# The splashback radius of halos from particle dynamics. I. The SPARTA algorithm

## TL;DR

This paper introduces SPARTA, a parallel code that analyzes particle orbits in cosmological simulations to accurately determine the splashback radius of dark matter halos, serving as a physical boundary.

## Contribution

The paper presents a novel algorithm and a parallel code for reliably measuring the splashback radius from particle trajectories in large cosmological simulations.

## Key findings

- Reliable measurement of splashback radii for 95% of halos above resolution limit
- Splashback radius and mass converge to better than 5% accuracy
- Algorithm accurately identifies first apocenters of particles after infall

## Abstract

Motivated by the recent proposal of the splashback radius as a physical boundary of dark matter halos, we present a parallel computer code for Subhalo and PARticle Trajectory Analysis (SPARTA). The code analyzes the orbits of all simulation particles in all host halos, billions of orbits in the case of typical cosmological N-body simulations. Within this general framework, we develop an algorithm that accurately extracts the location of the first apocenter of particles after infall into a halo, or splashback. We define the splashback radius of a halo as the smoothed average of the apocenter radii of individual particles. This definition allows us to reliably measure the splashback radii of 95% of host halos above a resolution limit of 1000 particles. We show that, on average, the splashback radius and mass are converged to better than 5% accuracy with respect to mass resolution, snapshot spacing, and all free parameters of the method.