The morphology of the Milky Way - II. Reconstructing CO maps from disc galaxies with live stellar distributions

Abstract

The arm structure of the Milky Way remains somewhat of an unknown, with observational studies hindered by our location within the Galactic disc. In the work presented here, we use smoothed particle hydrodynamics and radiative transfer to create synthetic longitude–velocity observations. Our aim is to reverse engineer a top-down map of the Galaxy by comparing synthetic longitude–velocity maps to those observed. We set up a system of N-body particles to represent the disc and bulge, allowing for dynamic creation of spiral features. Interstellar gas, and the molecular content, is evolved alongside the stellar system. A 3D-radiative transfer code is then used to compare the models to observational data. The resulting models display arm features that are a good reproduction of many of the observed emission structures of the Milky Way. These arms however are dynamic and transient, allowing for a wide range of morphologies not possible with standard density wave theory. The best-fitting models are a much better match than previous work using fixed potentials. They favour a four-armed model with a pitch angle of approximately 20°, though with a pattern speed that decreases with increasing Galactic radius. Inner bars are lacking, however, which appear required to fully reproduce the central molecular zone.