Instabilities of Spiral Shocks -- II. A quasi-steady State in the multi-phase inhomogeneous ISM

TL;DR

This paper uses advanced 3D hydrodynamic simulations to show that galactic shocks are inherently unstable but tend to settle into a quasi-steady, inhomogeneous spiral pattern with dynamic GMC-like structures.

Contribution

It advances previous models by incorporating self-gravity, radiative cooling, and star formation feedback, revealing the transient nature of galactic shocks and the formation of complex spiral structures.

Findings

Galactic shocks are unstable and transient.

Spiral patterns evolve into a quasi-steady, inhomogeneous state.

Supernova feedback influences vertical ISM structures but not the spiral pattern stability.

Abstract

The ``galactic shocks'' \citep{fujimoto68,roberts69} is investigated using a full three-dimensional hydrodynamic simulations, taking into account self-gravity of the ISM, radiative cooling, and star formation followed by energy feedback from supernovae. This is an essential progress from the previous numerical models, in which 2-D isothermal, non-self-gravitating gas is assumed. We find that the classic galactic shocks appears is unstable and transient, and it shifts to a globally quasi-steady, inhomogeneous pattern due to non-linear development of instabilities in the disk. The spiral patterns consists of many GMC-like dense condensations, but those local structures are not steady, and they evolves into irregular spurs in the inter-arm regions. Energy feedback from supernovae do not destroy the quasi-steady spiral arms, but it mainly contributes to vertical motion and structures of the ISM.