Systematic or Signal? How dark matter misalignments can bias strong lensing models of galaxy clusters

TL;DR

This paper investigates how dark matter misalignments in galaxy clusters can cause significant biases in strong lensing models, affecting image position predictions and potentially revealing dark matter properties.

Contribution

It introduces a method to quantify the impact of dark matter-baryon misalignments on lensing models and estimates the resulting errors in image position predictions.

Findings

Dark matter misalignments can shift image positions by over 1".

Assuming light traces mass causes an RMS error of 0.5" in models.

Potential to detect ~1kpc dark matter-baryon offsets indicating self-interacting dark matter.

Abstract

We explore how assuming that mass traces light in strong gravitational lensing models can lead to systematic errors in the predicted position of multiple images. Using a model based on the galaxy cluster MACSJ0416 (z = 0.397) from the Hubble Frontier Fields, we split each galactic halo into a baryonic and dark matter component. We then shift the dark matter halo such that it no longer aligns with the baryonic halo and investigate how this affects the resulting position of multiple images. We find for physically motivated misalignments in dark halo position, ellipticity, position angle and density profile, that multiple images can move on average by more than 0.2" with individual images moving greater than 1". We finally estimate the full error induced by assuming that light traces mass and find that this assumption leads to an expected RMS error of 0.5", almost the entire error budget observed in the Frontier Fields. Given the large potential contribution from the assumption that light traces mass to the error budget in mass reconstructions, we predict that it should be possible to make a first significant detection and characterisation of dark halo misalignments in the Hubble Frontier Fields with strong lensing. Finally, we find that it may be possible to detect ~1kpc offsets between dark matter and baryons, the smoking gun for self-interacting dark matter, should the correct alignment of multiple images be observed.