Lensing by galaxy halos in clusters of galaxies
  Weak shear maps of the outer regions of clusters have been successfully used to map the distribution of mass at large radii from the cluster center. The typical smoothing lengths employed thus far preclude the systematic study of the effects of galactic-scale substructure on the measured weak lensing signal. In this paper, we present two methods to infer the possible existence and extent of dark halos around bright cluster galaxies by quantifying the `local' weak lensing induced by them. The proposed methods are: direct radial averaging of the shear field in the vicinity of bright cluster members and a maximum-likelihood method to extract fiducial parameters characterizing galaxy halos. We demonstrate using simulations that these observed local weak-shear effects on galaxy scales within the cluster can be used to statistically constrain the mean mass-to-light ratio, and fiducial parameters like the halo size, velocity dispersion and hence mass of cluster galaxies. We compare the two methods and investigate their relative drawbacks and merits in the context of feasibility of application to HST cluster data, whereby we find that the prospects are promising for detection on stacking a minimum of 20 WFPC2 deep cluster fields. 
Priyamvada Natarajan (IoA, Cambridge) and Jean-Paul Kneib   (Observatoire Midi-Pyrenees, Toulouse)