Rare galaxies shed light on a dark universe

Researchers based at the Institute for Computational Cosmology (ICC) in Durham and at Caltech in California, have found striking proof that their computer simulations of the universe can accurately predict how galaxies are clustered, so helping to reveal the distribution of dark matter throughout the universe. Using a computer simulation to follow the formation and evolution of galaxies in a universe filled with dark energy and dark matter, they predicted that the most luminous galaxies should be associated with the most massive concentrations of dark matter and, as a consequence, these very bright galaxies should be clustered more tightly than average galaxies. After testing the predictions using data on thousands of galaxies from the Anglo-Australian telescope 2-degree Field Galaxy Redshift Survey (2dFGRS),Dr Peder Norberg of the University of Durham, working with other members of the 2dFGRS team, found that their predictions were spot on, proving that the Cosmology Machine supercomputer is a powerful tool for understanding how the universe works. These results will be presented on Wednesday 10 April 2002 at the National Astronomy Meeting in Bristol by team member Dr Ofer Lahav of Cambridge University.

For a long time, cosmologists have held the belief that the bulk of the mass in the universe is in the form of “cold dark matter”. This material does not emit any light but astronomers are convinced it really is there because something invisible exerts a gravitational pull on luminous objects astronomers can see, such as stars and galaxies. Several experiments, including one operated by the UK Dark Matter Collaboration at the Boulby mine in North Yorkshire, are racing to be the first to detect the exotic elementary particles that are the likely candidates for the cold dark matter. On top of that, recent observations have suggested that the universe is filled with a mysterious “dark energy”, which produces a repulsive force. Currently, dark energy is winning out over the gravitational pull of the matter, both dark and luminous, with the result that the expansion of the universe is speeding up.

Theorists expect the dark matter in the universe to be clumpy due to the influence of gravity, which boosts small primordial ripples in the distribution of mass. The formation of structure in the dark matter – dubbed “the cosmic web” – can be followed with computer simulations. However, one of the biggest challenges facing cosmologists is to predict where galaxies should form and light up the cosmic web of dark matter. The team from the Institute for Computational Cosmology (ICC) at the University of Durham (Dr Carlton Baugh, Dr Shaun Cole, Prof. Carlos Frenck and Dr Cedric Lacey) and the California Institute of Technology (Dr Andrew Benson) have now come up with a model that can follow the formation and evolution of galaxies in a universe filled with dark energy and dark matter. The results, involving some of the largest simulations run on the Cosmology Machine supercomputer at the ICC, predict how galaxies are arranged in space and how tightly they are clustered. One basic prediction is that the most luminous galaxies should be associated with the most massive dark matter structures. This in turn means that these very bright galaxies are expected to cluster together more than average galaxies.

Because these luminous galaxies are extremely rare, the theoretical predictions have been hard to test until now. The problem of finding enough very bright galaxies has been solved by an Anglo-Australian collaboration that has produced a map of the local universe of unprecedented size. The Two-Degree Field Galaxy Redshift Survey (2dFGRS), named after the instrument used to make the map on the Anglo-Australian Telescope at Siding Spring in New South Wales, Australia, contains over 250 000 galaxy redshifts, more than ten times more than any survey completed in the last millennium, and has uncovered plenty of the precious bright galaxies. The 2dFGRS team measured the degree of clustering of very luminous galaxies and found that these galaxies have a much clumpier distribution than more typical galaxies in the survey. Remarkably, the level of the enhancement in clustering was exactly that predicted by the computer model.

Aware that galaxies in which relatively few new stars are forming tend to be more numerous among the brightest galaxies, the team wanted to be quite sure their observational result was really due to changes in luminosity and not simply a case of `comparing apples with oranges`. So they went on to use the huge wealth of data from the 2dfGRS to see whether the clustering of galaxies depends as much on their type as their brightness. “This is only feasible in a survey with as many galaxies as the 2dFGRS” says 2dFGRS team member Carlton Baugh. “Whilst we found a small difference in the clustering strength of different types of galaxy with the same luminosity, the overriding trend is that the clustering signal is most sensitive to the luminosity of the galaxy. These exciting results mean that cosmologists are starting to get to grips with the problem of lighting up their dark universe.”