Observations of H _ { 2 } O masers from circumnuclear disks in active galaxies for the Megamaser Cosmology Project allow accurate measurement of the mass of supermassive black holes ( BH ) in these galaxies .
We present the Very Long Baseline Interferometry ( VLBI ) images and kinematics of water maser emission in six active galaxies : NGC 1194 , NGC 2273 , NGC 2960 ( Mrk 1419 ) , NGC 4388 , NGC 6264 and NGC 6323 .
We use the Keplerian rotation curves of these six megamaser galaxies , plus a seventh previously published , to determine accurate enclosed masses within the central \sim 0.3 pc of these galaxies , smaller than the radius of the sphere of influence of the central mass in all cases .
We also set lower limits to the central mass densities of between 0.12 and 60 \times 10 ^ { 10 } M _ { \odot } pc ^ { -3 } .
For six of the seven disks , the high central densities rule out clusters of stars or stellar remnants as the central objects , and this result further supports our assumption that the enclosed mass can be attributed predominantly to a supermassive black hole .
The seven BHs have masses ranging between 0.76 and 6.5 \times 10 ^ { 7 } M _ { \odot } .
The BH mass errors are \approx 11 % , dominated by the uncertainty of the Hubble constant .
We compare the megamaser BH mass determination with other BH mass measurement techniques .
The BH mass based on virial estimation in four galaxies is consistent with the megamaser BH mass given the latest empirical value of \langle f \rangle , but the virial mass uncertainty is much greater .
Circumnuclear megamaser disks allow the best mass determination of the central BH mass in external galaxies and significantly improve the observational basis at the low-mass end of the M - \sigma _ { \star } relation .
The M - \sigma _ { \star } relation may not be a single , low-scatter power law as originally proposed .
MCP observations continue and we expect to obtain more maser BH masses in the future .