding — that neutrinos are extremely light — means the dark matter problem is still unsolved.

In a second result, also being presented by Dr Lahav at the UK meeting, researchers based at the Institute for Computational Cosmology (ICC) in Durham and at Caltech in California used the 2dFGRS results to show the that their computer simulation of the Universe was 'spot on'.

The simulation predicted that the brightest galaxies are associated with the most massive concentrations of dark matter, and so they should be clustered more tightly than average galaxies.

Very bright galaxies are extremely rare. The 2dFGRS has been the first survey to map enough of them for the astronomers to test the prediction.

Because the prediction turned out to be right, astronomers can now have confidence that they have a good idea of how the invisible 'dark matter' is distributed throughout the Universe.


Australian 2dFGRS coordinator

Dr Matthew Colless, Research School of Astronomy and Astrophysics,
Australian National University. Tel. (+61) 2 6125 8030.

Last 2dFGRS observing run

Dr Scott Croom, Anglo-Australian Observatory
Control room, Anglo-Australian Telescope, night of 11 April 2002, 02-6842-6279
Siding Spring Observatory Lodge, 02-6842-6246


For comment:
Associate Professor Dr Ray Volkas,
University of Melbourne Office 03-8344-5464
Home 03 9489 1091 Mobile 0407 311 254

Dr. Ofer Lahav, Institute of Astronomy,
University of Cambridge, Madingley Road,
Cambridge CB3 0HA. Tel. (+44) (0) 1223 337540.

National Astronomy Meeting Press Room phones (9-12 April only):
+44 (0)117 928-4337, (0)117 928-4338, (0)117 954-5913, (0)117 928-7901

Dr Terry Bridges, Anglo-Australian Observatory, Sydney

Modelling of the Universe

Dr Carlton Baugh, Institute of Computational Cosmology, Dept of Physics,
University of Durham, DH1 3LE. Tel: (+44) (0)191 374 2142
email: c.m.baugh@durham.ac.uk

Dr Peder Norberg, Institute of Computational Cosmology, Dept of Physics,
University of Durham, DH1 3LE. Tel: (+44) (0)191 374 1662
email: peder.norberg@durham.ac.uk



The paper on which this finding is based is available from the astro-ph preprint server at http://xxx.lanl.gov/abs/astro-ph/0204152. It
has also been submitted to Physical Review Letters for publication.

Modelling of the Universe

1. The 2dF Galaxy Redshift Survey: luminosity dependence of galaxy clustering by P. Norberg et al. (the 2dFGRS team). Monthly
Notices of the Royal Astronomical Society, Volume 328, pages 64-70, November 2001.

2. The 2dF Galaxy Redshift Survey: The dependence of galaxy clustering on luminosity and spectral type by P. Norberg et al. (the 2dFGRS team). Monthly Notices of the Royal Astronomical Society, in press.
Abstract available at http://xxx.soton.ac.uk/abs/astro-ph/0112043


1. 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.

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

Helen Sim - Public Relations and Media Liaison
Anglo-Australian Observatory
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