Based on spectroscopic signatures , about one-third of known H _ { 2 } O maser sources in active galactic nuclei ( AGN ) are believed to arise in highly inclined accretion disks around central engines .
These “ disk maser candidates ” are of interest primarily because angular structure and rotation curves can be resolved with interferometers , enabling dynamical study .
We identify five new disk maser candidates in studies with the Green Bank Telescope , bringing the total number published to 30 .
We discovered two ( NGC 1320 , NGC 17 ) in a survey of 40 inclined active galaxies ( v _ { sys } < 20000 km s ^ { -1 } ) .
The remaining three disk maser candidates were identified in monitoring of known sources : NGC 449 , NGC 2979 , NGC 3735 .
We also confirm a previously marginal case in UGC 4203 .
For the disk maser candidates reported here , inferred rotation speeds are 130–500 km s ^ { -1 } .
Monitoring of three more rapidly rotating candidate disks ( CG 211 , NGC 6264 , VV 340A ) has enabled measurement of likely orbital centripetal acceleration , and estimation of central masses ( 2–7 \times 10 ^ { 7 } M _ { \odot } ) and mean disk radii ( 0.2–0.4 pc ) .
Accelerations may ultimately permit estimation of distances when combined with interferometer data .
This is notable because the three AGN are relatively distant ( 10000 < v _ { sys } < 15000 km s ^ { -1 } ) , and fractional error in a derived Hubble constant , due to peculiar motion of the galaxies , would be small .
As signposts of highly inclined geometries at galactocentric radii of \sim 0.1 –1 pc , disk masers also provide robust orientation references that allow analysis of ( mis ) alignment between AGN and surrounding galactic stellar disks , even without extensive interferometric mapping .
We find no preference among published disk maser candidates to lie in high-inclination galaxies .
This provides independent support for conclusions that in late-type galaxies , central engine accretion disks and galactic plane orientations are not correlated .