2010MNRAS.407..405D

We have created synthetic neutral hydrogen (Hi) Galactic Plane Survey data cubes covering 90° ≤ ℓ ≤ 180°, using a model spiral galaxy from smoothed particle hydrodynamics (SPH) simulations and the radiative transfer code torus. The density, temperature and other physical parameters are fed from the SPH simulation into torus, where the HI emissivity and opacity are calculated before the 21-cm line emission profile is determined. Our main focus is the observation of outer Galaxy `Perseus arm' Hi, with a view to tracing atomic gas as it encounters shock motions as it enters a spiral arm interface, an early step in the formation of molecular clouds. The observation of HI self-absorption features at these shock sites (in both real observations and our synthetic data) allows us to investigate further the connection between cold atomic gas and the onset of molecular cloud formation.