GALAXIES, GALAXIES, EVERYWHERE.
We live in a large spiral star system, a galaxy known as the Milky Way. Astronomers know that there is a multitude of other galaxies in the Universe, many so far away that they have yet to be detected. But exactly how many galaxies are there?
The latest effort to chart these distant stellar communities is the 2dF (two-degree field) galaxy survey, which has mapped more than 150,000 galaxies, using the Anglo-Australian Telescope near Coonabarabran in eastern Australia.
The survey will reach its target of 250,000 by the end of the year, making it ten times larger than the largest previous survey. The 2dF galaxy survey has already provided a detailed three-dimensional map of a section of the nearby Universe and detailed information on the intrinsic properties of the galaxies it contains.
A team of astronomers based in the UK and Australia is currently analysing these data in order to answer fundamental questions about the Universe.
New results from this analysis will be presented at the UK National Astronomy Meeting in Cambridge on Wednesday 4 April 2001 by Dr. Steve Maddox (Nottingham University) and Dr. William Percival (Edinburgh University).
THE GROWTH AND EVOLUTION OF GALAXY CLUSTERS By looking at the large- scale distribution of nearby galaxies, the 2dF team has been able to learn more about the early Universe.
According to current theory, small density fluctuations, produced shortly after the Big Bang, are expected to be the seeds that grow to become present-day galaxies due to the force of gravity. The distribution of these fluctuations depends on the overall properties of the Universe.
Variations in the distribution of these seeds are caused by oscillations (or waves) in the early Universe. The amplitude of these oscillations depends on what fraction of matter is in the form of baryons, the 'normal' matter that makes up our bodies and the world around us. Information about the Universe is therefore predicted to be contained in the distribution of galaxies that form the 2dF galaxy survey.
Analysis of the galaxy distribution has led the 2dF team to conclude that the Universe contains only about 30 % of the matter required to stop it from expanding and then collapse in a "Big Crunch". Approximately 5 % is in the form of baryons and the remaining 25 % in the form of a yet unknown dark matter. This means that the Universe will expand forever because there is too little mass to provide the gravitational force to rein it in.
Other research provides evidence (e.g. from the fluctuations in the Cosmic Microwave Background radiation) that the Universe has flat geometry. Together with the 2dF survey, this suggests that, in addition to the 30% mass density, 70% of the density of the Universe is in the form of 'dark energy'.
This result is in good agreement with other lines of evidence, and not only provides a remarkable confirmation of the standard picture of cosmology and galaxy formation, but also gives a new independent estimate of the baryon content of the Universe.
The 2dF galaxy survey has also revealed that brighter galaxies are more clustered than fainter galaxies This result is important because it provides observational support for a fundamental assumption which underlies all modern models of galaxy formation.
In order to make model Universes that match the observed galaxy distribution, the model galaxies must be more tightly clustered than the overall mass distribution. The natural way to introduce such a 'biasing' effect is to assume that galaxies form only in dense mass concentrations, with the most luminous galaxies forming in the densest of these. An unavoidable consequence of this assumption is that the biggest and brightest galaxies should be more tightly clustered than the smallest and faintest.
The 2dF galaxy survey is the first to contain enough galaxies to clearly show such a change in clustering for brighter galaxies. These results provide a direct observational justification of the basic assumptions made in biased galaxy formation models. Eventually, the details of how the clustering depends on luminosity will provide information about the physical processes at work in galaxy formation, and the efficiency with which collapsing gas can turn into observable galaxies.
Dr. Steve Maddox School of Physics and Astronomy University of Nottingham Nottingham NG7 2RD
RAS Press Officers Peter Bond
Dr Jacqueline Mitton