Observational evidence for massive black holes in the centers of active galaxies

Naturally occurring water vapor maser emission at 1.35 cm wavelength provid es an accurate probe for the study of accretion disks around highly compact objects, thought to be black holes, in the centers of active galaxies. Bec ause of the exceptionally fine angular resolution, 200 microarcseconds, obt ainable with very long baseline interferometry, accompanied by high spectra l resolution, < 0.1 km s(-1), the dynamics and structures of these disks ca n be probed with exceptional clarity. The data on the galaxy NGC 4258 are d iscussed here in detail. The mass of the black hole binding the accretion d isk is 3.9 x 10(7) M.. Although the accretion disk has a rotational period of about 800 years, the physical motions of the masers have been directly m easured with VLBI over a period of a few years. These measurements also all ow the distance from the earth to the black hole to be estimated to an accu racy of 4 per cent. The status of the search for other maser/black hole can didates is also discussed.