Effects of dark matter substructures on gravitational lensing: results from the Aquarius simulations

Abstract

We use the high-resolution Aquarius simulations of the formation of Milky Way-sized haloes in the cold dark matter cosmology to study the effects of dark matter substructures on gravitational lensing. Each halo is resolved with ∼10 8 particles (at a mass resolution m p ∼ 10 3 to 10 4 h −1 M ) within its virial radius. Subhaloes with masses m sub 10 5 h −1 M are well resolved, an improvement of at least two orders of magnitude over previous lensing studies. We incorporate a baryonic component modelled as a Hernquist profile and account for the response of the dark matter via adiabatic contraction. We focus on the 'anomalous' flux ratio problem, in particular on the violation of the cusp-caustic relation due to substructures. We find that subhaloes with masses less than ∼10 8 h −1 M play an important role in causing flux anomalies; such low-mass subhaloes have been unresolved in previous studies. There is large scatter in the predicted flux ratios between different haloes and between different projections of the same halo. In some cases, the frequency of predicted anomalous flux ratios is comparable to that observed for the radio lenses, although in most cases it is not. The probability for the simulations to reproduce the observed violations of the cusp lenses is ≈10 −3 . We therefore conclude that the amount of substructure in the central regions of the Aquarius haloes is insufficient to explain the observed frequency of violations of the cusp-caustic relation. These conclusions are based purely on our dark matter simulations which ignore the effect of baryons on subhalo survivability.

Gravitational detection of a low-mass dark satellite galaxy at cosmological distance

“…The predicted fraction of substructure from simulations within the same mass range and projected distance from the host halo centre is ∼ 0.1 +0.3 −0.1 % (ref. 19), which is marginally smaller than the lower limit implied by our detection of a substructure at the 68 per cent confidence level, independent of the prior set on the slope of the mass function. However, these simulations model the formation of a Milky Way halo at z = 0 and simulations of elliptical galaxies out to z ∼ 1 must be made before a more quantitative conclusion can be drawn.…”

“…Hence, the shape of the substructure mass function cannot be constrained and so far it has remained unclear whether the results from quasar flux ratio anomalies are in agreement or disagreement with numerical simulations 19 . Our new low-mass detection also allows us to constrain for the first time the slope of the substructure mass function for galaxies other than our own, when combined with the detection of the ∼18-fold more massive (i.e.…”

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

“…Dalal & Kochanek 2002;Bradač et al 2004;Dobler & Keeton 2006;Metcalf & Amara 2012) suggest consistency between the CDM model and observations, others (e.g. Mao et al 2004;Amara et al 2006;Macciò & Miranda 2006;Chen, Koushiappas & Zentner 2011) including those by us (Xu et al 2009(Xu et al , 2010 find that subhaloes from CDM simulations are actually insufficient to explain the observed radio flux-anomaly frequency.…”

“…For example, as shown by Keeton, Gaudi & Petters (2003), flux ratios are quite sensitive to the ellipticity of the main lens. Metcalf & Amara (2012) also pointed out that one of the reasons that our previous studies (Xu et al 2009(Xu et al , 2010 did not reproduce enough perturbations to match observations could be due to our adoption of a restricted ellipticity instead of the full range of ellipticities in the main lens models.…”

Constraints on the identity of the dark matter from strong gravitational lenses

“…Xu et al 2009Xu et al , 2015. For example, Hsueh et al (2015) have shown that the flux ratio anomaly of CLASS B1555+375, one of the most anomalous lens systems known, can be explained by the presence of a previously undetected edge-on disc in the lens galaxy.…”