Model-free analysis of quadruply imaged gravitationally lensed systems and substructured galaxies

TL;DR

This paper uses lensing invariants based on image angles to analyze quadruply imaged gravitational lens systems, revealing that smooth elliptical+shear models cannot account for the observed distribution, implying the need for substructure.

Contribution

It introduces a model-free method using image angle invariants to analyze lens populations, highlighting the limitations of smooth models in reproducing observed quads.

Findings

Smooth elliptical+shear lenses cannot reproduce the observed quad angle distribution.

Lensing invariants can distinguish between smooth models and real lens populations.

Substructure or nearby perturbers are likely necessary to explain observed quads.

Abstract

Multiple image gravitational lens systems, and especially quads are invaluable in determining the amount and distribution of mass in galaxies. This is usually done by mass modeling using parametric or free-form methods. An alternative way of extracting information about lens mass distribution is to use lensing degeneracies and invariants. Where applicable, they allow one to make conclusions about whole classes of lenses without model fitting. Here, we use approximate, but observationally useful invariants formed by the three relative polar angles of quad images around the lens center to show that many smooth elliptical+shear lenses can reproduce the same set of quad image angles within observational error. This result allows us to show in a model-free way what the general class of smooth elliptical+shear lenses looks like in the three dimensional (3D) space of image relative angles, and that this distribution does not match that of the observed quads. We conclude that, even though smooth elliptical+shear lenses can reproduce individual quads, they cannot reproduce the quad population. What is likely needed is substructure, with clump masses larger than those responsible for flux ratio anomalies in quads, or luminous or dark nearby perturber galaxies.