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 can not 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 50-60 km s ^ { -1 } kpc ^ { -1 } and an arm pattern speed of approximately 20 km s ^ { -1 } kpc ^ { -1 } , with a bar orientation of approximately 45 ^ { \circ } and arm pitch angle between 10 ^ { \circ } -15 ^ { \circ } .