Synthetic HI observations of a simulated spiral galaxy
  Using the Torus radiative transfer code we produce synthetic observations of
the 21 cm neutral hydrogen line from an SPH simulation of a spiral galaxy. The
SPH representation of the galaxy is mapped onto an AMR grid, and a ray tracing
method is used to calculate 21 cm line emission for lines of sight through the
AMR grid in different velocity channels and spatial pixels. The result is a
synthetic spectral cube which can be directly compared to real observations. We
compare our synthetic spectral cubes to observations of M31 and M33 and find
good agreement, whereby increasing velocity channels trace the main disc of the
galaxy. The synthetic data also show kinks in the velocity across the spiral
arms, evidence of non-circular velocities. These are still present even when we
blur our data to a similar resolution as the observations, but largely absent
in M31 and M33, indicating those galaxies do not contain significant spiral
shocks. Thus the detailed velocity structure of our maps better represent
previous observations of the grand design spiral M81.
