Simulations of spiral galaxies with an active potential: molecular cloud formation and gas dynamics

TL;DR

This paper presents simulations of gaseous discs responding to an active, time-evolving spiral potential, revealing differences from grand design spirals in gas dynamics, cloud formation, and spiral structure behavior.

Contribution

It introduces a novel simulation approach using an N-body derived potential to study gas response and molecular cloud formation in active, evolving spiral galaxies.

Findings

Gas forms asymmetric, long spiral arms similar to grand design galaxies.

Molecular cloud mass spectrum depends mainly on ambient pressure.

Largest molecular clouds form during spiral arm collisions.

Abstract

We describe simulations of the response of a gaseous disc to an active spiral potential. The potential is derived from an N-body calculation and leads to a multi-armed time-evolving pattern. The gas forms long spiral arms typical of grand design galaxies, although the spiral pattern is asymmetric. The primary difference from a grand-design spiral galaxy, which has a consistent 2/4-armed pattern, is that instead of passing through the spiral arms, gas generally falls into a developing potential minimum and is released only when the local minimum dissolves. In this case, the densest gas is coincident with the spiral potential, rather than offset as in the grand-design spirals. We would there fore expect no offset between the spiral shock and star formation, and no obvious co-rotation radius. Spurs which occur in grand-design spirals when large clumps are sheared off leaving the spiral arms, are rare in the active, time-evolving spiral reported here. Instead, large branches are formed from spiral arms when the underlying spiral potential is dissolving due to the N-body dynamics. We find that the molecular cloud mass spectrum for the active potential is similar to that for clouds in grand design calculations, depending primarily on the ambient pressure rather than the nature of the potential. The largest molecular clouds occur when spiral arms collide, rather than by agglomeration within a spiral arm.