On the Segregation of Dark Matter Substructure

TL;DR

This paper analyzes how dark matter subhaloes are spatially and dynamically segregated within host haloes, revealing insights into their formation, accretion history, and the effects of tidal stripping and dynamical friction.

Contribution

It provides the first comprehensive simulation-based analysis of dark matter subhalo segregation across multiple properties and their origins, including infall conditions and dynamical effects.

Findings

Subhaloes are strongly segregated by accretion redshift.

Massive subhaloes experience boosted segregation due to dynamical friction.

Tidal stripping diminishes present-day mass segregation.

Abstract

We present the first comprehensive analysis of the segregation of dark matter subhaloes in their host haloes. Using numerical simulations, we examine the segregation of twelve different subhalo properties with respect to both orbital energy and halo-centric radius (in real space as well as in projection). Subhaloes are strongly segregated by accretion redshift, which is an outcome of the inside-out assembly of their host haloes. Since subhaloes that were accreted earlier have experienced more tidal stripping, subhaloes that have lost a larger fraction of their mass at infall are on more bound orbits. Subhaloes are also strongly segregated in their masses and maximum circular velocities at accretion. We demonstrate that part of this segregation is already imprinted in the infall conditions. For massive subhaloes it is subsequently boosted by dynamical friction, but only during their first radial orbit. The impact of these two effects is counterbalanced, though, by the fact that subhaloes with larger accretion masses are accreted later. Because of tidal stripping, subhaloes reveal little to no segregation by present-day mass or maximum circular velocity, while the corresponding torques cause subhaloes on more bound orbits to have smaller spin. There is a weak tendency for subhaloes that formed earlier to be segregated towards the center of their host halo, which is an indirect consequence of the fact that (sub)halo formation time is correlated with other, strongly segregated properties. We discuss the implications of our results for the segregation of satellite galaxies in galaxy groups and clusters.