H2O megamaser emission from sub-parsec circumnuclear disks at the center of active galaxies allows a single-step, direct distance measurements to galaxies in the Hubble flow without any external calibration. Based on accurate distance determinations of six maser galaxies within 150 Mpc, the Megamaser Cosmology Project (MCP) team recently obtained H0 = 73.9± 3.0 km/s/Mpc (‘∼ 4% accuracy), independent of distance ladders and the cosmic microwave background. To further applying the megamaser technique to attain a 1% Ho measurement, detecting more high-quality disk maser systems is crucial. In this conference proceeding, we update the status of the MCP and discuss strategies of detecting additional high-quality disk maser galaxies within z ∼ 0.1. In addition, we show the prospects of reaching a 1% Ho measurement with the supreme sensitivity of the ngVLA. Finally, we demonstrate that applying the maser technique to distance measurements of high-z galaxies with future submm VLBI systems is promising and this will allow for investigation of the new tension between the ΛCDM model and the high-z Hubble diagram.

Water vapor maser emission from a Seyfert 2 galaxy IC2560 was observed with sensitive VLBI. The accurate black hole mass was determined by separating the mass of the disk surrounding the black hole. In addition, the distance of the galaxy was directly measured from the maser disk. The Hubble constant determined using the distances and recession velocities of IC2560 and othere megamasers is inconsistent with that determined with CMB by 3–6 km s-1 Mpc-1.

OH megamasers (OHMs) are luminous masers found in (ultra-)luminous infrared galaxies ([U]LIRGs). OHMs are signposts of major gas-rich mergers associated with some of the most extreme star forming regions in our universe. The dominant OH masing line, occurring at 1667 MHz, can spoof the 1420 MHz neutral hydrogen (HI) line in untargeted HI emission line surveys. While only ∼120 OHMs are currently known, HI surveys on next-generation radio telescopes, such as the Square Kilometre Array (SKA) and its precursors, will detect unprecedented numbers of OHMs. This surge in detections will not only fundamentally change what we know about the OHM population, but will also unlock our ability to implement OHMs as tracers of major mergers and extreme star formation on cosmic scales. Here we present predictions for the number of OHMs that will be detected by these surveys. We also present our novel methods for identifying these interlopers using a k-Nearest Neighbors machine learning algorithm. Preliminary data from HI surveys on precursor SKA telescopes is being used to vet and strengthen these methods as well as give us a first look at a new era in OHM science. From a detection of one of the most luminous OHMs to the discovery of a megamaser at a record-shattering redshift, these new sources are glimpses into how our understanding of the known OHM population will soon be expanding and shifting rapidly and how they will influence our understanding of galaxy evolution.

We present our systematic infrared and (sub)millimeter spectroscopic observations of gas/dust-rich merging ultraluminous infrared galaxies (ULIRGs) to scrutinize deeply buried AGNs (mass-accreting supermassive black holes [SMBHs]). We have found signatures of optically elusive, but intrinsically luminous buried AGNs in a large fraction of nearby (z < 0.3) ULIRGs, suggesting that SMBH mass growth is ongoing in the ULIRG population. Using ALMA, we have detected compact (<100 pc), very luminous (>104Lʘ), AGN-origin, 183 GHz (1.6 mm) H2O megamaser emission in one merging ULIRG, demonstrating that the megamaser emission can be a very powerful tool to dynamically estimate SMBH masses, with the smallest modeling uncertainty of kpc-wide stellar and gas mass distribution, at dusty ULIRGs’ nuclei, because of minimum extinction effects at millimeter. We present our current results and future prospect for the study of the SMBH mass growth in gas/dust-rich galaxy mergers, using (sub)millimeter AGN-origin H2O megamaser emission lines.

Supermassive stars have been proposed as the solution to a number of longstanding problems in globular cluster formation. The hypothetical stars have been suggested as potential polluters responsible for the observed chemical peculiarities within those clusters. In recent hydrodynamic simulations, we have demonstrated that accretion discs around such stars are stable even with large stellar accretion and flyby rates and produce H2O kilomasers. We propose that the W1 kilomaser, associated with a super star cluster in the starburst galaxy NGC 253, may arise in an accretion disc around a supermassive star with a mass of around 4000 Mʘ.

