Multiple Weak Deflections in Galaxy-Galaxy Lensing

TL;DR

This paper investigates the impact of multiple weak gravitational lensing deflections by galaxies on cosmic shear measurements, revealing that multiple deflections significantly increase shear estimates on small scales and depend on galaxy halo mass.

Contribution

It provides a detailed Monte Carlo simulation analysis of multiple weak deflections in galaxy-galaxy lensing, highlighting their importance for cosmic shear studies and the sensitivity to galaxy halo mass.

Findings

Multiple deflections increase net shear compared to single-deflection models.

Galaxy-galaxy lensing can significantly contribute to small-scale cosmic shear.

Contribution to cosmic shear diminishes rapidly beyond 5 arcmin, independent of halo mass.

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 order 1 arcmin. The contribution of galaxy-galaxy lensing to cosmic shear decreases rapidly with angular scale and extrapolates to zero at scales of order 5 arcmin. This last result is roughly independent of the halo mass and suggests that for scales greater than about 5 arcmin, cosmic shear is insensitive to the details of the gravitational potentials of large galaxies.