On the Relevance of the Tremaine-Weinberg Method Applied to an Hα Velocity Field: Pattern Speed Determination in M100 (NGC 4321)

The relevance of the Tremaine-Weinberg (TW) method is tested for measuring bar, spiral, and inner structure pattern speeds using a gaseous velocity field. The TW method is applied to various simulated barred galaxies in order to demonstrate its validity in seven different configurations, including star formation and/or dark matter halo. The reliability of the different physical processes involved and of the various observational parameters is also tested. The simulations show that the TW method could be applied to gaseous velocity fields to get a good estimate of the bar pattern speed, under the condition that regions of shocks are avoided and measurements are confined to regions where the gaseous bar is well formed. We successfully apply the TW method to the Hα velocity field of the Virgo Cluster galaxy M100 (NGC 4321) and derive pattern speeds of 55 ± 5 km s^-1 kpc^-1 for the nuclear structure, 30 ± 2 km s^-1 kpc^-1 for the bar, and 20 ± 1 km s^-1 kpc^-1 for the spiral pattern, in full agreement with published determinations using the same method or alternative ones.