The dynamics of spiral arms in pure stellar disks

TL;DR

This study demonstrates through high-resolution N-body simulations that pure stellar disks can sustain spiral arms for over 10 billion years without cooling mechanisms, due to a self-regulating process involving gravitational scattering and disk heating.

Contribution

It provides evidence that pure stellar disks can maintain long-lived spiral arms via a self-regulating mechanism, challenging previous assumptions about the necessity of dissipational effects.

Findings

Spiral arms can last over 10 Gyrs in high-resolution simulations.

A self-regulating mechanism maintains spiral amplitude and disk stability.

Lower particle numbers lead to faster spiral decay and disk heating.

Abstract

It has been believed that spirals in pure stellar disks, especially the ones spontaneously formed, decay in several galactic rotations due to the increase of stellar velocity dispersions. Therefore, some cooling mechanism, for example dissipational effects of the interstellar medium, was assumed to be necessary to keep the spiral arms. Here we show that stellar disks can maintain spiral features for several tens of rotations without the help of cooling, using a series of high-resolution three-dimensional $N$-body simulations of pure stellar disks. We found that if the number of particles is sufficiently large, e.g., $3\times 10^6$, multi-arm spirals developed in an isolated disk can survive for more than 10 Gyrs. We confirmed that there is a self-regulating mechanism that maintains the amplitude of the spiral arms. Spiral arms increase Toomre's $Q$ of the disk, and the heating rate correlates with the squared amplitude of the spirals. Since the amplitude itself is limited by the value of $Q$, this makes the dynamical heating less effective in the later phase of evolution. A simple analytical argument suggests that the heating is caused by gravitational scattering of stars by spiral arms, and that the self-regulating mechanism in pure-stellar disks can effectively maintain spiral arms on a cosmological timescale. In the case of a smaller number of particles, e.g., $3\times 10^5$, spiral arms grow faster in the beginning of the simulation (while $Q$ is small) and they cause a rapid increase of $Q$. As a result, the spiral arms become faint in several Gyrs.