The many boundaries of the stratified dark matter halo

TL;DR

This paper reviews the physics and observational aspects of various boundaries of dark matter halos, highlighting their stratified structure and implications for understanding cosmic structure formation.

Contribution

It introduces the concept of multiple halo boundaries, including splashback and depletion radii, and presents a Python package for modeling spherical collapse.

Findings

Multiple halo boundaries characterize different regions of halo growth.

Theoretical models explain the existence of these boundaries.

The new boundaries improve understanding of halo growth and structure formation.

Abstract

We review the physics of halo collapse giving rise to various halo boundaries, as well as their identification, observation, and applications. The classical halo is typically defined as a monolithic, virialized object enclosed within its virial radius -- a definition which, however, does not account for ongoing halo growth. Continuous accretion causes the orbits of infalling particles to shrink over time, confining newly accreted material in a growing layer outside the virialized region. Several novel halo boundaries, such as the splashback and depletion radii, have recently been proposed to characterize this growth layer from different perspectives. Along with the turnaround radius, which operates on an even larger scale to enclose the entire infall region, these multiple boundaries comprise an extended view of a dark matter halo as a stratified structure. Theoretical models can largely explain the existence of various boundaries, while challenges remain in providing unified and quantitative predictions of their properties. The multiple boundaries open new avenues for observing halo growth and may substantially improve our understanding and modeling of cosmic structure formation. We provide a python package, SpheriC, implementing the key spherical collapse models.