Towards an understanding of third-order galaxy-galaxy lensing

Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
e-mail: psimon@astro.uni-bonn.de

Received: 8 February 2012
Accepted: 17 October 2012

Abstract

Context. Third-order galaxy-galaxy lensing (G3L) is a next generation galaxy-galaxy lensing (GGL) technique that either measures the excess shear about lens pairs or the excess shear-shear correlations about lenses. From their definition it is clear that these statistics assess the three-point correlations between galaxy positions and projected matter density.

Aims. For future applications of these novel statistics, we aim at a more intuitive understanding of G3L to isolate the main features that possibly can be measured.

Methods. We construct a toy model (“isolated lens model”; ILM) for the distribution of galaxies and associated matter to determine the measured quantities of the two G3L correlation functions and traditional GGL in a simplified context. The ILM presumes single lens galaxies to be embedded inside arbitrary matter haloes that, however, are statistically independent (“isolated”) from any other halo or lens position. Clusters of galaxies and their common cluster matter haloes are a consequence of clustering smaller haloes. In particular, the average mass-to-galaxy number ratio of clusters of any size cannot change in the ILM.

Results. GGL and galaxy clustering alone cannot distinguish an ILM from any more complex scenario. The lens-lens-shear correlator in combination with second-order statistics enables us to detect deviations from a ILM, though. This can be quantified by a difference signal defined in the paper. We demonstrate with the ILM that this correlator picks up the excess matter distribution about galaxy pairs inside clusters, whereas pairs with lenses well separated in redshift only suppress the overall amplitude of the correlator. The amplitude suppression can be normalised. The lens-lens-shear correlator is sensitive to variations among matter haloes. In principle, it could be devised to constrain the ellipticities of haloes, without the need for luminous tracers, or maybe even random halo substructure.