The Fundamental Surface of Quad Lenses

TL;DR

This paper reveals a nearly invariant 2D surface in the angular configuration space of quad lens images, enabling new insights into lensing theory and simplifying the analysis of lens mass distributions.

Contribution

It introduces a universal surface in the 3D angle space of quads from simple lenses, providing a novel framework for studying and modeling gravitational lens systems.

Findings

Most observed quads deviate from the surface, indicating the need for external shear or substructure.

The surface is well-fit by SIS+elliptical lensing potential with minimal deviation.

The framework allows mass distribution inference solely from image positions.

Abstract

In a quadruply imaged lens system the angular distribution of images around the lens center is completely described by three relative angles. We show empirically that in the 3D space of these angles, spanning 180 x 180 x 90 degrees, quads from simple two-fold symmetric lenses of arbitrary radial density profile and arbitrary radially dependent ellipticity or external shear define a nearly invariant 2D surface. We give a fitting formula for the surface using SIS+elliptical lensing potential. Various circularly symmetric mass distributions with shear up to 0.4 deviate from it by typically, rms~0.1 deg, while elliptical mass distributions with ellipticity of up 0.4 deviate from it by rms~1.5 deg. The existence of a near invariant surface gives a new insight into the lensing theory and provides a framework for studying quads. It also allows one to gain information about the lens mass distribution from the image positions alone, without any recourse to mass modeling. As an illustration, we show that about 3/4 of observed galaxy-lens quads do not belong to this surface within observational error, and so require additional external shear or substructure to be modeled adequately.