Dissociation of dark matter and gas in cosmic large-scale structure

TL;DR

This paper introduces a dissociation index to quantify the separation of dark matter and gas in cosmic structures, showing most haloes are positively dissociated and linking dissociation to binary collision dynamics.

Contribution

It presents a new dimensionless dissociation index and demonstrates its effectiveness in analyzing dark matter and gas separation in large-scale structures using simulations.

Findings

90% of haloes are positively dissociated with DM more elongated than gas

The density of highly dissociated structures aligns with observations

Binary collision simulations explain the frequency of dissociation

Abstract

The partial spatial separation of cold dark matter (DM) and gas is a ubiquitous feature in the formation of cosmic large-scale structure. This separation, termed dissociation, is prominent in galaxy clusters that formed through collisions of massive progenitors, such as the famous `Bullet' cluster. A direct comparison of the incidence of such dissociated structures with theoretical predictions is challenged by the rarity of strongly dissociated systems and the difficulty to quantify dissociation. This paper introduces a well-defined dimensionless dissociation index $S\in[-1,1]$ that encodes the quadrupole difference between DM and gas in a custom region. Using a simulation of cosmic large-scale structure with cold DM and ideal non-radiating gas, in $\Lambda$CDM cosmology, we find that 90 per cent of the haloes are positively dissociated ($S>0$), meaning their DM is more elongated than their gas. The spatial density of highly dissociated massive structures appears consistent with observations. Through idealised $N$-body+SPH simulations of colliding gaseous DM haloes, we further explore the details of how ram-pressure causes dissociation in binary collisions. A suite of 300 such simulations reveals a scale-free relation between the orbital parameters of binary collisions and the resulting dissociation. Building on this relation, we conclude that the frequency of dissociated structures in non-radiative cosmological simulations is nearly fully accounted for by the major (mass ratio $>1:10$) binary collisions predicted by such simulations. In principle, our results allow us to constrain the orbital parameters that produced specific observed dissociated clusters.