Comparing Gaseous and Stellar Orbits in a Spiral Potential

TL;DR

This study uses MHD simulations to compare gaseous and stellar orbits in a spiral galaxy potential, revealing significant differences especially near resonances and highlighting the dynamic, non-steady nature of gas orbits.

Contribution

It provides the first detailed comparison between gas and stellar orbits in a spiral potential, challenging the assumption that gas follows stellar-like periodic orbits.

Findings

Gas orbits match stellar orbits far from resonances.

Gas exhibits nearly circular orbits near the 4:1 resonance.

Gas orbits do not necessarily close or settle into steady states.

Abstract

It is generally assumed that gas in a galactic disk follows closely non self-intersecting periodic stellar orbits. In order to test this common assumption, we have performed MHD simulations of a galactic-like disk under the influence of a spiral galactic potential. We also have calculated the actual orbit of a gas parcel and compared it to stable periodic stellar orbits in the same galactic potential and position. We found that the gaseous orbits approach periodic stellar orbits far from the major orbital resonances only. Gas orbits initialized at a given galactocentric distance but at different azimuths can be different, and scattering is conspicuous at certain galactocentric radii. Also, in contrast to the stellar behaviour, near the 4:1 (or higher order) resonance the gas follows nearly circular orbits, with much shorter radial excursions than the stars. Also, since the gas does not settle into a steady state, the gaseous orbits do not necessarily close on themselves.