How well can cold-dark-matter substructures account for the observed radio flux-ratio anomalies?

TL;DR

This study evaluates whether cold-dark-matter substructures can fully explain observed radio flux-ratio anomalies in gravitational lensing, finding they are unlikely to be the sole cause and suggesting other factors may contribute.

Contribution

The paper constructs realistic lens models with subhalo populations from simulations to assess their impact on flux anomalies, providing a detailed comparison with observed data.

Findings

CDM substructures can reproduce anomalies in some systems with 5-20% probability.

Most observed anomalies are likely caused by propagation effects or modeling oversimplifications.

CDM substructures alone are insufficient to explain all radio flux-ratio anomalies.

Abstract

Discrepancies between the observed and model-predicted radio flux ratios are seen in a number of quadruply-lensed quasars. The most favored interpretation of these anomalies is that CDM substructures present in lensing galaxies perturb the lens potentials and alter image magnifications and thus flux ratios. So far no consensus has emerged regarding whether or not the predicted CDM substructure abundance fully accounts for the lensing flux anomaly observations. Accurate modeling relies on a realistic lens sample in terms of both the lens environment and internal structures and substructures. In this paper we construct samples of generalised and specific lens potentials, to which we add (rescaled) subhalo populations from the galaxy-scale Aquarius and the cluster-scale Phoenix simulation suites. We further investigate the lensing effects from subhalos of masses several orders of magnitude below the simulation resolution limit. The resulting flux ratio distributions are compared to the currently best available sample of radio lenses. The observed anomalies in B0128+437, B0712+472 and B1555+375 are more likely to be caused by propagation effects or oversimplified lens modeling, signs of which are already seen in the data. Among the quadruple systems that have closely located image triplets/pairs, the anomalous flux ratios of MG0414+0534 can be reproduced by adding CDM subhalos to its macroscopic lens potential, with a probability of 5%-20%; for B0712+472, B1422+231, B1555+375 and B2045+265, these probabilities are only of a few percent. We hence find that CDM substructures are unlikely to be the whole reason for radio flux anomalies. We discuss other possible effects that might also be at work.