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Using the  torus  radiative transfer code, we produce synthetic observations of the 21‐cm neutral hydrogen line from a smoothed particle hydrodynamics (SPH) simulation of a spiral galaxy. The SPH representation of the galaxy is mapped on to an adaptive mesh refinement (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 to the observations, but largely absent in M31 and M33, indicating that these galaxies do not contain significant spiral shocks. Thus, the detailed velocity structure of our maps better represents previous observations of the grand design spiral M81.

Synthetic H i observations of a simulated spiral galaxy