Probing the Mass Distribution in Groups of Galaxies using Gravitational Lensing

In this paper, we present a numerical study of gravitational lensing by groups of galaxies. Since groups are abundant and therefore have a large covering fraction on the sky, lensing by groups is likely to be very important observationally. Besides, it has recently become clear that many models for strong lensing by individual galaxies require external shear to reproduce the observed image geometries; in many cases a nearby group is detected that could provide this shear. In this work, we study the expected lensing behavior of moderate-redshift analogs of compact galaxy groups in both the weak- and strong-lensing regimes. We show how measurements of the tangential shear can be used to determine whether groups possess a massive common group halo or whether most of the mass is associated with the individual galaxies. From our simulations, we find that the peak value of the weak-lensing shear signal is of the order of 3% and varies by a factor of about 2 for different assumed mass distributions. We demonstrate that these variations are large enough to be detectable with 3 σ confidence from space with the Advanced Camera for Surveys on the Hubble Space Telescope with only 100 groups, or alternatively with 2 σ confidence from the ground with about 200 groups, by stacking high-quality images. In the strong-lensing regime, we find that the image geometries and typical magnifications are sensitive to the group properties and that groups can easily provide enough external shear to produce quadruple images. We investigate the statistical properties of lensing galaxies that are near to or part of a group and find that properties such as the distribution of time delays are affected measurably by the presence of the group, which can therefore introduce an additional systematic error in the measurement of the Hubble constant from such systems. We conclude that both the detection of weak lensing by groups and accurate observations of strong galaxy lens systems near groups could provide important information on the total mass and matter distribution within galaxy groups.