THE AUTOCORRELATION FUNCTION OF THE EXTRAGALACTIC BACKGROUND LIGHT .1. MEASURING GRAVITATIONAL SHEAR

A new method for measuring the shear induced by gravitational light de flection is proposed. It is based on analyzing the anisotropy induced in the auto-correlation function of the extragalactic background light which is produced by very faint distant galaxies. The auto-correlatio n function can be measured 'locally', and its anisotropy is caused by the tidal gravitational field of the deflecting mass distribution in t he foreground of these faint background galaxies. Since the method doe s not require individual galaxy detection, it can be used to measure t he shear of extremely faint galaxies which are not detectable individu ally, but are present in the noise. The shear estimated from the auto- correlation function of the image provides an independent measurement which can be compared to the shear obtained from the distortion of ind ividual galaxy images. Combining these two independent estimates clear ly increases the sensitivity of shear measurements. In addition, our n ew method may allow to determine the local magnification caused by the deflector if the auto-correlation function is caused by a large numbe r density of faint galaxies; in this case, the intrinsic auto-correlat ion function may provide a 'standard source' with respect to which she ar and magnification can be obtained. Applications to real and synthet ic data are shown and the feasibility of our new method is demonstrate d. In particular, we present the shear maps obtained with our new meth od for the double QSO 2345+007 and the cluster C10024+16 and compare t hem to published shear maps.