  Pattern speeds are a fundamental parameter of the dynamical features (e.g.
bars, spiral arms) of a galaxy, setting resonance locations. Pattern speeds are
not directly observable, so the Tremaine-Weinberg (TW) method has become the
most common method used to measure them in galaxies. However, it has not been
tested properly whether this method can straightforwardly be applied to gas
tracers, despite this being widely done in the literature. When applied to
observations, the TW method may return invalid results, which are difficult to
diagnose due to a lack of ground truth for comparison. Although some works
applying the TW method to simulated galaxies exist, only stellar populations
have been tested. Therefore, here we explore the applicability of the TW method
for gas gracers, by applying it to hydrodynamical simulations of galaxies,
where we know the true value of the bar pattern speed. We perform some simple
tests to see if the TW method has a physically reasonable output. We add
different kinds of uncertainties (e.g. in position angle or flux) to the data
to mock observational errors based on the magnitude of uncertainty present in
the observations. Second, we test the method on 3D simulations with chemical
networks. We show that in general, applying TW to observations of gas will not
recover the true pattern speed. These results have implications for many
"pattern speeds" reported in the literature, and based on these tests we also
give some best practices for measuring pattern speeds using gas tracers going
forwards.
