Megamasers (the HO maser at 1.35cm) are very strong, of the order of 2000 Jy at 1 Mpc. They are uncommon: near the centers of starburst galaxies. The linewidths can be very large: (NGC4258) up to 1000 km/s rotation has been found on a length scale of 0.1 pc ( 2 solar masses ). The enhanced VLA could be used to survey the megamasers in known starburst IRAS bright galaxies. The enhanced VLA (sensitivity mJy/10min/100 km/s) and larger bandwidth 2000 km/s (156 MHz)-about 20 times faster than current VLA) could detect such megamasers out to 316 Mpc at the level. A large number of such systems could thus be identified.
Followup with the VLBA would be a promising way to identify extragalactic black hole candidates independently of their nuclear radio continuum. The VLBA could yield proper motions of many masers in the host galaxies, and/or the direction of rotation of these systems. The combination of the expanded VLA and the VLBA could therefore be a powerful tool for exploring the dynamics of galactic nuclei independently of their continuum radio luminosities, and thus for an unbiased study of the relationships, if any, between these dynamics and the formation of an AGN.