Is every strong lens model unhappy in its own way? Uniform modelling of a sample of 13 quadruply+ imaged quasars

TL;DR

This paper introduces a uniform modeling framework for quadruply-imaged quasar lens systems, applying it to the largest sample to date, and explores the alignment of mass and light distributions along with flux-ratio anomalies.

Contribution

It presents a general, automated modeling approach using Lenstronomy for a large sample of quads, enabling systematic studies of their properties and cosmological implications.

Findings

Mass and light position angles are well-aligned except with strong external shear.

No correlation between the ellipticity of light and mass distributions.

Flux ratios deviate from smooth model predictions, especially in blue bands, indicating microlensing and millilensing effects.

Abstract

Strong-gravitational lens systems with quadruply-imaged quasars (quads) are unique probes to address several fundamental problems in cosmology and astrophysics. Although they are intrinsically very rare, ongoing and planned wide-field deep-sky surveys are set to discover thousands of such systems in the next decade. It is thus paramount to devise a general framework to model strong-lens systems to cope with this large influx without being limited by expert investigator time. We propose such a general modelling framework (implemented with the publicly available software Lenstronomy) and apply it to uniformly model three-band Hubble Space Telescope Wide Field Camera 3 images of 13 quads. This is the largest uniformly modelled sample of quads to date and paves the way for a variety of studies. To illustrate the scientific content of the sample, we investigate the alignment between the mass and light distribution in the deflectors. The position angles of these distributions are well-aligned, except when there is strong external shear. However, we find no correlation between the ellipticity of the light and mass distributions. We also show that the observed flux-ratios between the images depart significantly from the predictions of simple smooth models. The departures are strongest in the bluest band, consistent with microlensing being the dominant cause in addition to millilensing. Future papers will exploit this rich dataset in combination with ground based spectroscopy and time delays to determine quantities such as the Hubble constant, the free streaming length of dark matter, and the normalization of the initial stellar mass function.