Extragalactic masers were discovered more than 20 years ago (Whiteoak & Gardner 1973). It soon became apparent that those extragalactic masers we can detect are intrinsically very powerful and are located in galactic nuclei. The term Megamaser was coined to describe the most luminous OH masers, which are a million times more powerful than any OH masers within our own Galaxy (Baan & Haschick 1984). The same word is nowadays applied to any powerful maser associated with an active galactic nucleus (AGN). Whereas normal Galactic masers would be barely detectable outside the Local Group, megamasers are detectable in principle out to redshifts of z ~ 2 (Baan 1997). The galaxies hosting megamasers in their nuclei are invariably active in some degree: they include ultraluminous infrared galaxies, starbursts and Seyfert galaxies.

The OH megamasers have a distinct parent population of dusty and FIR luminous merger galaxies. Optical, radio and molecular characteristics may be used to further identify the nature of the nuclear activity in the OH megamaser host galaxies. Recent interferometric data on OH megamasers Arp 220 and III Zw35 will be used to discuss the spatial distribution of the OH emission.

We would like to review the current status of OH megamaser pumping models. Contrary to the quantity and quality of observations of OH megamasers, surprisingly little theoretical modeling has been done on them so far.

An analysis of a sample of OH megamaser galaxies is presented. It is shown that the dependence of OH luminosity on FIR (far infrared) luminosity is not quadratic, as previously assumed, but closer to linear. Using a luminosity function of the luminous infrared galaxies, as well as linear relationship between OH and FIR luminosities the expected number of OH megamaser galaxies has been estimated. Spectral energy distributions (SED) of 16 megamaser galaxies have been constructed using multiwavelength (from radio to X-ray) data. The SED of megamasers look closely similar from radio to X-ray. It is shown that the maser amplification coefficient depends both on the OH main line to 60 micron ratio and the main line to 2.2 micron ratio. The OH main lines ratio (f1667/f1665) decreases, while the flux density at 10 micron is increasing.

We review correlations between water megamaser emission and other properties of AGN. Galaxies detected as water megamaser emitters tend to be nearer, brighter at mid-infrared and possibly centimeter wavelengths, and more highly inclined than undetected galaxies. Only Seyfert 2s and LINERs (no Seyfert Is) are detected. It is argued that the primary parameter determining whether an active galaxy is detectable as an H2O megamaser is the gas column along our line of sight to the nucleus.

Twenty-one H2O masers have been identified in the nuclei of active galaxies. The detection rate is about 7 percent. Very Long Baseline Interferometric data of four of these maser systems (NGC 4258, NGC 1068, NGC 4945, and NGC 3079) show evidence of Keplerian disks on a subparsec scale. The best example is NGC 4258. There the masers trace a thin warped disk in nearly perfect circular Keplerian motion. If the apparent thinness is indicative of hydrostatic equilibrium, then the kinetic temperature must be less than 1000K, the magnetic field less than 200 mG, and the mass accretion rate less than about α10-3 M⊙/yr, where a is the viscosity parameter. From direct Zeeman measurements the toroidal magnetic field strength is less than 300 mG. The proper motions of the systemic maser feature imply a distance to the galaxy of 7.3 ± 0.3 Mpc. The high-velocity features show no detectable proper motions or accelerations, which confirms the model of circular Keplerian motion, and puts severe constraints on any alternative model. A feature in the systemic group flared to 20 Jy, offering opportunities to study the physics of the maser emission.

Strong H2O maser emission is detected from active galactic nuclei (AGN) as well as Galactic objects such as star-forming regions and late-type stars. In spite of the widely different luminosities of the different masers, a common pump mechanism seems adequate: neutral collisions at densities of ˜ 108-1010 cm-3 and temperatures of ˜ 250-500 K. The different properties of the various masers can be attributed to geometry. Although disk rotation controls the AGN maser geometry it does not directly determine the dimensions. X-ray radiation and spiral shocks have been suggested as the heating sources of H2O megamasers. Both are capable of explaining the observations, and the radiative scenario seems best understood in terms of chance alignment of standard broad-line-region clouds. It is not yet clear whether these different proposals produce distinct maser signatures.

