Testing Density Wave Theory with Resolved Stellar Populations around Spiral Arms in M81

TL;DR

This study investigates the presence of age gradients across spiral arms in M81 to test density wave theory, using resolved stellar populations and star formation histories derived from HST data, finding no evidence supporting stationary density waves.

Contribution

It provides the first detailed star formation history analysis across M81's spiral arms, challenging the stationary density wave model and supporting a kinematic spiral pattern driven by tidal interactions.

Findings

No age gradients detected across the spiral arms.

Results do not support stationary density wave theory.

Suggests spiral arms are driven by tidal interactions.

Abstract

Stationary density waves rotating at a constant pattern speed $\Omega_{\rm P}$ would produce age gradients across spiral arms. We test whether such age gradients are present in M81 by deriving the recent star formation histories (SFHs) of 20 regions around one of M81's grand-design spiral arms. For each region, we use resolved stellar populations to determine the SFH by modeling the observed color-magnitude diagram (CMD) constructed from archival Hubble Space Telescope (HST) F435W and F606W imaging. Although we should be able to detect systematic time delays in our spatially-resolved SFHs, we find no evidence of star formation propagation across the spiral arm. Our data therefore provide no convincing evidence for a stationary density wave with a single pattern speed in M81, and instead favor the scenario of kinematic spiral patterns that are likely driven by tidal interactions with the companion galaxies M82 and NGC 3077.