Constraining Warm Dark Matter using QSO gravitational lensing

TL;DR

This paper uses gravitational lensing of quasars to set a lower mass limit on warm dark matter particles, finding that particles lighter than a few keV cannot account for observed flux anomalies, thus constraining WDM models.

Contribution

It introduces a new method to constrain warm dark matter particle mass using flux ratios in gravitationally lensed QSOs, focusing on the impact of intergalactic haloes.

Findings

WDM particles lighter than ~10 keV cannot reproduce observed flux anomalies.

The effect of intergalactic haloes on flux ratios peaks for halo masses of 10^6-10^8 solar masses.

Results support previous constraints from Lyman-alpha forest and CMB analyses.

Abstract

Warm Dark Matter (WDM) has been invoked to resolve apparent conflicts of Cold Dark Matter (CDM) models with observations on subgalactic scales. In this work we provide a new and independent lower limit for the WDM particle mass (e.g. sterile neutrino) through the analysis of image fluxes in gravitationally lensed QSOs. Starting from a theoretical unperturbed cusp configuration we analyze the effects of intergalactic haloes in modifying the fluxes of QSO multiple images, giving rise to the so-called anomalous flux ratio. We found that the global effect of such haloes strongly depends on their mass/abundance ratio and it is maximized for haloes in the mass range $10^6-10^8 \Msun$. This result opens up a new possibility to constrain CDM predictions on small scales and test different warm candidates, since free streaming of warm dark matter particles can considerably dampen the matter power spectrum in this mass range. As a consequence, while a ($\Lambda$)CDM model is able to produce flux anomalies at a level similar to those observed, a WDM model, with an insufficiently massive particle, fails to reproduce the observational evidences. Our analysis suggests a lower limit of a few keV ($m_{\nu} \sim 10$) for the mass of warm dark matter candidates in the form of a sterile neutrino. This result makes sterile neutrino Warm Dark Matter less attractive as an alternative to Cold Dark Matter, in good agreement with previous findings from Lyman-$\alpha$ forest and Cosmic Microwave Background analysis.