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.
