Large-scale structures in the Universe can act as weak lenses and therefore cause an amplification bias on flux-limited counts of background sources provided their luminosity function is sufficiently steep. If galaxies are associated with these structures, as assumed in the biasing hypothesis of galaxy formation, then this weak lensing effect can be detected by searching for correlations between background sources and foreground galaxies on scales of some 10 arcminutes. We have recently investigated this effect theoretically and have shown that highly significant large-scale correlations between 1-Jansky radio quasars and Lick galaxies are present (Bartelmann & Schneider 1993a,b). Here, we extend the analysis, correlating high-redshift 1-Jansky radio quasars with IRAS galaxies. Highly significant correlations are found also in this case; specifically, we find that 1-Jy radio quasars with z greater than or similar to 1.25 are correlated with IRAS galaxies at the 95% confidence level, which is increased to more than 99% for z greater than or similar to 1.5. We make use of individual correlation coefficients and demonstrate that they may provide interesting information on individual background sources; for example, this procedure reveals that the I-Jy source J2216-038 is weakly lensed by two X-ray bright galaxy clusters in the foreground. A comparison of the sky distribution of those 1-Jy sources which exhibit large correlation coefficients with either Lick or IRAS galaxies with the sky distribution of X-ray bright galaxy clusters from the EMSS tentatively indicates that highly correlated background sources are located preferentially in regions of higher-than-average cluster density. We interpret these results as further evidence for weak lensing by extended structures in the universe which are traced by optical, infrared, and X-ray emission.
