Exploiting flux ratio anomalies to probe warm dark matter in future large scale surveys

TL;DR

This paper proposes using flux ratio anomalies in double imaged quasars from future large surveys to distinguish between cold and warm dark matter models, highlighting the potential of this method with upcoming data.

Contribution

It introduces a statistically efficient method using double imaged quasars to probe dark matter nature, incorporating realistic simulations and systematics.

Findings

WDM predicts ~6% fewer small flux ratios than CDM.

Approximately 600 doubly imaged quasars are needed to differentiate models.

Future surveys like Euclid can provide the necessary data.

Abstract

Flux ratio anomalies in strong gravitationally lensed quasars constitute a unique way to probe the abundance of non-luminous dark matter haloes, and hence the nature of dark matter. In this paper we identify double imaged quasars as a statistically efficient probe of dark matter, since they are 20 times more abundant than quadruply imaged quasars. Using N-body simulations that include realistic baryonic feedback, we measure the full distribution of flux ratios in doubly imaged quasars for cold (CDM) and warm dark matter (WDM) cosmologies. Through this method, we fold in two key systematics - quasar variability and line-of-sight structures. We find that WDM cosmologies predict a ~6 per cent difference in the cumulative distribution functions of flux ratios relative to CDM, with CDM predicting many more small ratios. Finally, we estimate that ~600 doubly imaged quasars will need to be observed in order to be able to unambiguously discern between CDM and the two WDM models studied here. Such sample sizes will be easily within reach of future large scale surveys such as Euclid. In preparation for this survey data we require discerning the scale of the uncertainties in modelling lens galaxies and their substructure in simulations, plus a strong understanding of the selection function of observed lensed quasars.