INT+IDS, JKT+CCD

According to the 'cosmological principle', the large-scale Universe should be smooth and well behaved. Distant galaxies ought to be evenly distributed in space, and their motions should correspond to a pure 'Hubble flow', a uniform expansion of space in all directions. In other words, the Universe, in some average sense, is homogeneous and isotropic. But galaxies have other "peculiar velocities", over and above the general cosmic expansion.

Although the cosmological principle is one of the central tenets of cosmological theory, it is obvious that the Universe is not exactly homogeneous and isotropic. Matter is not smoothly distributed, but organized into galaxies, galaxy clusters and even superclusters of galaxy clusters. This complex hierarchy of density fluctuations is, according to 'inflation theory', a result of the gravitational amplification of low-amplitude 'ripples' that were present in the very Early Universe. But there should be a scale beyond which gravity has not had sufficient time to produce structures, and beyond which the Universe should therefore appear homogenous.

Besides generating spatial patterns, gravity also generates velocities. In a perfectly uniform universe, everything moves away from everything else with a velocity that is proportional to the distance between. This is known as the Hubble law. But the presence of density fluctuations distorts this uniform Hubble flow by introducing peculiar motions.

All galaxies execute some kind of peculiar motion, as a consequence of the gravitational influence of the lumpy distribution of material around them. In the densest galaxy clusters - known as Abell clusters - where gravitational forces are very strong, galaxies move around with peculiar velocities of ~1,000 km/s generated by the deep potential well in which they reside. On scales larger than individual clusters, the concerted action of entire superclusters produces a calmer, more coherent flow towards regions of above-average density, and away from regions of below-average density. These 'streaming' motions contain clues to the size of the largest structures doing the pulling and thus furnish an important test of cosmological models.

In 1988, a study of streaming motions in a sample of elliptical galaxies revealed evidence for a systematic flow, simple modelling of which suggested that it could be explained by a hypothetical object about 60 megaparsecs away from the Milky Way, which became known as the 'Great Attractor'.

To map cluster motions, astronomers have to work out how much their velocity - easy to determine from redshift - departs from the velocity that the overall cosmic expansion would give to an object at that distance. That means determining their distance without relying on redshift, a much tougher requirement. The usual strategy is to find some observable feature of galaxies that is thought to indicate their actual brightness or size, then compare it with the brightness or size observed from Earth to get distances.

The Streaming Motions of Abell Clusters (SMAC) Collaboration looked at elliptical galaxies and determined their absolute size by measuring the mean surface brightness in the central part of the galaxy and how fast stars are darting around within it - indicated by the broadening of spectral lines. Then they compared these to similar known galaxies close to Earth. They applied the constructed distance indicator to about 700 galaxies in 56 rich clusters spanning a volume some 1.2 billion light years in diameter. Many telescopes were used in this survey, including the INT and the JKT. 

The SMAC survey went far beyond the proposed location of the Great Attractor and they still see outward motion of galaxies beyond it. The reported bulk flow is of amplitude 630±200 km/s with respect to the cosmic microwave background. This flow is robust against the effects of individual clusters and data subsets, the choice of Galactic extinction maps, Malmquist bias, and stellar population effects. The direction of the SMAC flow is about 90º away from the flow found by other astronomers, but it is in good agreement with the gravity dipole predicted from the distribution of X-ray-luminous clusters.
 

Image of the galaxy cluster Abell 1656 studied by the SMAC team with the Isaac Newton Group of Telescopes. The SMAC survey was based on measurements of 699 galaxies in 56 galaxy clusters like Abell 1656. [ JPEG | TIFF ]

Some references: 

• P. Coles, 1999, "The pulling power of galaxy clusters", Nature news, 398, 288.
• A. Hellemans, 1999, "The mistery of the migrating galaxy clusters", Science, 283, 625.
• M. J. Hudson, R. J. Smith, J. R. Lucey, D. J. Schlegel, R. L. Davies, 1999, "A large-scale bulk flow of galaxy clusters", ApJ Letters, 512, 79.
• "Durham team discover a cosmic flow of galaxies across one billion light years of the universe", University of Durham Press Release, 28 January 2000.