Discriminating among theories of spiral structure using Gaia DR2

TL;DR

This study uses Gaia DR2 data to compare star distributions with various spiral galaxy theories, finding evidence supporting the transient spiral mode model and suggesting that observed spirals result from superimposed modes.

Contribution

It introduces a method of analyzing Gaia data in action-angle variables to distinguish between different spiral structure theories in disc galaxies.

Findings

Gaia data shows features consistent with transient spiral mode predictions.

Reprojection into action-angle variables enhances the identification of resonant scattering.

Superposition of spiral modes explains the observed spiral structures.

Abstract

We compare the distribution in position and velocity of nearby stars from the Gaia DR2 radial velocity sample with predictions of current theories for spirals in disc galaxies. Although the rich substructure in velocity space contains the same information, we find it more revealing to reproject the data into action-angle variables, and we describe why resonant scattering would be more readily identifiable in these variables. We compute the predicted changes to the phase space density, in multiple different projections, that would be caused by a simplified isolated spiral pattern, finding widely differing predictions from each theory. We conclude that the phase space structure present in the Gaia data shares many of the qualitative features expected in the transient spiral mode model. We argue that the popular picture of apparently swing-amplified spirals results from the superposition of a few underlying spiral modes.