Abell 2744: too much substructure for ΛCDM?

Abstract

The massive substructures found in Abell 2744 by Jauzac et al. (2016) present a challenge to the cold dark matter paradigm due to their number and proximity to the cluster centre. We use one of the biggest N-body simulations, the Millennium XXL, to investigate the substructure in a large sample of massive dark matter haloes. A range of effects which influence the comparison with the observations is considered, extending the preliminary evaluation carried out by Jauzac et al. (2016). There are many tens of haloes in the simulation with a total mass comparable with or larger than that of Abell 2744. However, we find no haloes with a number and distribution of massive substructures (> 5 × 10 13 M ) that is close to that inferred from the observations of Abell 2744. The application of extreme value statistics suggests that we would need a simulation of at least ten times the volume of the Millennium XXL to find a single dark matter halo with a similar internal structure to Abell 2744. Explaining the distribution of massive substructures in clusters is a new hurdle for hierarchical models to negotiate, which is not weakened by appeals to baryonic physics or uncertainty over the nature of the dark matter particle.

hbt+: an improved code for finding subhaloes and building merger trees in cosmological simulations

“…The detection of this hardening requires the ability to identify subhalos in the inner regions of the host halo, which hbt+ does, but which subhalo finders that work in configuration space alone have difficulty identifying. Recent lensing observations of galaxy clusters have resulted in reports of discrepancies in the observed subhalo distribution compared to that of ΛCDM predictions, including the excess of massive subhalos reported by Jauzac et al (2016) and Schwinn et al (2017) and the more concentrated subhalo radial distribution reported by Natarajan et al (2017). These discrepancies can be explained, at least in part, by the blending problem present in the Millennium-XXL and Illustris subfind catalogues used in their comparisons.…”

“…The deficiency of massive subhalos near the centre of the host halo in catalogues constructed using subfind explains, at least in part, the disagreement noted by Schwinn et al (2017) and Natarajan et al (2017) between the distrubtion of subhalos identified using subfind in ΛCDM simulations and the distribution of subhalos inferred from lensing studies in galaxy clusters. Mao et al (2017) have argued that the mismatch between the simulation result of Millennium-XXL (Angulo et al 2012) and the lensing results of Jauzac et al (2016); Schwinn et al (2017) is also affected by the use of different masses in the comparison: subfind masses in the simulation but projected aperture masses from lensing that also include mass contributions from the host.…”

“…Mao et al (2017) have argued that the mismatch between the simulation result of Millennium-XXL (Angulo et al 2012) and the lensing results of Jauzac et al (2016); Schwinn et al (2017) is also affected by the use of different masses in the comparison: subfind masses in the simulation but projected aperture masses from lensing that also include mass contributions from the host. Even with the use of deblended subhalo mass estimates in the strong lensing analysis of Natarajan et al (2017), however, the observed spatial distribution of massive subhalos is still found to be more centrally concentrated than that from the subfind catalogues of the Illustris simulations (Vogelsberger et al 2014), which can be attributed to this radial-dependent blending issue in the subfind catalogues.…”

“…Recent lensing observations suggest an excess of massive subhalos in galaxy clusters. Using a combined weak and strong lensing analysis, Jauzac et al (2016) and Schwinn et al (2017) compared the distribution of massive subhalos inferred in Abell 2744 with those in the Millennium-XXL (Angulo et al 2012) simulation. They could not find any halo in Millennium-XXL that hosts as many massive subhalos as are observed in Abell 2744.…”

A Very Large (θE ≳ 40″) Strong Gravitational Lens Selected with the Sunyaev–Zel’dovich Effect: PLCK G287.0+32.9 (z = 0.38)

“…The high concentrations and large Einstein radii found for several massive clusters have been previously claimed to challenge ΛCDM Broadhurst & Barkana 2008), although updated analyses (and account of projection biases) have alleviated this tension Umetsu et al 2016). In a similar fashion, the amount of substructure within massive clusters can also be compared to numerical simulations and expectations from ΛCDM (Jauzac et al 2016;Schwinn et al 2017). Given its extreme properties, PLCKG287 is another useful laboratory for similar studies.…”