Resonance Related Spiral Substructure in a Galactic Gaseous Disk

TL;DR

This study uses high-resolution hydrodynamic simulations to explore how spiral substructures like spurs and shocks form in galactic gaseous disks due to resonances and spiral potentials, revealing complex gas responses.

Contribution

It demonstrates the role of ultraharmonic resonances and spiral potential strength in generating spiral substructures in galactic gas disks, using detailed 2D simulations.

Findings

Spurs form after several disk revolutions as spiral potential increases.

Shocks bifurcate and overlap with spurs, creating complex structures.

A smooth background potential can produce highly complex gaseous responses.

Abstract

We use high resolution 2D hydrodynamic simulations to study the formation of spiral substructure in the gaseous disk of a galaxy. The obtained gaseous response is driven by a self-consistent non-axisymmetric potential obtained from an imposed spiral mass distribution. We highlight the importance of ultraharmonic resonances in generating these features. The temporal evolution of the system is followed with the parallel ZEUS-MP code, and we follow the steepening of perturbations induced by the spiral potential until large-scale shocks emerge. These shocks exhibit bifurcations that protrude from the gaseous arms and continue to steepen until new shocks are formed. When the contribution from the spiral potential relative to the axisymmetric background is increased from our default value, spurs protrude from the main arms after several revolutions of the gaseous disk. Such spurs overlap on top of the aforementioned shocks. These results support the hypothesis that a complicated gaseous response can coexist with an orderly spiral potential term, in the sense that the underlying background potential can be smooth yet drive a gaseous response that is far more spatially complex.