Radial distributions of arm-gas offsets as an observational test of spiral theories

TL;DR

This paper compares steady and dynamic spiral galaxy models by analyzing arm-gas offsets, proposing a new observational method to determine whether spiral arms are long-lived or transient.

Contribution

It introduces a radial profile method of arm-gas offsets to distinguish between steady and dynamic spiral models using hydrodynamic simulations.

Findings

Steady spiral models show radius-dependent arm-gas offsets.

Dynamic spiral models exhibit no systematic radial dependence.

The method can help identify the nature of spiral arms in observed galaxies.

Abstract

Theories of stellar spiral arms in disk galaxies can be grouped into two classes based on the longevity of a spiral arm. Although the quasi-stationary density wave theory supposes that spirals are rigidly-rotating, long-lived patterns, the dynamic spiral theory predicts that spirals are differentially-rotating, transient, recurrent patterns. In order to distinguish between the two spiral models from observations, we performed hydrodynamic simulations with steady and dynamic spiral models. Hydrodynamics simulations in steady spiral models demonstrated that the dust lane locations relative to the stellar spiral arms (hereafter, arm-gas offsets) depend on radius, regardless of the strength and pitch angle of the spiral and the model of the inter-stellar medium (ISM). In contrast, we found that the dynamic spiral models show no systematic radial dependence of the arm-gas offsets. The arm-gas offset radial profile method, together with the other test methods, will help us to distinguish between the two spiral models in observed spiral galaxies.