Abstract
Water vapor masers emitting at a frequency of 22 GHz are often associated with active galactic nuclei (AGN), where they are called "megamasers" because of their large luminosities. Very long baseline interferometric (VLBI) observations of these megamasers reveal that they reside in a thin, edge-on accretion disk orbiting the supermassive black hole (SMBH) at sub-parsec radii. The research presented in this thesis has leveraged the unique geometry and simple dynamics of disk megamaser systems to provide powerful astrophysical tools for studying AGN, SMBHs, and cosmology. Using the large dataset of GBT megamaser spectra collected by the Megamaser Cosmology Project (MCP) we have investigated a mechanism for maser excitation, explored the prospects of disk reverberation, discovered an instance of interstellar scintillation, and placed limits on the presence of disk magnetization. We have presented 321 GHz ALMA observations of several AGNs, detecting for the first time H2O megamaser emission at this frequency towards NGC 4945. We have also introduced the idea of using H2O accretion disk megamasers as dynamical tracers for measuring SMBH peculiar motion, and we have measured the galaxy recession velocities for a sample of 10 maser disk systems using a combination of spatially resolved neutral hydrogen (HI) disk modeling, spatially integrated HI profile fitting, and optical spectral line and continuum fitting. Our technique achieves a typical precision of ~10 km/s in the SMBH peculiar velocity measurement. As part of the MCP, we have conducted spectral monitoring and VLBI mapping observations of the megamaser disk galaxy CGCG 074-064 to measure its distance. In our preliminary fitting of a three-dimensional warped-disk model to the data, we measure a SMBH mass of 22.8 (+2.0,-1.8) million solar masses and a geometric distance to the system of 82.98 (+7.33,-6.48) Mpc. From the results of the disk modeling, we constrain the Hubble constant to be H0 = 83.91 (+7.83,-7.45) km/s/Mpc.
