# The Phase Space Structure of Dark Matter Halos

**Authors:** Han Aung, Daisuke Nagai, Eduardo Rozo, Rafael Garcia  
**arXiv:** [2003.11557](https://arxiv.org/abs/2003.11557)  
**Date:** 2020-03-25  
**Summary page:** https://www.summarizepaper.com/en/arxiv-id/2003.11557v1/

## Key points

- Phase space structure of dark matter halos is crucial for understanding their properties and dynamics
- Recent study investigates phase space structure and its implications for measuring halo mass, inferring mass accretion rates, and probing modified gravity effects
- Splashback radius can be measured using density profile slope in position space, but phase space structure extends beyond this radius
- Mixing of infalling, splashback, and virialized halos has important implications for dark matter distribution and dynamics within halos
- Researchers modelled three kinematically distinct populations within halos: infalling particles, particles near splashback radius, and virialized particles
- Analysis reveals an "edge radius" beyond which a halo has no orbiting substructures; this edge radius is a fixed multiple of the splashback radius
- Edge radius contains a fixed fraction of apocenters of dark matter particles
- Findings provide insights into dark matter halo structure and interpretation of observational data related to satellite galaxies
- Study serves as companion paper to another work analyzing phase space distribution of SDSS redMaPPer clusters

## Summary

The phase space structure of dark matter halos plays a crucial role in understanding their properties and dynamics. In a recent study, researchers investigated the phase space structure of dark matter halos and its implications for measuring halo mass, inferring mass accretion rates, and probing the effects of modified gravity. Previous studies have shown that the splashback radius, which marks the outer boundary of dark matter halos, can be measured in position space using the slope of the density profile. However, using N-body simulations, the authors of this study demonstrate that the phase space structure of dark matter halos extends beyond this splashback radius. In fact, they find that there exists a region where infalling, splashback and virialized halos are mixed spatially. This mixing has important implications for understanding the distribution and dynamics of dark matter within halos. To further investigate this phenomenon, the researchers modelled the distribution of three kinematically distinct populations within dark matter halos: infalling particles that are still approaching the halo; particles at or near the splashback radius; and particles that have already become virialized within the halo. Their analysis reveals an intriguing result - there exists an "edge radius" beyond which a dark matter halo has no orbiting substructures. This edge radius is found to be a fixed multiple of the splashback radius as defined in previous works. Importantly, it can be interpreted as a radius that contains a fixed fraction of apocenters (the farthest points in an orbit) of dark matter particles. These findings provide valuable insights into our understanding of dark matter halos and their internal structure. They also offer a theoretical foundation for interpreting observational data related to satellite galaxies within these halos. This study serves as a companion paper to another work by Tomooka et al., where they analyzed the phase space distribution of SDSS redMaPPer clusters. Together with this companion paper, these results contribute to a more comprehensive understanding of the phase space structure of dark matter halos and its implications for various astrophysical phenomena.