Flux-ratio anomalies in cusp quasars reveal dark matter beyond CDM

TL;DR

This study uses flux-ratio anomalies in cusp quasars to provide strong evidence for fuzzy dark matter, challenging traditional cold and self-interacting dark matter models, based on extensive simulations and analysis of specific lens configurations.

Contribution

It introduces a new region in lens parameter space where flux anomalies strongly favor fuzzy dark matter over CDM and SIDM, supported by Bayesian analysis.

Findings

J1042+1641 is >3σ incompatible with CDM and SIDM.

Bayes factor exceeds 100 favoring FDM.

Extending sample size is crucial for statistical confirmation.

Abstract

Strongly lensed quasars in cusp configurations provide a uniquely sensitive probe of small-scale dark matter structure. Using the largest microlensing-free flux ratios for 17 quadruply imaged cusps, we combine these with extensive Monte Carlo simulations of mock lens realizations under cold dark matter (CDM), self-interacting dark matter (SIDM), and fuzzy dark matter (FDM) scenarios. Building on this, we propose a region (minor-axis and narrow major-axis cusp lenses) where flux-ratio anomalies persist even under globally parameterized models ("macromodels") with multipole freedom (capturing disk, asymmetric, or merger-driven structures). Within this region, J1042+1641 is $>3\sigma$ incompatible with both CDM and SIDM. Our results yield a Bayes factor exceeding $100$, providing very strong evidence for FDM over even the most optimistic CDM and SIDM scenarios. As only 11 cusp lenses lie within this region, extending to larger samples will be essential for assessing its statistical generality and for decisively confirming these findings with future microlensing-free flux ratio data.