How massive are central black holes in AGNs?
To date, only two supermassive black holes, in the Milky Way and NGC 4258, have had their masses measured directly by motions of gas within a parsec of the center. Several weak water megamasers with strong indications of Keplerian rotation as in NGC 4258, have been discovered recently by the GBT and the 70m NASA antenna in Australia. Imaging of these sources with the VLBA and other large antennas may provide additional high-accuracy black-hole masses. Direct distance measurements also may be made at distances several times larger than for NGC 4258, providing better calibration of the expansion rate of the Universe. • How are relativistic, collimated flows generated? In the most favorable cases, such as M87 where a wide opening angle is seen at high resolution, it appears that the region in which jets are collimated is starting to be resolved. Observations of the structure, polarization and dynamical properties of such regions provide constraints on magnetohydrodynamical models and simu