High resolution (0."26 × 0."13 (70 × 35 pc)) L-band (18 cm) OH megamaser (OHM) e-Merlin observations of the LIRG Zw049.057 show that the emission is emerging from a low velocity outflowing structure - which is foreground to a fast, dense and collimated molecular outflow detected by ALMA. The extremely dusty compact obscured nucleus (CON) of Zw049.057 has no (or only little) OHM emission associated with it - possibly because of too high number densities that quench the OHM. In contrast we detect 6 cm H2CO emission primarily from the CON-region. We suggest that the OHM-region of Zw049.057 is not directly associated with star formation, but instead occurs in a wide-angle, slow outflow that surrounds the fast and dense outflow. The OHM is pumped by IR emission that likely stems from activities in the nucleus. We briefly discuss how OHM emission can be used as a probe of LIRG-CON galaxies.

Fundamental physical quantities of the nuclear regions of the Active Galactic Nuclei can be obtained using megamaser studies. In particular, disk-masers associated with accretion disks around the supermassive black holes are used, through high angular resolution measurements, to trace the disk geometry, to estimate the BH mass and to measure accurate distances to their host galaxies. In this contribution, we present the first results in continuum and spectral-line mode of a high-sensitivity, multi-epoch VLBI study of the nuclear region of the megamaser LINER galaxy IC 485.

Extragalactic maser sources are unique tools to derive fundamental physical quantities of the host galaxies, e.g, geometry of accretion disks around super-massive black holes and precise black hole masses, and study in detail the interaction region of nuclear jets/outflows with the interstellar medium, in nearby and distant Active Galactic Nuclei. So far, however, extragalactic maser searches have yielded detection of few percent, and only relatively few maser sources have been found. Because of their unprecedented sensitivity, new upcoming facilities, like the SKA and the ngVLA, will allow to significantly increase the number of known (water) maser sources. This will lead to the chance of performing statistically-relevant studies of the maser phenomenon (and its occurrence), derive extragalactic masers luminosity functions, and ultimately (in particular, through the aid of longer-baselines arrays options) to perform the studies described above for larger samples and up to cosmological distances.

We took profit of the availability of large catalogs of active galact nuclei (AGNs) selected in the hard X-ray from satellite missions (e. g., INTEGRAL, Swift/BAT) to investigate the relation between the occurrence of water maser emission and the X-ray properties of the nuclei on a statistically meaningful basis. Our studies demonstrate that the hard X-ray selection may significantly enhance the maser detection rate over comparably large optical or infrared surveys. Here, we report on a recent survey to search for water maser emission with the Sardinia Radio Telescope (SRT) in a sample of heavily absorbed AGN taken from the 70 months Swift/BAT catalog.

If water megamaser disk activity is intimately related to the circumnuclear activity from accreting supermassive black holes, a thorough understanding of the co-evolution of galaxies with their central black holes should consider the degree to which the maser production correlates with traits of their host galaxies. This contribution presents an investigation of multiwavelength nuclear and host properties of galaxies with and without water megamasers, that reveals a rather narrow multi-dimensional parameter space associated with the megamaser emission. This “goldilocks” region embodies the availability of gas, the degree of dusty obscuration and reprocessing of the central emission, the black hole mass, and the accretion rate, suggesting that the disk megamaser emission in particular is linked to a short-lived phase in the intermediate-mass galaxy evolution, providing new tools for both 1) further constraining the growth process of the incumbent AGN and its host galaxy, and 2) significantly boosting the maser disk detection by efficiently confining the 22 GHz survey parameters.

The mass of the black hole separated from the mass of the maser disk is calculated using the mega-maser technique for 15 maser-galaxies and the corresponding Magorrian relationship (inline1) is analysed.

We present dual-frequency VLBI observations of a nearby radio galaxy NGC 4261 at 22 and 43 GHz using the East Asia VLBI Network. In particular, the first sub-pc scale image of the 22 GHz water megamaser line in the circumnuclear region of NGC 4261 is shown. Our results suggest that the megamaser emission in NGC 4261 can be associated with the inner radius of the obscuring disk, as it is proposed for the nearest radio-loud megamaser source NGC 1052. An alternative hypothesis on the megamaser association is the shock region of the interaction between the jet and ambient molecular clouds.

Astrometric very long baseline interferometry (VLBI) observations of stellar masers are an excellent method to determine distances and proper motions in our Galaxy. Large maser astrometry surveys, the Bar and Spiral Structure Legacy survey and the VLBI Exploration for Radio Astrometry, allowed astronomers to determine fundamental Galactic parameters, such as the rotation curve and the distance to the Galactic centre, as well as to trace the spiral arms. In this review, the results of these surveys will be summarised and compared with astrometric measurements using other methods.

