The Megamaser Cosmology Project VIII. A Geometric Distance to NGC 5765b

TL;DR

This paper presents a new geometric distance to galaxy NGC 5765b using water megamaser observations, enabling a measurement of the Hubble Constant and insights into the galaxy's maser disk structure.

Contribution

First precise geometric distance measurement to NGC 5765b using VLBI and maser data, refining Hubble Constant estimates and disk properties.

Findings

Hubble Constant H0 = 66.0 ± 6.0 km/s/Mpc

Distance to NGC 5765b = 126.3 ± 11.6 Mpc

Detection of spiral density wave features in the maser disk

Abstract

As part of the Megamaser Cosmology Project (MCP), here we present a new geometric distance measurement to the megamaser galaxy NGC 5765b. Through a series of VLBI observations, we have confirmed the water masers trace a thin, sub-parsec Keplerian disk around the nucleus, implying an enclosed mass of 4.55 $\pm$ 0.40 $\times~10^{7}M_\odot$. Meanwhile, from single dish monitoring of the maser spectra over two years, we measured the secular drifts of maser features near the systemic velocity of the galaxy with rates between 0.5 and 1.2 km s$^{-1}$ yr$^{-1}$. Fitting a warped, thin disk model to these measurements, we determine a Hubble Constant $H_{0}$ of 66.0 $\pm$ 6.0 km s$^{-1}$ Mpc$^{-1}$ with the angular-diameter distance to NGC 5765b of 126.3 $\pm$ 11.6 Mpc. Apart from the distance measurement, we also investigate some physical properties related to the maser disk in NGC 5765b. The high-velocity features are spatially distributed into several clumps, which may indicate the existence of a spiral density wave associated with the accretion disk. For the red-shifted features, the envelope defined by the peak maser intensities increases with radius. The profile of the systemic masers in NGC 5765b is smooth and shows almost no structural changes over the two years of monitoring time, which differs from the more variable case of NGC 4258.