These images show an example of the mapping of the dark mass distribution in one of the 50 sky fields observed with the VLT and FORS1. At the top is the original image, a 36-min exposure in a near-infrared wavelength band. At the bottom is the reconstructed map of the mass (a "mass photo") in this direction, based on an analysis of the weak shear effect seen in the field; that is, on the measured elongations and directions of the axes of the galaxy images in this field. The brighter areas indicate the directions in which there is most mass along the line of sight. The circle in the left photo surrounds the images of a distant cluster (or group) of galaxies, seen in this direction. Note that there is a corresponding concentration of mass in the "mass photo"; this is obviously the mass of that cluster. The mass reconstruction map shows the (mostly) dark matter responsible for the cosmic shear found on the small scales, now measured with the VLT. Technical information about these photos is available below. Photo: ESO

An international team of astronomers has succeeded in mapping the "dark" (invisible) matter in the Universe, as seen in 50 different directions from the Earth. They find that, within the uncertainty, it is unlikely that mass alone would stop the current expansion of the Universe.

This fundamental result is based on the powerful, but challenging method of "cosmic shear". It depends on very accurate measurements of the apparent, weak distortion and preferential orientation of images of distant galaxies.

This effect is caused by deflection of the light from those galaxies by the large mass concentrations in the Universe it encounters on its way to us. The larger these masses are, the larger are the apparent image distortions and the more pronounced are the alignments of neigbouring galaxy images.

The new analysis was made possible by means of unique observational data, obtained under excellent conditions with the the ESO 8.2-m VLT ANTU telescope and the multi-mode FORS1 instrument at the Paranal Observatory.

Led by astronomers at the Institut d'Astrophysique de Paris, the team used for the first time the VLT to probe the mass density of dark matter in the Universe, by means of weak gravitational lensing effects.

The team selected 50 different sky fields which were then observed in service mode by the ESO staff at the Paranal Observatory. Long exposures of these fields were made with the FORS1 instrument (in its imaging mode) on the VLT 8.2-m ANTU telescope and only during nights with the very best observing conditions. In fact, 90% of the fields have image quality better than 0.65 arcsec, guaranteeing a superb basis for the subsequent study.

Clumps of dark matter
The unprecedented quality of these data enabled the astronomers to measure the shapes and orientations of the images of more than 70,000 galaxies with very high precision. After a careful statistical analysis, they were able to demonstrate that the distant galaxies are not randomly oriented on the sky - they show a a certain degree of alignment over substantial sky areas (to distances of several arcmin).

The astronomers refer to this as a coherent orientation. It can only be explained by gravitational lensing effects produced by clumps of dark matter in space, distributed along huge "filaments". The above photo demonstrates this, by means of the VLT exposure (right) and the deduced mass distribution in the same direction, based on these measurements

The weak lensing effect
The gravitational lensing effect was predicted by Einstein's theory of general relativity at the beginning of the century.

When the light of a distant galaxy passes close to a concentration of matter in space, it will be (more or less) deflected, due to the effect of the field of gravity of this matter. The observed image of the galaxy is therefore distorted.

Very strong gravitational lensing effects (by very heavy objects) produce spectacular gravitational arcs observed in some rare clusters of galaxies, cf. the VLT images of CL2244-0 and Abell 370.

Much weaker lensing effects (by less massive objects) are in fact present everywhere in the Universe, but they are not easy to detect. This was the effect the astronomers searched for. It manifests itself as a small stretching in a particular direction of the images of all galaxies that are located behind the gravitational lens. This phenomenon may then be observed as an alignment of galaxies in that particular sky area. The existence of the lens and its overall mass and extension can then be determined, albeit with some uncertainty only.

An important contribution to the map of the Universe
Thanks to the large light collecting power of the VLT and the superb quality of the present images, the team succeeded to detect large-scale, weak lensing effects in the Universe, in a large number of different (and thus independent) directions. Moreover, the analysis of this large data sample enabled the astronomers, for the first time, to set limits to the overall mass density of the universe, by means of the gravitational lensing by large scale structures. It turns out that their results are in remarkable agreement with the current constraints obtained by other cosmological considerations.

This kind of investigation is rather difficult and cannot be based on individual sky fields alone. The final result, in terms of the inferred mass density of the Universe, only emerges when "adding" all of the 50 observed fields.

Making the reasonable assumption that the distribution of galaxies and dark matter in space are similar, the new investigation shows that the total matter density is less than half of what is needed to stop the current cosmic expansion. The new result also supports the existence of a non-zero "cosmological constant" (vacuum energy), already indicated by supernova observations.

In the ongoing quest for establishing the first true mass map of the Universe from the gravitational lensing effects caused by this mass, the VLT has now demonstrated its great potential with bravour.

The light collecting power and, not least, its excellent image quality provides what it likely to be the best observing configuration for this very challenging research programme. It was also made possible because of the opportunity to use the VLT Service Mode during which ESO staff astronomers at Paranal are responsible for carrying out the actual observations, at the moment of the very best atmospheric conditions.