Weighing Supermassive Black Holes

Understanding the formation and evolution of galaxies is central to much of contemporary astrophysics. The black hole mass - galaxy mass relation and its derivatives imply a tight link between the growth of central supermassive black holes (SMBHs) and that of galaxies, and these relations now underlie a staggering number of observations and simulations. However, the number of reliable SMBH mass measurements is small, and the number of independent measuring methods even smaller. Prof Bureau and collaborators have recently shown that the dense molecular gas of galaxies is the best tracer of their circular velocity, and thus of their mass. Most importantly, as published in Nature and reported in early studies, they have now carried out the first ever (and most accurate) SMBH mass measurements using molecular gas.

The ease of these measurements and the abundance of molecular gas in galaxies mean that these measurements are easily scalable. The PhD project of Smith thus aims to exploit the method's advantages and use molecular gas to measure SMBH masses in a large sample of local galaxies spanning a range of morphological types, masses, and nuclear activities.

As part of the WISDOM team (mm-Wave Interferometric Survey of Dark Object Masses), Smith will use current mm/sub-mm telescopes to pursue a programme of SMBH mass measurements. This will primarily use ALMA, the largest ground-based telescope project in existence, on which we recently received a large allocation of time for such a purpose. With the revolutionary sensitivity and angular resolution of ALMA, Smith will probe the physical conditions and kinematics of the molecular gas in galaxies to the smallest spatial scales yet, well within the spheres of influence of the SMBHs. The SMBH mass measurements are then straightforward and robust, and the tools to model the velocity fields obtained have already been developed (e.g. KinMS). In parallel, the student will use lower resolution observations from ALMA and other telescopes (e.g. NOEMA) to survey nearby galaxies and identify promising targets for future measurements. The project will thus significantly increase the number of reliable SMBH masses available, and will revolutionise our understanding of the co-evolution of SMBHs and galaxies.

Finally, Smith will also explore ways in which the modelling tools may be improved, particularly with regard to modelling in the uv plane of interferometric observations, where the data points are independent.