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

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 (SPH) 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 4-armed model with a pitch angle of approximately 20 degrees, 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.

The morphology of the Milky Way - I. Reconstructing CO maps from simulations in fixed potentials

We present an investigation into the morphological features of the Milky Way. We use smoothed particle hydrodynamics (SPH) to simulate the interstellar medium (ISM) in the Milky Way under the effect of a number of different gravitational potentials representing spiral arms and bars, assuming the Milky Way is grand design in nature. The gas is subject to ISM cooling and chemistry, enabling us to track the evolution of molecular gas. We use a 3D radiative transfer code to simulate the emission from the SPH output, allowing for the construction of synthetic longitude-velocity (l-v) emission maps as viewed from the Earth. By comparing these maps with the observed emission in CO from the Milky Way, we infer the arm/bar geometry that provides a best fit to our Galaxy. We find that it is possible to reproduce nearly all features of the l-v diagram in CO emission. There is no model, however, that satisfactorily reproduces all of the features simultaneously. Models with 2 arms cannot reproduce all the observed arm features, while 4 armed models produce too bright local emission in the inner Galaxy. Our best fitting models favour a bar pattern speed within 50-60km/s/kpc and an arm pattern speed of approximately 20km/s/kpc, with a bar orientation of approximately 45 degrees and arm pitch angle between 10-15 degrees.