Observations of H 2 O masers from circumnuclear disks in active galaxies for the Megamaser Cosmology Project allow accurate measurement of the mass of supermassive black holes (BH) in these galaxies. We present the Very Long Baseline Interferometry (VLBI) images and kinematics of water maser emission in six active galaxies: NGC 1194, NGC 2273, NGC 2960 (Mrk 1419), NGC 4388, NGC 6264 and NGC 6323. We use the Keplerian rotation curves of these six megamaser galaxies, plus a seventh previously published, to determine accurate enclosed masses within the central ∼ 0.3 pc of these galaxies, smaller than the radius of the sphere of influence of the central mass in all cases. We also set lower limits to the central mass densities of between 0.12 and 60 ×10 10 M ⊙ pc −3 . For six of the seven disks, the high central densities rule out clusters of stars or stellar remnants as the central objects, and this result further supports our assumption that the enclosed mass can be attributed predominantly to a supermassive black hole. The seven BHs have masses ranging between 0.76 and 6.5×10 7 M ⊙ . The BH mass errors are ≈ 11%, dominated by the uncertainty of the Hubble constant. We compare the megamaser BH mass determination with other BH mass measurement techniques. The BH mass based on virial estimation in four galaxies is consistent with the megamaser BH mass given the latest empirical value of f , but the virial mass uncertainty is much greater. Circumnuclear megamaser disks allow the best mass determination of the central BH mass in external galaxies and significantly improve the observational basis at the low-mass end of the M − σ ⋆ relation. The M − σ ⋆ relation may not be a single, low-scatter power law as originally proposed. MCP observations continue and we expect to obtain more maser BH masses in the future.

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We present the first constraint on Faraday rotation measure (RM) at submillimeter wavelengths for the nucleus of M 87. By fitting the polarization position angles (χ) observed with the SMA at four independent frequencies around ∼230 GHz and interpreting the change in χ as a result of external Faraday rotation associated with accretion flow, we determine the rotation measure of the M 87 core to be between −7.5×10 5 and 3.4×10 5 rad/m 2 . Assuming a density profile of the accretion flow that follows a power-law distribution and a magnetic field that is ordered, radial, and has equipartition strength, the limit on the rotation measure constrains the mass accretion rateṀ to be below 9.2×10 −4 M ⊙ yr −1 at a distance of 21 Schwarzchild radii from the central black hole. This value is at least two orders of magnitude smaller than the Bondi accretion rate, suggesting significant suppression of the accretion rate in the inner region of the accretion flow. Consequently, our result disfavors the classical advection dominated accretion flow (ADAF) and prefers the adiabatic inflow-outflow solution (ADIOS) or convection-dominated accretion flow (CDAF) for the hot accretion flow in M 87.

In 2017 April, the Event Horizon Telescope (EHT) observed the near-horizon region around the supermassive black hole at the core of the M87 galaxy. These 1.3 mm wavelength observations revealed a compact asymmetric ring-like source morphology. This structure originates from synchrotron emission produced by relativistic plasma located in the immediate vicinity of the black hole. Here we present the corresponding linear-polarimetric EHT images of the center of M87. We find that only a part of the ring is significantly polarized. The resolved fractional linear polarization has a maximum located in the southwest part of the ring, where it rises to the level of ∼15%. The polarization position angles are arranged in a nearly azimuthal pattern. We perform quantitative measurements of relevant polarimetric properties of the compact emission and find evidence for the temporal evolution of the polarized source structure over one week of EHT observations. The details of the polarimetric data reduction and calibration methodology are provided. We carry out the data analysis using multiple independent imaging and modeling techniques, each of which is validated against a suite of synthetic data sets. The gross polarimetric structure and its apparent evolution with time are insensitive to the method used to reconstruct the image. These polarimetric images carry information about the structure of the magnetic fields responsible for the synchrotron emission. Their physical interpretation is discussed in an accompanying publication.

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As part of the Megamaser Cosmology Project, here we present a new geometric distance measurement to the megamaser galaxy NGC 5765b. Through a series of very long baseline interferometry observations, we have confirmed the water masers trace a thin, sub-parsec Keplerian disk around the nucleus, implying an enclosed mass of 4.55 ± 0.40 × 107 M ⊙. 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 ± 6.0 km s−1 Mpc−1 with an angular-diameter distance to NGC 5765b of 126.3 ± 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 redshifted 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.

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±9 km s −1 Mpc −1 , which corresponds to an angular-diameter distance of 144±19 Mpc. In addition, the fit also gives a mass of the central black hole of (3.09±0.42)×10 7 M ⊙ . The result demonstrates the feasibility of measuring distances to galaxies located well into the Hubble flow by using circumnuclear megamaser disks.

As part of the Megamaser Cosmology Project (MCP), we present VLBI maps of nuclear water masers toward five galaxies. The masers originate in sub-parsec circumnuclear disks. For three of the galaxies, we fit Keplerian rotation curves to estimate their supermassive black hole (SMBH) masses, and determine (2.9 ± 0.3) × 10 6 M for J0437+2456, (1.7 ± 0.1) × 10 7 M for ESO 558−G009, and (1.1 ± 0.2) × 10 7 M for NGC 5495. In the other two galaxies, Mrk 1029 and NGC 1320, the geometry and dynamics are more complicated and preclude robust black hole mass estimates. Including our new results, we compiled a list of 15 VLBIconfirmed disk maser galaxies with robust SMBH mass measurements. With this sample, we confirm the empirical relation of R out ∝ 0.3M SM BH reported in Wardle & Yusef-Zadeh (2012). We also find a tentative correlation between maser disk outer radii and WISE luminosity. We find no correlations of maser disk size with X-ray 2-10 keV luminosity or [O III] luminosity.-3imaged, they may become an important sub-sample to study the outflows and interactions of the central AGN with its environment.Here we present high-resolution VLBI images and rotation curves of 5 new maser galaxies: J0437+2456, ESO 558−G009, NGC 5495, Mrk 1029, and NGC 1320. The first three have spectral profiles indicative of "classical" Keplerian disk masers, while the last two have profiles suggesting more complicated nuclear structure (e.g. Pesce et al. 2015). We note that J0437+2456 and ESO 558−G009 are also current targets for distance measurements with more extensive MCP observations. Here we present preliminary VLBI maps and SMBH masses, based on fitting the rotation curve of high-velocity maser features. We describe our observations and data reduction in Section 2. Then Section 3 shows the VLBI images and the rotation curves, followed by our analysis of the SMBH masses on each source. Using these results, in Section 4, we define a sample of 15 "clean" disk masers and explore the relation between maser disk radius, SMBH mass and AGN luminosities. Then we give our conclusions in Section 5.