The Rockstar Phase-Space Temporal Halo Finder and the Velocity Offsets of Cluster Cores

TL;DR

This paper introduces Rockstar, a new adaptive phase-space halo finder that improves substructure detection in large simulations and reveals significant velocity offsets in dark matter halo cores across various masses and redshifts.

Contribution

We present Rockstar, a novel, highly efficient phase-space halo finder that enhances substructure recovery and demonstrates velocity offsets in dark matter halo cores.

Findings

Rockstar achieves superior substructure detection compared to other halo finders.

Dark matter halo cores exhibit significant velocity offsets from their bulk velocities.

Offsets can reach up to 350 km/s in massive clusters at z=0.

Abstract

We present a new algorithm for identifying dark matter halos, substructure, and tidal features. The approach is based on adaptive hierarchical refinement of friends-of-friends groups in six phase-space dimensions and one time dimension, which allows for robust (grid-independent, shape-independent, and noise-resilient) tracking of substructure; as such, it is named Rockstar (Robust Overdensity Calculation using K-Space Topologically Adaptive Refinement). Our method is massively parallel (up to 10^5 CPUs) and runs on the largest current simulations (>10^10 particles) with high efficiency (10 CPU hours and 60 gigabytes of memory required per billion particles analyzed). A previous paper (Knebe et al 2011) has shown Rockstar to have class-leading recovery of halo properties; we expand on these comparisons with more tests and higher-resolution simulations. We show a significant improvement in substructure recovery as compared to several other halo finders and discuss the theoretical and practical limits of simulations in this regard. Finally, we present results which demonstrate conclusively that dark matter halo cores are not at rest relative to the halo bulk or satellite average velocities and have coherent velocity offsets across a wide range of halo masses and redshifts. For massive clusters, these offsets can be up to 350 km/s at z=0 and even higher at high redshifts. Our implementation is publicly available at http://code.google.com/p/rockstar .