Quantifying substructures in {\it Hubble Frontier Field} clusters: comparison with $\Lambda CDM$ simulations

TL;DR

This study compares the substructure power spectrum of Hubble Frontier Field galaxy clusters, derived from lensing data, with that of simulated clusters, revealing differences likely due to observational limitations or simulation overestimations.

Contribution

It introduces a method to measure cluster substructure using the 2D power spectrum from lensing data without light-traces-mass assumptions and compares it with simulations.

Findings

Lensing-derived power spectra show lower small-scale power than simulations.

LTM assumptions inflate small-scale power in reconstructions.

Lower redshift clusters and deeper data yield higher small-scale power.

Abstract

The Hubble Frontier Fields (HFF) are six clusters of galaxies, all showing indications of recent mergers, which have recently been observed for lensed images. As such they are the natural laboratories to study the merging history of galaxy clusters. In this work, we explore the 2D power spectrum of the mass distribution $P_{\rm M}(k)$ as a measure of substructure. We compare $P_{\rm M}(k)$ of these clusters (obtained using strong gravitational lensing) to that of $\Lambda$CDM simulated clusters of similar mass. To compute lensing $P_{\rm M}(k)$, we produced free-form lensing mass reconstructions of HFF clusters, without any light traces mass (LTM) assumption. The inferred power at small scales tends to be larger if (i)~the cluster is at lower redshift, and/or (ii)~there are deeper observations and hence more lensed images. In contrast, lens reconstructions assuming LTM show higher power at small scales even with fewer lensed images; it appears the small scale power in the LTM reconstructions is dominated by light information, rather than the lensing data. The average lensing derived $P_{\rm M}(k)$ shows lower power at small scales as compared to that of simulated clusters at redshift zero, both dark-matter only and hydrodynamical. The possible reasons are: (i)~the available strong lensing data are limited in their effective spatial resolution on the mass distribution, (ii)~HFF clusters have yet to build the small scale power they would have at $z\sim 0$, or (iii)~simulations are somehow overestimating the small scale power.