A robust mass estimator for dark matter subhalo perturbations in strong gravitational lenses

TL;DR

This paper introduces a robust method to estimate the mass of dark matter subhalos in strong gravitational lensing, improving accuracy despite biases in density profile assumptions.

Contribution

The authors derive a characteristic scale for lensing perturbations and propose an effective subhalo lensing mass that remains accurate under various biases.

Findings

Effective subhalo lensing mass is accurate within 10%

Total subhalo mass estimates can be biased by nearly an order of magnitude

The method is validated with multiple gravitational lens simulations

Abstract

A few dark matter substructures have recently been detected in strong gravitational lenses though their perturbations of highly magnified images. We derive a characteristic scale for lensing perturbations and show that this is significantly larger than the perturber's Einstein radius. We show that the perturber's projected mass enclosed within this radius, scaled by the log-slope of the host galaxy's density profile, can be robustly inferred even if the inferred density profile and tidal radius of the perturber are biased. We demonstrate the validity of our analytic derivation by using several gravitational lens simulations where the tidal radii and the inner log-slopes of the density profile of the perturbing subhalo are allowed to vary. By modeling these simulated data we find that our mass estimator, which we call the effective subhalo lensing mass, is accurate to within about 10\% or smaller in each case, whereas the inferred total subhalo mass can potentially be biased by nearly an order of magnitude. We therefore recommend that the effective subhalo lensing mass be reported in future lensing reconstructions, as this will allow for a more accurate comparison with the results of dark matter simulations.