Parity Dependence in Strong Lens Systems as a Probe of Dark Matter Substructure

TL;DR

This paper investigates how parity dependence in flux ratios of strongly lensed images can reveal the presence and distribution of dark matter substructures within galaxies, challenging some existing dark matter models.

Contribution

It demonstrates that observed parity dependence requires aligned dark matter clumps and suggests a higher substructure mass fraction than some simulations predict.

Findings

Observed parity dependence implies significant alignment of dark matter clumps with images.

Results suggest a larger substructure mass fraction than in some dark matter simulations.

Luminous satellites are unlikely to account for the observed parity dependence.

Abstract

The amount of mass in small, dark matter clumps within galaxies (substructure) is an important test of cold dark matter. One approach to measuring the substructure mass fraction is to analyze the fluxes of images that have been strongly lensed by a galaxy. Flux ratios between images that are anomalous with respect to smooth (no substructure) models have previously suggested that there is a greater amount of substructure than found in dark matter simulations. One measure of anomalous flux ratios is parity dependence -- that the fluxes of different images of a source are perturbed differently. In this paper, we discuss parity dependence as a probe of dark matter substructure. We find that reproducing the observed parity dependence requires a significant alignment between concentrated dark matter clumps and images. The results may imply a larger fraction of mass in substructures than suggested by some dark matter simulations and that the observed parity dependence is unlikely to be reproduced by luminous satellites of lens galaxies.