Astronomers using the UK Schmidt Telescope in New South Wales, Australia, have completed a detailed map of more than 83% of the local galaxies in the southern sky (D. H. Jones et al. Preprint at <http://arxiv.org/abs/0903.5451>; 2009). For six years, the 6dF Galaxy Survey (6dFGS) collected 136,304 spectra, including redshift data for 125,071 galaxies up to two billion light years away. The combination of these distance measurements and the survey's peculiar velocity measurements — velocity relative to the uniform expansion of the cosmic background frame of reference described by Hubble's law — can differentiate motion due to the expansion of the universe from that due to gravitation. Hence the motion of invisible matter becomes visible.

Credit: CHRIS FLUKE, SWINBURNE UNIVERSITY OF TECHNOLOGY

To map half the sky in a reasonable time frame, the Six-degree Field (6dF) instrument uses optical fibres and robotic positioning to increase the telescope's observational power 100-fold, thus mapping 150 stars or galaxies in one go. This broad and shallow survey extends the coverage of the older 6dF Galaxy Redshift Survey by an order of magnitude and covers more than twice the area of the Sloan Digital Sky Survey. Astronomers may now concentrate on the evolution of large-scale structures, such as clusters and filaments, as well as numerous voids in the nearby Universe. About 500 empty regions have been identified or verified.

The precision mapping has also revealed new intervening structures between superclusters, such as Shapley and Hydra-Centaurus. Both of these clusters of galaxy clusters lie close to the constellation Centaurus, which our own Galaxy — as well as our neighbours — is speeding towards. This conglomeration of matter is known as the Great Attractor. Its gravitational pull is stronger than it should be based on the amount of visible matter. And as it lies behind the plane of the Milky Way, which obscures our view of the receding mass, the Great Attractor has been difficult to study; in fact, its presence was first reported only in 1973. The data from 6dFGS will help us understand the history — and, hopefully, the future — of such inhomogeneous mass distributions.

The database is available online at www-wfau.roe.ac.uk/6dFGS.