PROBING ACTIVE GALACTIC NUCLEI WITH H2O MEGAMASERS

We describe the characteristics of the rapidly rotating molecular disk in the nucleus of the mildly active galaxy NGC4258. The morphology and kinematics of the disk are delineated by the point-like water-vapor emission sources at 1.35-cm wavelength, High angular resolution [200 mu as where as is arcsec, corresponding to 0.006 parsec (pc) at 6.4 million pc] and high spectral resolution (0.2 km . s(-1) or nu/Delta nu = 1.4 x 10(6)) with the Very-Long-Baseline Array allow precise definition of the disk. The disk is very thin, but slightly warped, and is viewed nearly edge-on. The masers show that the disk is in nearly perfect Keplerian rotation within the observable range of radii of 0.13-0.26 pc. The approximately random deviations from the Keplerian rotation curve among the high-velocity masers are approximate to 3.5 km . s(-1) (rms), These deviations may be due to the masers lying off the midline by about +/-4 degrees or variations in the inclination of the disk by +/-4 degrees. Lack of systematic deviations indicates that the disk has a mass of <4 x 10(6) solar mass (Mo), The gravitational binding mass is 3.5 x 10(7) M., which must lie within the inner radius of the disk and requires that the mass density be >4 x 10(9) M.. pc(-3). If the central mass were in the form of a star cluster with a density distribution such as a Plummer model, then the central mass density would be 4 x 10(12) M.. pc(-3). The lifetime of such a cluster would be short with respect to the age of the galaxy [Maoz, E. (1995) Astrophys. J. Lett. 147, L91-L94]. Therefore, the central mass may be a black hole. The disk as traced by the systemic velocity features is unresolved in the vertical direction, indicating that its scale height is <0.0003 pc (hence the ratio of thickness to radius, H/R, is <0.0025). For a disk in hydrostatic equilibrium the quadrature sum of the sound speed and Alfven velocity is <2.5 km . s(-1), so that the temperature of the disk must be <1000 K and the toroidal magnetic field component must be <250 mG. If the molecular mass density in the disk is 10(10) cm(-3), then the disk mass is approximate to 10(4) M., and the disk is marginally stable as defined by the Toomre stability parameter Q (Q = 6 at the inner edge and 1 at the outer edge), The inward drift velocity is predicted to be <0.007 km . s(-1), for a viscosity parameter of 0.1, and the accretion rate is <7 x 10(-5) M.. yr(-1). At this value the accretion would be sufficient to power the nuclear x-ray source of 4 x 10(40) ergs(-I) (1 erg = 0.1 mu J). The volume of individual maser components may be as large as 10(46) cm(3), based on the velocity gradients, which is sufficient to supply the observed luminosity, The pump power undoubtedly comes from the nucleus, perhaps in the form of x-rays, The warp may allow the pump radiation to penetrate the disk obliquely [Neufeld, D. A. & Maloney, P. R. (1995) Astrophys. J. Lett. 447, L17-L19], A total of 15 H2O megamasers have been identified out of >250 galaxies searched, Galaxy NGC4258 may be the only case where conditions are optimal to reveal a well-defined nuclear disk Future measurement of proper motions and accelerations for NGC4258 will field an accurate distance and a more precise definition of the dynamics of the disk.