The Radial Orbit Instability in Collisionless N-Body Simulations

TL;DR

This study systematically investigates the radial orbit instability in collisionless N-body simulations, identifying key initial conditions that influence its onset and development, and highlighting the role of self-reinforcing torques.

Contribution

It provides a comprehensive analysis of how initial velocity anisotropy, virial ratio, and density profile shape affect ROI in collisionless systems, advancing understanding of its dynamics.

Findings

ROI begins after systems reach their most compact state

Fastest evolution occurs with radially anisotropic orbits and few isotropic cores

Self-reinforcing torques are crucial to ROI onset

Abstract

Using a suite of self-gravitating, collisionless N-body models, we systematically explore a parameter space relevant to the onset and behavior of the radial orbit instability (ROI), whose strength is measured by the systemic axis ratios of the models. We show that a combination of two initial conditions, namely the velocity anisotropy and the virial ratio, determines whether a system will undergo ROI and exactly how triaxial the system will become. A third initial condition, the radial shape of the density profile, plays a smaller, but noticeable role. Regarding the dynamical development of the ROI, the instability a) begins after systems collapse to their most compact configuration and b) evolves fastest when a majority of the particles have radially anisotropic orbits while there is a lack of centrally-concentrated isotropic orbits. We argue that this is further evidence that self-reinforcing torques are the key to the onset of the ROI. Our findings support the idea that a separate orbit instability plays a role in halting the ROI.