The Megamaser Cosmology Project. XIII. Combined Hubble constant constraints

Abstract

We present a measurement of the Hubble constant made using geometric distance measurements to megamaser-hosting galaxies. We have applied an improved approach for fitting maser data and obtained better distance estimates for four galaxies previously published by the Megamaser Cosmology Project: UGC 3789, NGC 6264, NGC 6323, and NGC 5765b. Combining these updated distance measurements with those for the maser galaxies CGCG 074-064 and NGC 4258, and assuming a fixed velocity uncertainty of 250 km s${}^{-1}$ associated with peculiar motions, we constrain the Hubble constant to be $H_0=73.9\pm 3.0$ km s${}^{-1}$ Mpc${}^{-1}$ independent of distance ladders and the cosmic microwave background. This best value relies solely on maser-based distance and velocity measurements, and it does not use any peculiar velocity corrections. Different approaches for correcting peculiar velocities do not modify $H_0$ by more than $\pm 1\ddot{I}ƒ$, with the full range of best-fit Hubble constant values spanning 71.8-76.9 km s${}^{-1}$ Mpc${}^{-1}$. We corroborate prior indications that the local value of $H_0$ exceeds the early-Universe value, with a confidence level varying from 95-99% for different treatments of the peculiar velocities.

13 pages, 1 figure, accepted for publication in ApJL