How well can cold dark matter substructures account for the observed 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 favoured 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 modelling 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/improper lens modelling, 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.

Dark Matter Haloes and Subhaloes

“…These smooth-lens relations are violated if there are intervening structures or substructures in the lens giving rise to flux-ratio anomalies, which probe the total amount of mass in structures along the line of sight to the lens [341,343,344]. Flux-ratio anomalies have been observed in several quadruply-lensed quasars but dark substructures alone are insufficient to explain the observed anomalies [345], implying that other effects such as inadequate lens modelling may be at work. With better modelling of the lens (including stellar discs and luminous satellites), it has been possible to set a lower limit to the mass of a thermal WDM particle (see [346] and Harvey et al, in preparation), similar to the limits from satellite counts discussed above and also to those derived from the observed inhomogeneity of the gas distribution at high redshift probed by the Lyman-α forest [347].…”

Shaken and stirred: the Milky Way's dark substructures

“…However, much lower masses, down to ∼10 6 M , may be probed through strong gravitational lensing, either via flux-ratio anomalies (e.g. Mao & Schneider 1998;Xu et al 2009Xu et al , 2015, or detectable perturbations of observed Einstein rings by substructures in the lens itself or along the line of sight (Mao & Schneider 1998;Metcalf & Madau 2001;Dalal & Kochanek 2002;Vegetti et al 2012Vegetti et al , 2014). On these scales, different dark matter models may be clearly distinguished, provided that the expected abundances and distributions of substructures for different models can be reliably predicted.…”

Prospects for Observations of Gravitationally Lensed Sources by Submillimeter Space Observatories

“…The same problem was also considered in Metcalf, Zhao (2002). At the same time another analysis by Xu et al (2009Xu et al ( , 2015 showed that the population of substructures predicted by numerical methods is insufficient to explain the observed anomalies in the ratio of fluxes of lensed images. These anomalies can be probably explained by the presence of complex baryonic structures in lensing galaxies and also by a presence of dark subhaloes on the line of sight.…”