Formation of disks with long-lived spiral arms from violent gravitational dynamics

TL;DR

This study uses dynamical experiments to explore how violent gravitational interactions lead to the formation of long-lived spiral arms in galactic disks, highlighting the roles of gas cooling and phase-space correlations.

Contribution

It demonstrates how gravitational collapse and gas dynamics produce persistent spiral arms with specific phase-space signatures, advancing understanding of galaxy formation.

Findings

Long-lived spiral arms form from gravitational collapse.

Gas cooling results in flatter, coherent disks.

Phase-space correlations indicate violent origins of arms.

Abstract

By means of simple dynamical experiments we study the combined effect of gravitational and gas dynamics in the evolution of an initially out-of-equilibrium, uniform and rotating massive over-density thought of as in isolation. The rapid variation of the system mean-field potential makes the point like particles (PPs), which interact only via Newtonian gravity, form a quasistationary thick disk dominated by rotational motions surrounded by far out-of-equilibrium spiral arms. On the other side, the gas component is subjected to compression shocks and radiative cooling so as to develop a much flatter disk, where rotational motions are coherent and the velocity dispersion is smaller than that of PPs. Around such gaseous disk long-lived, but nonstationary, spiral arms form: these are made of gaseous particles that move coherently because have acquired a specific phase-space correlation during the gravitational collapse phase. Such a phase-space correlation represents a signature of the violent origin of the arms and implies both the motion of matter and the transfer of energy. On larger scales, where the radial velocity component is significantly larger than the rotational one, the gas follows the same out-of-equilibrium spiral arms traced by PPs. We finally outline the astrophysical and cosmological implications of our results.