2020ApJ...890..118P

As part of the survey component of the Megamaser Cosmology Project, we have discovered a disk megamaser system in the galaxy CGCG 074-064. Using the Green Bank Telescope and the Very Large Array, we have obtained spectral monitoring observations of this maser system at a monthly cadence over the course of two years. We find that the systemic maser features display line-of-sight accelerations of ∼4.4 km s^–1 yr^–1 that are nearly constant with velocity, while the high-velocity maser features show accelerations that are consistent with zero. We have also used the High-Sensitivity Array to make a high-sensitivity very long baseline interferometric map of the maser system in CGCG 074-064, which reveals that the masers reside in a thin, edge-on disk with a diameter of ∼1.5 mas (0.6 pc). Fitting a three-dimensional warped disk model to the data, we measure a black hole mass of 2.42_–0.20^+0.22×10⁷M_☉ and a geometric distance to the system of 87.6_–7.2^+7.9 Mpc. Assuming a cosmic microwave background-frame recession velocity of 7308 ± 150 km s^–1, we constrain the Hubble constant to H₀=81.0_–6.9^+7.4 (stat.) ± 1.4 (sys.) km s^–1 Mpc^–1.