Supermassive Black Holes and Precision Cosmology with Megamasers

In the gaseous and dusty accretion disks that surround supermassive black holes in the nuclei of nearby galaxies, water molecules emit maser radiation at a wavelength of 1.3 cm.  Applying the technique of Very Long Baseline Interferometry, we can map the distribution of individual masers clouds in these disks and determine their positions with an accuracy better than 0.01 milliarcseconds, and their line-of-sight velocities with an accuracy better than 1 km/s.  The masers thus provide a powerful tool for tracing the dynamics of the disk.  We use these data to measure precise masses of the central black holes, a measurement which has important applications in understanding how galaxies evolve.  We also use these observations to measure direct, geometric distances to the host galaxies.  By measuring galaxy distances in this manner, we aim to determine the expansion rate of the universe at the few-percent level.  This measurement will place a significant constraint on models of dark energy.