Formation of spiral structure from the violent relaxation of self-gravitating disks

TL;DR

This study numerically investigates how spiral structures form rapidly in self-gravitating disks through violent relaxation, revealing dependencies on initial conditions like angular speed, shape, and particle number.

Contribution

It demonstrates that spiral arms can form quickly via violent relaxation and details how initial disk properties influence their number, shape, and winding, providing new insights into galaxy formation processes.

Findings

Spiral arms form within a few free-fall times.

Initial angular speed and disk shape affect arm properties.

Particle number influences the number of arms in circular disks.

Abstract

We present the numerical study of the formation of spiral structure in the context of violent relaxation. Initial conditions are the out-of-equilibrium disks of self-gravitating particles in rigid rotation. By that mechanism, robust and non-stationary spiral arms can be formed within a few free-fall times by the shearing of the mass ejection following the collapse. With a closer look, we find different properties of the arms in connection with the initial configuration. The winding degree tends to increase with initial angular speed provided that a disk is thin. If disk surface is circular, both number and position of arms are governed by the Poissonian density fluctuations that produce more arms as more particles are introduced. On the contrary, if the surface ellipticity is imposed, the number of arms and their placement are effectively controlled. Otherwise, the increase of thickness leads to a complicated outcome since the number of arms and winding degree are less effectively controlled. We speculate that this complexity is caused by a strong non-axisymmetric field during the violent relaxation that disorganizes the pre-collapse motion and the concentration of particles.