Gravitationaìconvergence' and Galaxy Cluster Masses

Two colour photometry of the cluster A1689 reveals a `relative magniication-bias' between lensed blue and red background galaxies, arising from a dependence of the faint galaxy count-slope on colour. The colour distribution is skewed blueward of the far eld, allowing us to measure the cluster magniication and to understand the notorious blueness of large arcs. We show that the magniication information can be combined with the usual image distortion measurements to isolate the locaìconvergence' component of lensing and hence derive the projected mass. This is achieved through a simple local relation between the convergence and the observables, which can be applied generally over the surface a cluster. In the weak lensing limit, the convergence reduces to a dependence on the magniication alone, so that in the outskirts of clusters the surface-density of matter is obtained directly from the surface-density of background galaxies. Hence, useful lensing work requires colour information but not necessarily good seeing. Interestingly, convergence varies slowly at high redshift, saturating at a level depending on the Horizon distance, allowing a useful model-independent measurement of the Global Geometry.