Galaxy-lens determination of $H_0$: the effect of the ellipse+shear modeling assumption

TL;DR

This study investigates how assuming elliptical mass distributions with external shear in galaxy lens models can bias the measurement of the Hubble constant, using synthetic data and highlighting discrepancies with observed quads.

Contribution

The paper demonstrates that common modeling assumptions can lead to significant biases in $H_0$ estimation and introduces tests with synthetic quads to evaluate these effects.

Findings

Synthetic quads with non-elliptical shapes can bias $H_0$ by about 10%.

Observed quads show a wider range of image distance ratios than models predict.

Model assumptions may cause systematic errors in cosmological parameter estimation.

Abstract

Galaxy lenses are frequently modeled as an elliptical mass distribution with external shear and isothermal spheres to account for secondary and line-of-sight galaxies. There is statistical evidence that some fraction of observed quads are inconsistent with these assumptions, and require a dipole-like contribution to the mass with respect to the light. Simplifying assumptions about the shape of mass distributions can lead to the incorrect recovery of parameters such as $H_0$. We create several tests of synthetic quad populations with different deviations from an elliptical shape, then fit them with an ellipse+shear model, and measure the recovered values of $H_0$. Kinematic constraints are not included. We perform two types of fittings -- one with a single point source and one with an array of sources emulating an extended source. We carry out two model-free comparisons between our mock quads and the observed population. One result of these comparisons is a statistical inconsistency not yet mentioned in the literature: the image distance ratios with respect to the lens center of observed quads appear to span a much wider range than those of synthetic or simulated quads. Bearing this discrepancy in mind, our mock populations can result in biases on $H_0$ $\sim10\%$.