Relaxation in N-body simulations of disk galaxies

TL;DR

This paper demonstrates that N-body simulations of disk galaxies experience higher-than-expected collisional relaxation, significantly influencing disk thickness and velocity distribution, which impacts the interpretation of galaxy evolution models.

Contribution

It reveals that relaxation effects are more prominent in galaxy simulations than previously thought, especially affecting vertical structure and velocity ellipsoids.

Findings

Relaxation impacts vertical disk thickness.

Velocity ellipsoid becomes moderately flattened.

Higher particle numbers reduce relaxation effects.

Abstract

I use N-body simulations with two mass species of particles to demonstrate that disk galaxy simulations are subject to collisional relaxation at a higher rate than is widely assumed. Relaxation affects the vertical thickness of the disk most strongly, and drives the velocity ellipsoid to a moderately flattened shape similar to that observed for disk stars in the solar neighborhood. The velocity ellipsoid in simulations with small numbers of particles quickly approaches this shape, but shot noise also dominates the in-plane behavior. Simulations with higher, but reachable, numbers of particles relax slowly enough to be considered collisionless, allowing the in-plane dispersions to rise due to spiral activity without heating the vertical motions. Relaxation may have affected many previously published simulations of the formation and evolution of galaxy disks.