Constraining the mass distribution of galaxies using galaxy-galaxy   lensing in clusters and in the field
  We present a maximum-likelihood analysis of galaxy-galaxy lensing effects in galaxy clusters and in the field. The aim is to determine the accuracy and robustness of constraints that can be obtained on galaxy halo properties in both environments - the high density cluster and the low density field. This paper is theoretically motivated, therefore, we work exclusively with simulated data (nevertheless defined to match observations) to study the accuracy with which input parameters for mass distributions for galaxies can be extracted. We model galaxies in the cluster and the field using a wide range of mass profiles: the truncated pseudo isothermal elliptical mass distribution, the Navarro, Frenk and White profile, and a Power Law model with a core radius. We find that independent of the choice of profile the mean mass of galaxies (of the order of 10^{12}Mo) can be estimated to within 15% from ground-based data and with an error of less than 10% with space observations. Additionally robust constraints can be obtained on the mean slope of the mass profile. The two standard parameters that characterise galaxy halo models, the central velocity dispersion and the truncation radius can also be retrieved reliably from the maximum-likelihood analysis. Furthermore, going beyond the usual formulation, we propose a re-parameterisation of the mass models that allows us to put yet stronger constraints on the aperture mass of a galaxy halo (with less than 10% error). The gain in signal to noise using space observations, expected for instance with the proposed SNAP satellite compared to ground based data in terms of accuracy of retrieving input parameters is highly significant. 
Marceau Limousin, Jean Paul Kneib and Priyamvada Natarajan