Quantitative Analysis of Galaxy‐Galaxy Lensing

Gravitational light deflection due to mass along the line-of-sight will distort the images of background sources. This effect has been used successfully to investigate the mass distribution of galaxy clusters. Although an individual galaxy is not massive enough to cause a detectable lensing distortion in the background population, this effect can be measured statistically for a population of galaxies, and a first detection was reported recently by Brainerd, Blandford and Smail (BBS). In this paper we explore a quantitative and efficient method to constrain the halo properties of distant galaxy populations through galaxy-galaxy lensing and show that the mean masses and sizes of halos can be estimated accurately, without excessive data requirements. Specifically, we propose a maximum-likelihood analysis which takes full account of the actual image ellipticities, positions and apparent magnitudes. We apply it to simulated observations, using the same model for the lensing galaxy population as in BBS, where the galaxy halos are described by isothermal spheres with velocity dispersion #sigma#, truncated at a radius s. Both parameters are assumed to scale with the luminosity of the galaxy. The best fitting values, #sigma#* and s*, corresponding to an L*-galaxy, are then determined with the maximum-likelihood analysis