TDCOSMO. VII. Boxyness/discyness in lensing galaxies : Detectability and impact on $H_0$

TL;DR

This study investigates how azimuthal structures in lensing galaxies, like boxy or discy features, affect gravitational lensing observations and the measurement of the Hubble constant, finding their impact is generally small but can be significant in worst-case scenarios.

Contribution

It characterizes the detectability of multipolar azimuthal structures in lensing galaxies and quantifies their potential bias on H0 inference, a previously unexplored aspect.

Findings

Multipole deformations are usually detectable in high S/N data.

Undetected multipoles can bias H0 by up to 10-12 km/s/Mpc in worst cases.

For a realistic galaxy population, the systematic bias on H0 is less than 1%.

Abstract

In the context of gravitational lensing, the density profile of lensing galaxies is often considered to be perfectly elliptical. Potential angular structures are generally ignored, except to explain flux ratios anomalies. Surprisingly, the impact of azimuthal structures on extended images of the source has not been characterized, nor its impact on the H0 inference. We address this task by creating mock images of a point source embedded in an extended source, lensed by an elliptical galaxy on which multipolar components are added to emulate boxy/discy isodensity contours. Modeling such images with a density profile free of angular structure allow us to explore the detectability of image deformation induced by the multipoles in the residual frame. Multipole deformations are almost always detectable for our highest signal-to-noise mock data. However the detectability depends on the lens ellipticity and Einstein radius, on the S/N of the data, and on the specific lens modeling strategy. Multipoles also introduce small changes to the time delays. We therefore quantify how undetected multipoles would impact H0 inference. When no multipoles are detected in the residuals, the impact on H0 for a given lens is in general less than a few km/s/Mpc, but in the worst case scenario, combining low S/N in the ring and large intrinsic boxyness/discyness, the bias on H0 can reach 10-12 km/s/Mpc. If we now look at the inference on H0 from a population of lensing galaxies, having a distribution of multipoles representative of what is found in the light-profile of elliptical galaxies, we then find a systematic bias on H0 < 1%. The comparison of our mock systems to the state-of-the-art time delay lens sample studied by the H0LiCOW and TDCOSMO collaborations, indicates that multipoles are currently unlikely to be a source of substantial systematic bias on the inferred value of H0 from time-delay lenses.