4.8.2 Weak Shear and Amplification Bias Near Radiosources
Bartelmann & Schneider (1993a, b) definitely confirmed the correlation found by Fugmann (1990) between radio-sources from the 1-Jy catalog and the distribution of galaxies from the Lick catalogue. They also found a strong correlation with the IRAS catalog (Bartelmann & Schneider 1994) which they interpreted as a lensing effect on distant radio sources by foreground dark matter overdensities associated with the distribution of galaxies. The correlation is observed on 10 arcmin. scales and its strength is rather compatible with that expected from cluster-mass systems. If such dark matter inhomogeneities are distributed on large scales, they could magnify and, in exceptional cases, split images of distant radio-sources and quasars. A few overbright objects or unexplained multiply-imaged quasars could be such lensed systems. The interpretation of Bartelmann & Schneider may have found a first spectacular confirmation through the measurement of an almost circular weak shear near the field of Q2345+007 by Bonnet et al. (1993). It is interpreted as gravitational distortion by a cluster-like system of matter. This explains the large angular separation of the quasar pairs. Although no distant cluster is visible so far on the deepest CCD images, the result is reinforced by the observation of an arc with a center of curvature exactly at the center of the shear field. And to some extent, the detection of an arc in the field of the double quasar Q2345+007 (Bernstein et al. 1993) is almost comparable, although the deflecting galaxy has been clearly seen.
Furthermore, by observing the weak gravitational shear in the vicinity of a selected sample of radio-sources and quasars at a redshift of about 1 (the mean redshift of background sources), we could try to test the Bartelmann & Schneider hypothesis. The best observational strategy is to preselect the brightest objects both in the optical and at radio wavelengths in order to benefit from the possible double magnification bias suggested by Borgeest, Linde & Refsdal (1991). This would significantly increase the probability that the selected source is really a magnified object. If a dark cluster-mass is responsible for the magnification, we could measure a weak shear in the field of the radio-sources and quasars (7 x 7 arcmin for available CCDs at CFHT) with an amplitude of about 10-2. This would provide a direct proof that the correlation observed by Bartelmann & Schneider is a lensing effect!
In summary, the possibility of measuring the weak shear around bright isolated galaxies, compact groups of galaxies and clusters or large scale inhomogeneities could allow us to directly investigate the mass spectrum of the largest gravitational systems of the universe from their lensing effects. Therefore, this new gravitational lensing topic should play a major role in the understanding of the formation of large scale structure and the distribution and possible nature of the dark matter.