First Mass-Selected Dark Matter Halo |
deutsch |
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.
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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.
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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
Further reading:
-
Mass-detection of a matter
concentration projected near the cluster Abell 1942: Dark clump or
high-redshift cluster?, T.Erben, L.van Waerbeke, Y.Mellier,
P.Schneider, J.C.Cuillandre, F.J. Castander, M.Dantel-Fort, A&A
submitted, Preprint
astro-ph 9907134
Last modified: Fri Jul 30 16:26:01 MDT 1999
by Markus Rampp
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