Gravitational `Convergence' and Cluster Masses

TL;DR

This paper introduces a method combining colour-dependent magnification bias and image distortion to measure the local convergence and mass distribution in galaxy clusters, providing insights into cluster mass and cosmic geometry.

Contribution

It presents a novel approach to determine cluster convergence and mass using combined magnification bias and distortion measurements, applicable over entire cluster surfaces.

Findings

Magnification bias depends on galaxy colour, enabling mass measurements.

Convergence can be derived from magnification alone in weak lensing regions.

Convergence varies slowly at high redshift, informing global geometry.

Abstract

Two colour photometry of the cluster A1689 reveals a `relative magnification-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 field, allowing us to measure the cluster magnification and to understand the notorious blueness of large arcs. We show that the magnification information can be combined with the usual image distortion measurements to isolate the local `convergence' 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 magnification 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.