We perform a combined X-ray and strong lensing analysis of RX J1347.5-1145, one of the most luminous galaxy clusters at X-ray wavelengths. We show that evidence from strong lensing alone, based on published VLT and new HST data, strongly argues in fa
vor of a complex structure. The analysis takes into account arc positions, shapes and orientations and is done thoroughly in the image plane. The cluster inner regions are well fitted by a bimodal mass distribution, with a total projected mass of $M_{tot} = (9.9 pm 0.3)times 10^{14} M_odot/h$ within a radius of $360 mathrm{kpc}/h$ ($1.5$). Such a complex structure could be a signature of a recent major merger as further supported by X-ray data. A temperature map of the cluster, based on deep Chandra observations, reveals a hot front located between the first main component and an X-ray emitting South Eastern sub-clump. The map also unveils a filament of cold gas in the innermost regions of the cluster, most probably a cooling wake caused by the motion of the cD inside the cool core region. A merger scenario in the plane of the sky between two dark matter sub-clumps is consistent with both our lensing and X-ray analyses, and can explain previous discrepancies with mass estimates based on the virial theorem.
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 ($m_{ u} sim 10$) 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.
