DETECTION OF WEAK LENSING BY A MASSIVE DARK HALO IN Q-2345+007

We report the detection of several arclets, a small arc, and a. cohere nt gravitational shear field centered on the vicinity of two bright ga laxies, almost-equal-to 45'' away from the double quasar 2345+007. In addition we discovered a new QSO at z = 2.653 and at almost-equal-to 2 ' from the center indicated by the shear. The shear is found from weak distortion of background galaxies and is representative of a cluster- like mass. This suggests a possibility that 2345+007 A, B are two imag es of the same source, split by a cluster-like mass. The two galaxies have redshift z = 0.28, and the field is too poor in bright galaxies t o indicate a rich cluster at this redshift. In addition, if there were a cluster-like (dark) mass at z = 0.25 with a critical line passing b etween the A and B QSO images, the shear field would be significantly stronger than what we observe. However, deep images show a. small exce ss of faint blue galaxies in the area. on which the coherent shear is centered. Thus, it is possible that a rich cluster at a higher redshif t induces the coherent shear. In this case, it would be able to produc e a critical line between A and B of 2345+007. To our knowledge, this is the first direct measurement of a weak shear near a double quasar. We find that this is a powerful technique to locate large masses on li nes of sight, even where concentrations of galaxies are not previously discovered. In particular, this technique will enable us to detect hy pothetical concentrations of dark matter which are not traced by galax ies. It can also provide a new efficient approach to the problem of la rge scale quasar-galaxy associations, raised by Fugmann (1990) and Bar telmann & Schneider(1993).