Multiple Weak Deflections in Galaxy-Galaxy Lensing

Abstract

The frequency and effects of multiple weak deflections in galaxy-galaxy lensing are investigated via Monte Carlo simulations. The lenses in the simulations are galaxies with known redshifts and known rest-frame blue luminosities. The frequency of multiple deflections above a given threshold shear value is quantified for discrete source redshifts, as well as for a set of sources that are broadly distributed in redshift space. In general, the closest lens in projection on the sky is not the only lens for a given source. In addition, ~50% of the time the closest lens is not the most important lens for a given source. Compared to a naive single-deflection calculation in which only the lensing due to the closest weak lens is considered, a full multiple-deflection calculation yields a higher net shear for individual sources, as well as a higher mean tangential shear around the lens centers. The full multiple-deflection calculation also shows that galaxy-galaxy lensing may contribute a substantial amount to cosmic shear on small angular scales. The degree to which galaxy-galaxy lensing contributes to the small-scale cosmic shear is, however, quite sensitive to the mass adopted for the halos of L* _B galaxies. Changing the halo mass by a factor of ~2.5 changes the contribution of galaxy-galaxy lensing to the cosmic shear by a factor of ~3 on scales of θ ~ 1'. The contribution of galaxy-galaxy lensing to cosmic shear decreases rapidly with angular scale and extrapolates to zero at θ ~ 5'. This last result is roughly independent of the halo mass and suggests that for scales θ >~ 5', cosmic shear is insensitive to the details of the gravitational potentials of large galaxies.