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 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±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 ±1σ, 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.

Based on measurements with the Effelsberg 100-m telescope, a multi-line study of molecular species is presented toward the gravitational lens system PKS 1830-211, which is by far the best known target to study dense cool gas in absorption at intermediate redshift.Determining average radial velocities and performing Large Velocity Gradient radiative transfer calculations, the aims of this study are (1) to determine the density of the gas, (2) to constrain the temperature of the cosmic microwave background (CMB), and (3) to evaluate the proton-to-electron mass ratio at redshift z ∼ 0.89. Analyzing data from six rotational HC 3 N transitions (this includes the J = 7 ← 6 line, which is likely detected for the first time in the interstellar medium) we obtain n(H 2 ) ∼ 2600 cm −3 for the gas density of the south-western absorption component, assuming a background source covering factor, which is independent of frequency. With a possibly more realistic frequency dependence proportional to ν 0.5 (the maximal exponent permitted by observational boundary conditions), n(H 2 ) ∼ 1700 cm −3 . Again toward the south-western source, excitation temperatures of molecular species with optically thin lines and higher rotational constants are, on average, consistent with the expected temperature of the cosmic microwave background, T CMB = 5.14 K. However, individually, there is a surprisingly large scatter which far surpasses expected uncertainties. A comparison of CS J = 1 ← 0 and 4 ← 3 optical depths toward the weaker north-western absorption component results in T ex = 11 K and a 1-σ error of 3 K. For the main component, a comparison of velocities determined from ten optically thin NH 3 inversion lines with those from five optically thin rotational transitions of HC 3 N, observed at similar frequencies, constrains potential variations of the proton-to-electron mass ratio μ to Δμ/μ < 1.4 × 10 −6 with 3-σ confidence. Also including optically thin rotational lines from other molecular species, it is emphasized that systematic errors are ΔV < 1 km s −1 , corresponding to Δμ/μ < 1.0 × 10 −6 .

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A survey for megamaser emission from 354 active galaxies has resulted in the detection of 10 H 2 O new sources, making 16 known altogether. The galaxies surveyed include a distance-limited sample (covering Seyferts and LINERs with recession velocities less than 7000 km s~1) and a magnitude-limited sample (covering Seyferts and LINERs withIn order to determine whether the m B ¹ 14.5). H 2 O-detected galaxies are "" typical ÏÏ active galactic nuclei (AGNs) or have special properties that facilitate the production of powerful masers, we have accumulated a database of physical, morphological, and spectroscopic properties of the observed galaxies. The most signiÐcant Ðnding is that megamasers are H 2 O detected only in Seyfert 2 and LINER galaxies, not Seyfert 1Ïs. This lack of detection in Seyfert 1Ïs indicates either that they do not have molecular gas in their nuclei with physical conditions appropriate to produce 1.3 cm masers or that the masers are beamed away from Earth, presumably in the plane H 2 O of the putative molecular torus that hides the Seyfert 1 nucleus in Seyfert 2Ïs. LINERs are detected at a similar rate to Seyfert 2Ïs, which constitutes a strong argument that at least some nuclear LINERs are AGNs rather than starbursts, since starbursts have not been detected as megamasers. We prefer-H 2 O entially detect emission from the nearer galaxies and from those that are apparently brighter at H 2 O mid-and far-infrared and centimeter radio wavelengths. There is also a possible trend for the H 2 Ogalaxies to be more intrinsically luminous in nuclear 6 cm radio emission than the undetected detected ones, though these data are incomplete. We Ðnd evidence that Seyfert 2Ïs with very high (N H [ 1024 cm~2) X-rayÈabsorbing columns of gas are more often detected as maser emitters than Seyfert 2Ïs H 2 O with lower columns. It may be that the probability of detecting maser emission in Seyfert galaxies H 2 O increases with increasing column of cool gas to the nucleus, from Seyfert 1Ïs through narrow-line X-ray galaxies to Seyfert 2Ïs.

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The Megamaser Cosmology Project (MCP) seeks to measure the Hubble Constant (H 0 ) in order to improve the extragalactic distance scale and constrain the nature of dark energy. We are searching for sources of H 2 O maser emission from active galactic nuclei with sub-pc accretion disks, as in NGC 4258, and following up these discoveries with very long baseline interferometric (VLBI) imaging and spectral monitoring. Here we present a VLBI map of the H 2 O masers toward UGC 3789, a galaxy well into the Hubble Flow. We have observed masers moving at rotational speeds up to 800 km s −1 at radii as small as 0.08 pc. Our map reveals masers in a nearly edge-on disk in Keplerian rotation about a 10 7 M supermassive black hole. When combined with centripetal accelerations, obtained by observing spectral drifts of maser features (to be presented in Paper II), the UGC 3789 masers may provide an accurate determination of H 0 , independent of luminosities and metallicity and extinction corrections.