Concentrating the Dark Matter in Galaxy Clusters through Tidal Stripping of Baryonically-Compressed Galactic Halos

TL;DR

This paper proposes a model where tidal stripping of baryonically compressed galactic halos during cluster mergers explains the observed steep dark matter density profiles in galaxy clusters, addressing discrepancies with simulations.

Contribution

It introduces a new explanation for dark matter concentration in clusters based on merger histories and tidal stripping of compressed galactic halos.

Findings

The model accounts for the higher observed dark matter density within 150 kpc.

Tidal stripping of compressed halos can modify cluster profiles significantly.

Feedback disperses baryons, aligning with observations of baryon distribution.

Abstract

Gravitational lensing observations of massive X-ray clusters imply a steep characteristic density profile marked by a central concentration of dark matter. The observed mass fraction within a projected radius of 150 kpc is twice that found in state-of-the-art dark matter simulations of the standard Lambda-CDM cosmology. A central baryon enhancement that could explain this discrepancy is not observed, leaving a major puzzle. We propose a solution based on the merger histories of clusters. A significant fraction of the final dark matter content of a cluster halo originates within galaxy-sized halos, in which gas can cool and compress the dark matter core to high densities. The subsequent tidal stripping of this compressed dark matter occurs in denser regions that are closer to the center of the cluster halo. Eventually, the originally cooled gas must be dispersed into the intracluster medium through feedback, for consistency with observations that do not find central baryon enhancements in clusters. Still, the early adiabatic compression of the galactic dark matter leaves a net effect on the cluster. Using a simple model for this process, we show that the central cluster profile is substantially modified, potentially explaining the observed discrepancy.