Warm Dark Haloes Accretion Histories and their Gravitational Signatures

TL;DR

This study compares Warm Dark Matter (WDM) and Cold Dark Matter (CDM) galaxy clusters, revealing that WDM clusters, despite having fewer substructures, exhibit higher gravitational lensing efficiencies due to larger, more extended subhaloes and increased smooth mass accretion.

Contribution

The paper demonstrates that WDM clusters have higher lensing efficiencies than CDM clusters, challenging previous expectations and highlighting the impact of subhalo size and distribution on gravitational lensing.

Findings

WDM clusters have fewer but larger and more extended subhaloes.

WDM clusters exhibit higher gravitational lensing efficiency than CDM clusters.

Radial distribution of subhaloes in WDM clusters enhances lensing cross-sections.

Abstract

We study clusters in Warm Dark Matter (WDM) models of a thermally produced dark matter particle $0.5$ keV in mass. We show that, despite clusters in WDM cosmologies having similar density profiles as their Cold Dark Matter (CDM) counterparts, the internal properties, such as the amount of substructure, shows marked differences. This result is surprising as clusters are at mass scales that are {\em a thousand times greater} than that at which structure formation is suppressed. WDM clusters gain significantly more mass via smooth accretion and contain fewer substructures than their CDM brethren. The higher smooth mass accretion results in subhaloes which are physically more extended and less dense. These fine-scale differences can be probed by strong gravitational lensing. We find, unexpectedly, that WDM clusters have {\em higher} lensing efficiencies than those in CDM cosmologies, contrary to the naive expectation that WDM clusters should be less efficient due to the fewer substructures they contain. Despite being less dense, the larger WDM subhaloes are more likely to have larger lensing cross-sections than CDM ones. Additionally, WDM subhaloes typically reside at larger distances, which radially stretches the critical lines associated with strong gravitational lensing, resulting in excess in the number of clusters with large radial cross-sections at the $\sim2\sigma$ level. Though lensing profile for an individual cluster vary significantly with the line-of-sight, the radial arc distribution based on a sample of $\gtrsim100$ clusters may prove to be the crucial test for the presence of WDM.