Resolution of the apparent discrepancy between the number of massive subhaloes in Abell 2744 and {\Lambda}CDM

TL;DR

This paper resolves the discrepancy between observed massive subhaloes in Abell 2744 and { extLambda}CDM predictions by clarifying the role of projection effects in aperture mass measurements, showing consistency with simulations.

Contribution

It demonstrates that aperture mass measurements are significantly affected by line-of-sight projection, reconciling observations with { extLambda}CDM simulations.

Findings

Aperture mass is dominated by projected mass along the line-of-sight.

Projection effects can enhance apparent subhalo masses by factors up to 100.

Observed data for Abell 2744 aligns with { extLambda}CDM predictions when accounting for projection.

Abstract

Schwinn et al. (2017) have recently compared the abundance and distribution of massive substructures identified in a gravitational lensing analysis of Abell 2744 by Jauzac et al. (2016) and N-body simulation and found no cluster in {\Lambda}CDM simulation that is similar to Abell 2744. Schwinn et al.(2017) identified the measured projected aperture masses with the actual masses associated with subhaloes in the MXXL N-body simulation. We have used the high resolution Phoenix cluster simulations to show that such an identification is incorrect: the aperture mass is dominated by mass in the body of the cluster that happens to be projected along the line-of-sight to the subhalo. This enhancement varies from factors of a few to factors of more than 100, particularly for subhaloes projected near the centre of the cluster. We calculate aperture masses for subhaloes in our simulation and compare them to the measurements for Abell 2744. We find that the data for Abell 2744 are in excellent agreement with the matched predictions from {\Lambda}CDM. We provide further predictions for aperture mass functions of subhaloes in idealized surveys with varying mass detection thresholds.