The discovery of megamasers in nearby active galactic nuclei (AGN) is an unexpected boon for AGN researchers. The most immediate benefit, brought out in observations of NGC4258 (Miyoshi et al 1995, Moran, these proceedings) is that it allows an accurate mass measurement for the central black hole, in this case of 3.6 x 107 M⊙, with unprecedented accuracy. (I shall not belabor the point that we have not proven rigorously that it is a black hole because this is surely on much firmer footing than what follows!) Masers also allow us to discover second order features of the kinematics, specifically that the disk is warped. Thirdly, and this is what I have been asked to review here, it should enable us to choose between several possible accretion modes by treating masers as diagnostic probes of physical conditions in the disk.

We propose that the observed high-velocity maser emission from circumnuclear disks arises in spiral shocks, which can naturally account for several peculiar findings in the spectrum and spatial distribution of the maser emission.

We present some results of observations of H2O masers (1.35 cm line) in Star-Forming Regions. The observations have been made from the beginning of the eighties at the 22 m radiotelescope at Pushchino, Russia (Lekht et al. 1995). The time interval between two consecutive observations is about one month. The spectral resolution is 0.1 km/sec and the sensitivity, for a time integration of 15 minutes, is about 10 Jy. The sources with which we deal in this work are: ONI, ON2, W31A, W75S, S128 and S252A. We pay attention to results of flux variations that are either common to all the sources or are results that allow to establish differences between them.

NGC 3079 has very luminous water megamaser from the nucleus, the peak of the spectrum being blueshifted by 180 km s-1 from the systemic velocity of the galaxy (Vsys = 1131 km s-1) (Henkel et al. 1984, Haschick & Baan 1985). Core-jet like continuum structure is also found in the nuclear region (Irwin & Seaquist 1988). No velocity drift for main features of water maser (VLSR — 941-975 km s1) has been shown (Nakai et al. 1995). However, the drift was recently detected for the maser of 1190 km s-1 (Nakai 1997). HI and OH absorptions were detected in the nucleus (Haschick & Baan 1985, Irwin & Seaquist 1991). Thus this galaxy is very unique object to investigate water masers, continuum structure and absorption features all together with VLBI.

Lunar astronomical facilities can be robotically emplaced or form an adjunct to a human base. Connected facilities are a means to increase science-per-dollar. Near-Earth and Centaur asteroids may also serve as venues for astronomy.

Lunar surface is, in spite of disadvantages on payload compared with those in the distant orbits, extremely attractive for future astronomical observations with large and complicated observing instruments. The Moon will provide firm and large base, very stable temperature and low background in some selected sites. In addition, the aids of human activities on the Moon can be expected in the near future. We have been discussing the small but realistic steps toward the future Moon-based astronomy, as part of Japanese lunar exploration plans with HI rockets. The fundamental Policy of Japan’s Space Development which was approved by Space Activities Commission in 1996 described that Japan will continuously promote unmanned lunar exploration and study the feasibility of a systematic exploration program for science and possible activities on the Moon, under a phase-by-phase approach. The plan includes a possibility of astronomical observations on the lunar surface.

Some pilot-type astronomical plans, starting from very small-size telescopes in optical/IR and mm-wave etc. are under discussion. We report here on the general background, discussion including proposed plans, and steps toward the future moon-based observatories.

