At the center of every massive galaxy is a supermassive black hole (SMBH). The masses of these objects are connected to other properties of the host galaxy through galaxy scaling relationships. Two methods for obtaining the supermassive black hole masses of galaxies include reverberation mapping and stellar dynamical modeling.
Reverberation mapping is a technique that can only be applied to active galactic nuclei (AGN), where matter is accreting onto the SMBH. In reverberation mapping, variance in continuum emission of the central accretion disk is compared to narrow line variation of the broad line clouds in order to determine a time delay corresponding to the radius of the broad line region. Both these light curves can be created through spectroscopy, while photometry can be used to aid the former. Reverberation mapping requires intensive time commitments, with observations needed almost each day for a period of months if not longer.
Stellar dynamical modeling is the simulation of stellar orbits in a gravitational potential. In order to match the observed kinematics and surface brightness distribution of a system, grids of models are run, varying SMBH mass and mass-to-light ratio until a fit of the true observations and simulations are found. Stellar dynamical modeling can be applied to quiescent galaxies and not just active ones, but requires that the black hole sphere of influence be resolved and so the target galaxy must be relatively nearby.
Ideally, reverberation mapping and stellar dynamical modeling can be applied to the same target to check the internal consistencies of these methods. My work at Georgia State has looked at these two methods and the comparison of them for the individual target NGC 4151.