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\nobservational studies hindered by our location within the Galactic disc. In the\nwork presented here we use smoothed particle hydrodynamics (SPH) and radiative\ntransfer to create synthetic longitude-velocity observations. Our aim is to\nreverse-engineer a top down map of the Galaxy by comparing synthetic\nlongitude-velocity maps to those observed. We set up a system of N-body\nparticles to represent the disc and bulge, allowing for dynamic creation of\nspiral features. Interstellar gas, and the molecular content, is evolved\nalongside the stellar system. A 3D-radiative transfer code is then used to\ncompare the models to observational data. The resulting models display arm\nfeatures that are a good reproduction of many of the observed emission\nstructures of the Milky Way. These arms however are dynamic and transient,\nallowing for a wide range of morphologies not possible with standard density\nwave theory. The best fitting models are a much better match than previous work\nusing fixed potentials. They favour a 4-armed model with a pitch angle of\napproximately 20 degrees, though with a pattern speed that decreases with\nincreasing Galactic radius. Inner bars are lacking however, which appear\nrequired to fully reproduce the central molecular zone.\n