The Gini Coefficient as a Morphological Measurement of Strongly Lensed Galaxies in the Image Plane

TL;DR

This paper investigates whether the Gini coefficient can serve as a reliable morphological measure for strongly lensed galaxies directly in the image plane, avoiding complex lens modeling and source reconstruction.

Contribution

It demonstrates through simulations that the Gini coefficient remains sufficiently conserved under lensing, PSF effects, and pixelization to be used as a morphological indicator in the image plane.

Findings

Gini coefficient is largely preserved under strong lensing.

PSF convolution and pixelization have limited impact on Gini measurement.

Using Gini in the image plane simplifies morphological analysis of lensed galaxies.

Abstract

Characterization of the morphology of strongly lensed galaxies is challenging because images of such galaxies are typically highly distorted. Lens modeling and source plane reconstruction is one approach that can provide reasonably undistorted images from which morphological measurements can be made, although at the expense of a highly spatially variable telescope PSF when mapped back to the source plane. Unfortunately, modeling the lensing mass is a time and resource intensive process, and in many cases there are too few constraints to precisely model the lensing mass. If, however, useful morphological measurements could be made in the image plane rather than the source plane, it would bypass this issue and obviate the need for a source reconstruction process for some applications. We examine the use of the Gini coefficient as one such measurement. Because it depends on the cumulative distribution of the light of a galaxy, but not the relative spatial positions, the fact that surface brightness is conserved by lensing means that the Gini coefficient may be well-preserved by strong gravitational lensing. Through simulations, we test the extent to which the Gini coefficient is conserved, including by effects due to PSF convolution and pixelization, to determine whether it is invariant enough under lensing to be used as a measurement of galaxy morphology that can be made in the image plane.