A Green Bank Telescope Search for Water Masers in Nearby Active Galactic Nuclei

Using the Green Bank Telescope, we have conducted a survey for 1.3 cm water maser emission toward the nuclei of nearby active galaxies, the most sensitive large survey for H2O masers to date. Among 145 galaxies observed, maser emission was newly detected in 11 sources and confirmed in one other. Our survey targeted nearby (v < 12,000 km s-1), mainly type 2 active galactic nuclei (AGNs) north of δ = -20° and includes a few additional sources as well. We find that more than one-third of Seyfert 2 galaxies have strong maser emission, although the detection rate declines beyond v ~ 5000 km s-1 because of sensitivity limits. Two of the masers discovered during this survey are found in unexpected hosts: NGC 4151 (Seyfert 1.5) and NGC 2782 (starburst). We discuss the possible relations between the large X-ray column to NGC 4151 and a possible hidden AGN in NGC 2782 to the detected masers. Four of the masers discovered here, NGC 591, NGC 4388, NGC 5728, and NGC 6323, have high-velocity lines symmetrically spaced about the systemic velocity, a likely signature of molecular gas in a nuclear accretion disk. The maser source in NGC 6323, in particular, reveals the classic spectrum of a "disk maser" represented by three distinct groups of Doppler components. Future single-dish and VLBI observations of these four galaxies could provide a measurement of the distance to each galaxy and of the Hubble constant, independent of standard candle calibrations.