VERA has been regularly conducting astrometry of Galactic maser sources for ∼ 20 years, producing more than 100 measurements of parallaxes and proper motions of star-forming regions as well as AGB stars. By combining the observational results obtained by VLBA BeSSeL, EVN, and LBA, maser astrometry provides a unique opportunity to explore the fundamental structure of the Galaxy. Here we present the view of the Galaxy revealed by the maser astrometry, and also discuss the importance of maser astrometry in the era of GAIA by comparing the results obtained by VLBI and GAIA. We also present our view of “proper motions toward the future” of the relevant field, expected in the next decade based on global collaborations.

Gaia space mission of the European Space Agency was launched at the end of 2013 and will continue operations until 2025. The published data releases revolutionize the view of the Milky Way galaxy and beyond thanks to its unprecedented astrometry, photometry and spectroscopy. The paper reviews the products of the last data release of the Gaia mission and some of the scientific impacts of the data. We also discuss the future perspectives of Galaxy astrometry from space.

The Perseus arm has a gap in Galactic longitudes (l) between 50° and 80° where the arm has little star formation activity. To understand the gap, we conducted VERA (VLBI Exploration of Radio Astrometry) astrometry and analyzed archival H <SC>I</SC> data. We report on parallax and proper motion results from four star-forming regions, of which G050.28–00.39 and G070.33+01.59 are associated with the gap. Perseus-arm sources G049.41+00.32 and G050.28–00.39 lag relative to a Galactic rotation by 77 ± 17 km s-1 and 31 ± 10 km s-1, respectively. The noncircular motion of G049.41+00.32 cannot be explained by the gravitational potential of the arm. We discovered rectangular holes with integrated brightness temperatures less than 30 K arcdeg in l vs. VLSR of the H <SC>I</SC> data. Also, we found extended H <SC>I</SC> emission on one side of the Galactic plane when integrating the H <SC>I</SC> data over the velocity range covering the hole. G049.41+00.32 and G050.28–00.39 are moving toward the emission. The Galactic H <SC>I</SC> disk at the same velocity range showed an arc structure, indicating that the disk was pushed from the lower side of the disk. All the observational results might be explained by a cloud collision with the Galactic disk.

Stellar SiO masers are found in the atmospheres of asymptotic giant branch (AGB) stars with several maser transitions observed around 43 and 86 GHz. At least 28 SiO maser stars have been detected within ∼2 pc projected distance from Sgr A* by the Very Large Array (VLA) and Atacama Millimeter/submillimeter Array (ALMA). A subset of these masers have been studied for several decades and form the basis of the radio reference frame that anchors the reference frame for infrared stars in the Galactic Center (GC). We present new observations of the GC masers from VLA and ALMA. These new data combined with extant maser astrometry provide 3D positions, velocities, and acceleration limits. The proper motions and Doppler velocities are measured with unprecedented precision for these masers. We further demonstrate how these measurements may be used to trace the stellar and dark matter mass distributions within a few pc of Sgr A*.

We present the trigonometric parallax, proper motion, and structure of three methanol masers from the Southern Hemisphere Parallax Interferometric Radio Astrometry Legacy Project (SπRALS). All three masers have better than 5% parallax accuracy, which we attribute to the new inverse MultiView calibration technique.

Great progress has been made using VLBI techniques to measure trigonometric parallaxes to masers associated with young, high-mass stars, in order to map the spiral structure of the Milky Way. However, large numbers of parallax distance have only been obtained over about half of the Galaxy. Here I discuss the use of 3-dimensional kinematic distances for completing the map with many sources well past the Galactic Center.

We present a method to estimate distances to AGB stars, utilizing the rich infrared data sets available for these infrared-bright targets. The method is based on the assumption that stars with intrinsically similar properties (metallicity, initial mass, etc.) produce similar spectral energy distributions (SEDs) and similar luminosities. We here discuss the results for AGB stars belonging to the BAaDE survey sample whose distances were calibrated using the template SEDs of stars with their VLBI parallaxes. As VLBI parallaxes are only known for a handful of sources, the resulting templates only cover a small subset of the BAaDE sample. Additional methods to derive suitable templates will therefore also be required. The work on expanding the template set is promising, although more fine tuning is still needed.