UCL News


Largest 3D map of the galaxies

25 October 2006

Professor Ofer Lahav (UCL Physics and Astronomy) is among the international team of leading astronomers who have released maps from the largest full-sky, three-dimensional survey of galaxies ever conducted.

Their detailed maps show the 'local' cosmos out to a distance of 600 million light years, identifying all the major superclusters of galaxies and voids. They also provide important clues regarding the distribution of the mysterious 'dark matter' and 'dark energy' which are thought to account for up to 96 per cent of the apparent mass of the universe.

The maps are based on the observation that, as the universe expands, the colours of galaxies change as their emitted light waves are stretched or 'redshifted'. By measuring the extent of this redshift, astronomers are able to calculate approximate distances to galaxies.

The new survey, known as the 2MASS Redshift Survey (2MRS), has combined two dimensional positions and colours from the Two Micron All Sky Survey (2MASS), with redshifts of 25,000 galaxies over most of the sky. These redshifts were either measured specifically for the 2MRS or they were obtained from an even deeper survey of the southern sky, the 6dF Galaxy Redshift Survey (6dFGS).

The great advantage of 2MASS is that it detects light in the near-infrared, at wavelengths slightly longer than the visible light. The near-infrared waves are one of the few types of radiation that can penetrate gases and dust and that can be detected on the Earth's surface. Although the 2MRS does not probe as deeply into space as other recent narrow-angle surveys, it covers the entire sky.

Within this vast volume, the most massive galaxy supercluster is 400 million light years away. The Shapley supercluster is so big that it takes light at least 20 million years to travel from its one end to the other.

In order to map the dark matter probed by the survey, the team used a
novel technique borrowed from image processing. The method was partly developed
by Professor Lahav, a co-author of the paper and Head of the UCL Astrophysics
Group. The technique, applied by the lead author Dr Pirin Erdogdu (University of Nottingham), utilises the relationship between galaxy velocities and the total distribution of mass.

"It is like reconstructing the true street map of London just from a satellite image of London taken at night. The street lights, like the luminous galaxies, act as beacons of the underlying roads," said Professor Lahav.

"This extraordinarily detailed map of the Milky Way's cosmic neighbourhood provides a benchmark against which theories for the formation of structure in the universe can be tested," commented Professor Matthew Colless, director of the Anglo-Australian Observatory and leader of the 6dF Galaxy Survey. "In the near future, the predicted motions derived from this map will be confronted with direct measurements of galaxies' velocities obtained by the 6dF Galaxy Survey, providing a new and stringent test of cosmological models."

The findings are presented in the journal 'Monthly Notices of the Royal Astronomical Society' and also here.

Image: The reconstructed density field, evaluated on a thin shell at 450 million light years. The overdense regions are shown in red, with voids in blue.