The Dynamical Relationship Between the Bar and Spiral Patterns of NGC 1365

TL;DR

This study measures the pattern speeds of the bar and spiral arms in NGC 1365, providing evidence that they are dynamically distinct features with different rotation rates, challenging some existing theories.

Contribution

It offers the first detailed radial profile of pattern speeds in NGC 1365, testing theoretical predictions and revealing the bar and spiral as separate dynamical entities.

Findings

The bar rotates faster than the spiral pattern.

Evidence for mode coupling is unreliable and inconsistent.

The pattern speed profile suggests the bar and spiral are distinct features.

Abstract

Theories that attempt to explain the dynamical relationship between bar and spiral patterns in galactic disks make different predictions about the radial profile of the pattern speed. These are tested for the H-alpha bar and spiral patterns of NGC 1365. The radial profile of the pattern speed is measured by fitting mathematical models that are based on the Tremaine-Weinberg method. The results show convincing evidence for the bar rotating at a faster rate than the spiral pattern, inconsistent with a global wave mode or a manifold. There is evidence for mode coupling of the bar and spiral patterns at the overlap of corotation and inner Lindblad resonances, but the evidence is unreliable and inconsistent. The results are the most consistent with the bar and spiral patterns being dynamically distinct features. The pattern speed of the bar begins near an ILR and ends near the corotation resonance. The radial profile of the pattern speed beyond the bar most closely resembles what is expected for coupled spiral modes and tidal interactions.