Astronomers pin down Dark Matter distribution
ANGLO-AUSTRALIAN OBSERVATORY NEWS RELEASE
Posted: December 12, 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.
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.
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:
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).