An Improved Distance to NGC 4258 and Its Implications for the Hubble Constant

NGC 4258 is a critical galaxy for establishing the extragalactic distance scale and estimating the Hubble constant (${H}_{0}$). Water masers in the nucleus of the galaxy orbit about its supermassive black hole, and very long baseline interferometric observations of their positions, velocities, and accelerations can be modeled to give a geometric estimate of the angular-diameter distance to the galaxy. We have improved the technique to obtain model parameter values, reducing both statistical and systematic uncertainties compared to previous analyses. We find the distance to NGC 4258 to be 7.576 ± 0.082 (stat.) ± 0.076 (sys.) Mpc. Using this as the sole source of calibration of the Cepheid-SN Ia distance ladder results in ${H}_{0}=72.0\pm 1.9$ km s−1 Mpc−1, and in concert with geometric distances from Milky Way parallaxes and detached eclipsing binaries in the LMC we find ${H}_{0}=73.5\,\pm 1.4$ km s−1 Mpc−1. The improved distance to NGC 4258 also provides a new calibration of the tip of the red giant branch of ${M}_{F814W}=-4.01\pm 0.04$ mag, with reduced systematic errors for the determination of ${H}_{0}$ compared to the LMC-based calibration, because it is measured on the same Hubble Space Telescope (HST) photometric system and through similarly low extinction as SN Ia host halos. The result is ${H}_{0}=71.1\pm 1.9$ km s−1 Mpc−1, in good agreement with the result from the Cepheid route, and there is no difference in ${H}_{0}$ when using the same calibration from NGC 4258 and the same SN Ia Hubble diagram intercept to start and end both distance ladders.