Measuring Supermassive Black Hole Masses with H₂O Megamasers: Observations, Methods, and Implications for Black Hole Demographics

Measuring supermassive black hole (SMBH) masses is fundamental to understanding active galactic nuclei (AGN) and their coevolution with host galaxies. Among existing techniques, H2O megamaser observations with Very Long Baseline Interferometry (VLBI) provide the most direct and geometric determinations of SMBH masses by tracing molecular gas in sub-parsec Keplerian disks. Over the past two decades, the Megamaser Cosmology Project (MCP) has surveyed thousands of nearby AGNs and obtained high-sensitivity VLBI maps of dozens of maser disks that lead to accurate SMBH masses with uncertainties typically below 10%. In this paper, we present a comprehensive review that summarizes the essential elements required to obtain accurate black hole masses with the H2O megamaser technique — including the physical conditions for maser excitation, observational requirements, disk modeling, and sources of SMBH mass uncertainty — and we discuss the implications of maser-based measurements for exploring SMBH demographics. In particular, we will show that maser-derived black hole masses, largely free from the systematic biases of stellar or gas-dynamical methods, provide critical anchors at the low-mass end of the SMBH population (MBH∼107M⊙) and reveal possible deviations from the canonical MBH–σ* relation. With forthcoming spectroscopic surveys and advances in millimeter/submillimeter VLBI, the maser technique promises to extend precise dynamical mass measurements to both larger local samples and high-redshift galaxies.