Applying the Tremaine-Weinberg Method to Nearby Galaxies: Stellar Mass-Based Pattern Speeds, and Comparisons with ISM Kinematics

Abstract

We apply the Tremaine-Weinberg method to 19 nearby galaxies using stellar mass surface densities and velocities derived from the PHANGS-MUSE survey, to calculate (primarily bar) pattern speeds (\(\Omega_{\rm P}\)). After quality checks, we find that around half (10) of these stellar mass-based measurements are reliable. For those galaxies, we find good agreement between our results and previously published pattern speeds, and use rotation curves to calculate major resonance locations (co-rotation radii and Lindblad resonances). We also compare these stellar-mass derived pattern speeds with H\(\alpha\) (from MUSE) and CO(\(J=2{-}1\)) emission from the PHANGS-ALMA survey. We find that in the case of these clumpy ISM tracers, this method erroneously gives a signal that is simply the angular frequency at a representative radius set by the distribution of these clumps (\(\Omega_{\rm clump}\)), and that this \(\Omega_{\rm clump}\) is significantly different to \(\Omega_{\rm P}\) (\(\sim\)20% in the case of H\(\alpha\), and \(\sim\)50% in the case of CO). Thus, we conclude that it is inadvisable to use "pattern speeds" derived from ISM kinematics. Finally, we compare our derived pattern speeds and co-rotation radii, along with bar properties, to the global parameters of these galaxies. Consistent with previous studies, we find that galaxies with a later Hubble type have a larger ratio of co-rotation radius to bar length, more molecular-gas rich galaxies have higher \(\Omega_{\rm P}\), and more bulge-dominated galaxies have lower \(\Omega_{\rm P}\). Unlike earlier works, however, there are no clear trends between the bar strength and \(\Omega_{\rm P}\), nor between the total stellar mass surface density and the pattern speed.