Weighing supermassive black holes with molecular gas kinematics

Supermassive black hole (SMBH) masses correlate tightly with various properties of their host galaxies, implying close co-evolution. However, the mass measurements underpinning this conclusion have large uncertainties and are limited by selection biases. With the advent of modern millimetre/sub-millimetre interferometers, molecular gas dynamics are now suitable tracers of the SMBH-dominated regions of galaxy potentials. In this thesis, I present new mass measurements for the SMBHs in the nuclei of the local elliptical galaxies NGC 524 and NGC 7052. These measurements exploit exceptionally high angular resolution observations of carbon monoxide (CO) J=2−1 emission using the Atacama Large Millimeter/sub-millimeter Array (ALMA). I refine earlier dynamical modelling methods, allowing for non-axisymmetric gas distributions and accounting for previously underestimated formal uncertainties when fitting models with very large numbers of constraints. The measurement in NGC 524 is consistent with an earlier stellar dynamical measurement, while that in NGC 7052 shows that an earlier gas dynamical measurement underestimated the SMBH mass (possibly by neglecting pressure support). The dynamical models of these galaxies also reveal a small inflow/warp in NGC 524, and that the central hole in NGC 7052 is consistent with the region within which tidal forces disrupt molecular clouds. Using spatially-integrated CO spectra from both single-dish telescopes and interferometers I further show a correlation exists between large-scale CO emission line widths and SMBH masses. This new correlation is no less tight than most, allowing it to be used to estimate SMBH masses in the absence of the (often harder to obtain) data required to exploit tighter correlations. Finally, I discuss the dynamical features revealed in new ALMA observations of 11 other SMBH mass measurement candidate galaxies, and estimate the SMBH masses in the three most promising. This thesis demonstrates the potential of molecular gas dynamics to advance our understanding of SMBH–host coevolution, and indicates the next steps in understanding the molecular contribution to AGN fuelling and feedback.