First Mass-Selected Dark Matter Halo

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Images of faint background galaxies get distorted by the tidal gravitational field of a foreground mass concentration; the statistical properties of the distortion field can be obtained from deep high-quality images and used to reconstruct the mass distribution of the deflecting intervening matter. Previously, this method has been used to study the mass distribution of known clusters of galaxies which generate a coherent alignment of the images of background galaxies. Fig.1 shows a particularly dramatic case of image distortions caused by the massive cluster of galaxies Abell 1689.

Figure 1:
    A very deep Hubble Space Telescope image of the cluster Abell 1689. Most of the bright galaxies seen belong to the cluster at redshift z=0.18. The cluster is surrounded by a large number of highly elongated images of faint background galaxies; their images are distorted by the strong tidal field of the cluster acting on the corresponding light bundles. Further out in angular separation from the cluster center, the distortions become weaker and are not immediately obvious, but can be measured by averaging over several of the faint galaxy images. The distortion pattern signifies the presence, and allows to quantify the mass of the cluster.

The distortion effect, called weak gravitational lensing, can also be used to make a blind search for mass concentrations in the Universe; by this, one might detect dark matter halos with little light emission which would be missed by the conventional selection methods for galaxy clusters. In a weak lensing analysis of the cluster Abell 1942 on a wide-field image, we detected, beside the cluster itself, a second highly significant mass peak, separated by about 7' from the cluster (see Fig.2). This second mass peak has been confirmed by obtaining a second wide-field image of the region, using a different camera and a different filter. The probability that this mass peak is caused by chance alignment of the galaxies is about 1 part in a million. This mass concentration is not related to any overdensity of galaxies, and only little X-ray emission is seen in this direction. This mass clump could either be a cluster of galaxies at medium redshift, but with an extremely low light-to-mass ratio, or a high-redshift cluster with more normal mass-to-light ratio, but extremely high mass. Future X-ray and infrared observations will be able to distinguish between these possibilities.

Figure 2:
    Reconstructed surface mass density contours, plotted on deep V-band (left panel, field size 14' x 14') and I-band (right panel, field size 7.5' x 15') image. The cluster Abell 1942 at redshift z=0.22 is located in the center of the V-band, and close to the upper edge of the I-band image. About 7' to the South, a second mass concentration is seen, which, in various statistical tests, turns out to be about as significantly detected as the cluster itself. No overdensity of galaxies is seen to be associated with this mass concentration.

If confirmed, this would be the first mass-selected dark matter halo. Existing and upcoming wide-field imaging facilities will enable a systematic search for such halos, with an expected number density of about 10 per square degrees. A mass-selected halo sample will be invaluable for testing models of cosmology and structure evolution.

Th. Erben, P. Schneider

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Last modified: Fri Jul 30 16:26:01 MDT 1999 by Markus Rampp
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