As stressed at the second International Lunar Exploration Working Group meeting (Kyoto, October 1996), the Moon, if kept free from pollution, contains a series of remarkable astronomical sites. In particular the following fields of instrumentation and research emerge :

(1) very low frequency radio-astronomical arrays to be located on the lunar far side for surveying an entirely new spectrum, albeit at fairly low angular resolution;

(2) interferometers in several wavebands to search for extrasolar planets as well as to perform other observations (morphological studies e.g.);

(3) transit optical telescopes for the detailed observation of dark matter and other targets;

(4) millimeter-wave telescopes for high sensitivity cosmic background mapping;

(5) infrared telescopes in permanently cryogenic environment (e.g. the lunar South pole);

(6) gravitational wave detectors;

(7) cosmic-ray and high energy detectors;

Very Long Baseline Interferometry (VLBI) technique occupies a special place among tools for studying the Universe due to its record high angular resolution. The latter is in the inverse proportion to the length of interferometer baseline at any given wavelength. Until recently, the available angular resolution in radio domain of about 1 milliarcsecond at centimeter wavelengths was limited by the diameter of the Earth. However, many astrophysical problems require a higher angular resolution. The only way to achieve this at a given wavelength is to create an interferometer with the baseline larger than the Earth’s diameter by placing at least one telescope in space. In February 1997, the first dedicated Space VLBI mission, VLBI Space Observatory Program (VSOP), led by the Institute of Space and Astronautical Sciences (Japan) has been launched (Hirabayashi 1997). The VSOP mission opens a new dimension in the development of radio astronomy of extremely high angular resolution and will be followed by other Space VLBI missions. A review of scientific drives and technological challenges of the next generation Space VLBI mission have been discussed, for example, by Gurvits et al. (1996) and Ulvestad et al. (1997).

The radio sky at frequencies of several MHz and below is essentially unexplored with high angular resolution due to refraction and opacity in the Earth’s ionosphere. An interferometer array in space providing arcminute resolution images would allow a wide range of problems in solar, planetary, galactic, and extragalactic astronomy to be attacked. These include the evolution of solar and planetary radio bursts, interplanetary and interstellar scintillation, the distribution of low energy cosmic rays and diffuse ionized hydrogen in our galaxy, the determination of spectral turnover frequencies and magnetic field strengths in galactic and extragalactic radio sources, searches for “fossil” radio galaxies which are no longer detectable by high frequency surveys, and searches for new sources of coherent radio emission. In addition, it is likely that unexpected objects and emission processes will be discovered by such an instrument, as has often happened when high resolution observations first become possible in a new spectral region. The Moon can provide shielding from terrestrial interference (and from the Sun half of the time) and consequently the lunar farside surface offers an ideal site of a low frequency radio array.

The Moon combines some of the most attractive features for astronomical observations from space (no atmospheric absorption, perfect seeing, etc.) and those from ground (large steady optical bench, ease of control from Earth, etc.). Astronomers have planned for telescopes on the Moon for decades but, due to its primary obstacle – high cost – they have not yet been built, save one small telescope that was carried to the Moon on an Apollo mission.

We have a plan to a radioastronomy mission at the Moon. This is dedicated to the mapping of the CBR anisotropy in order to study the properties of the early Universe, especially galaxy formation. This is a candidate for the Moon mission of NASDA, launched by an HIIA Rocket. The mission carries an 1.5 m (min.) offset parabola antenna with radiation shield to the north polar region of the Moon. This will observe CBR anisotropy at three (min.) frequencies between 30 to 90 GHz. The angular resolution is 9’ at 90 GHz. The telescope will observe the donut-shaped sky between 5 to 30 degree (not fixed) from the Moon celestial north pole. The resultant sensitivity will reach ΔT/T ⋍ 106K in 30’x30’ pixel and ΔT/T ⋍ 105K in 9’x9’ pixel.

Because of the ever increasing human-made radio frequency interference (RFI), we propose to IAU a Resolution for protecting the well singled out lunar farside 100 km diameter SAHA crater from any future RFI for the scientific benefit of the coming decades high-sensitivity radioastronomy at large. Immediate and pragmatic action is strongly recommended. Our strategy, different from the ones of a recent ITU Recommendation, could increase our bargaining possibilities.