The Megamaser Cosmology Project. V. An Angular Diameter Distance to NGC 6264 at 140 Mpc

Abstract: We present the direct measurement of the Hubble constant, yielding the direct measurement of the angular-diameter distance to NGC 6264 using the H${2}$O megamaser technique. Our measurement is based on sensitive observations of the circumnuclear megamaser disk from four observations with the Very Long Baseline Array, the Green Bank Telescope and the Effelsberg Telescope. We also monitored the maser spectral profile for 2.3 years using the Green Bank Telescope to measure accelerations of maser lines by tracking their line-of-sight velocities as they change with time. The measured accelerations suggest that the systemic maser spots have a significantly wider radial distribution than in the archetypal megamaser in NGC 4258. We model the maser emission as arising from a circumnuclear disk with orbits dominated by the central black hole. The best fit of the data gives a Hubble constant of $H{0} =$ 68$\pm$9 km s$^{-1}$ Mpc$^{-1}$, which corresponds to an angular-diameter distance of 144$\pm$19 Mpc. In addition, the fit also gives a mass of the central black hole of (3.09$\pm$0.42)$\times10^{7}$ $M_{\odot}$. The result demonstrates the feasibility of measuring distances to galaxies located well into the Hubble flow by using circumnuclear megamaser disks.