Measuring \(H_0\) with H \(_2\) O Megamasers

Observations of 22 GHz circumnuclear water vapor megamasers in the accretion disks of low-z active galactic nuclei contribute to the measurement of the Hubble Constant \(H_0\) in two ways. First, the maser disk in the nucleus of the nearby Seyfert 2 galaxy NGC 4258 determines its distance to 1.5% and is used to calibrate both the classical Cepheid period-luminosity relationship as well as the tip of the red giant branch. The maser distance thereby provides an anchor for the extragalactic distance ladders that connect to type Ia supernovae in the Hubble Flow. Second, water megamasers determine distances to galaxies directly in the Hubble Flow and thus provide a one-step geometric measurement of \(H_0\) independent of standard candles and distance ladders. The megamasers used for distance measurements originate in thin, pc-scale, edge-on accretion disks. To determine a galaxy’s distance, the spatial distribution of the individual maser clouds within a disk must be determined using Very Long Baseline Interferometry (VLBI) observations. The VLBI map is combined then with measurements of the line-of-sight accelerations of individual maser components, derived from the secular velocity drifts observed by monitoring its spectral profile. A thin, warped accretion disk model is fit to the observed positions and line-of-sight velocities and accelerations of each maser spot. The global parameters for the model include the mass of the central supermassive black hole as well as both the distance to the galaxy and its recession velocity, thus determining the Hubble Constant. The best current direct measurement using the megamaser technique is H \(_0\) = 73.9 ± 3.0 km s \(^{-1}\) Mpc \(^{-1}\) , based on distance measurements to 6 megamaser galaxies and using a conservative treatment of peculiar velocities with no corrections for galaxy flow models.