Systematic errors induced by the elliptical power-law model in galaxy-galaxy strong lens modeling

TL;DR

This study quantifies biases in galaxy-galaxy strong lens modeling caused by the elliptical power-law model, highlighting the need for more complex models in future cosmography to improve accuracy in measuring galaxy and cosmological parameters.

Contribution

The paper systematically assesses the biases introduced by the elliptical power-law model in strong lensing analyses using realistic galaxy simulations, emphasizing the importance of advanced models.

Findings

Source morphology is accurately recovered, with slight bias towards more compact sources.

Einstein radius and density slope are robustly measured with minimal systematic errors.

Biases in time delay and Hubble constant measurements can significantly affect cosmological inferences.

Abstract

The elliptical power-law (EPL) model of the mass in a galaxy is widely used in strong gravitational lensing analyses. However, the distribution of mass in real galaxies is more complex. We quantify the biases due to this model mismatch by simulating and then analysing mock {\it Hubble Space Telescope} imaging of lenses with mass distributions inferred from SDSS-MaNGA stellar dynamics data. We find accurate recovery of source galaxy morphology, except for a slight tendency to infer sources to be more compact than their true size. The Einstein radius of the lens is also robustly recovered with 0.1% accuracy, as is the global density slope, with 2.5% relative systematic error, compared to the 3.4% intrinsic dispersion. However, asymmetry in real lenses also leads to a spurious fitted `external shear' with typical strength, $\gamma_{\rm ext}=0.015$. Furthermore, time delays inferred from lens modelling without measurements of stellar dynamics are typically underestimated by $\sim$5%. Using such measurements from a sub-sample of 37 lenses would bias measurements of the Hubble constant $H_0$ by $\sim$9%. Although this work is based on a particular set of MaNGA galaxies, and the specific value of the detected biases may change for another set of strong lenses, our results strongly suggest the next generation cosmography needs to use more complex lens mass models.