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 H 2 O masers to date. Among 145 galaxies observed, maser emission was newly detected in eleven sources and confirmed in one other. Our survey targeted nearby (v < 12,000 km s −1 ), mainly type 2 AGNs north of δ = -20 • , and includes a few additional sources as well. We find that more than a third of Seyfert 2 galaxies have strong maser emission, though the detection rate declines beyond v ∼ 5000 km s −1 due to 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 1 The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement with Associated Universities, Inc.2 -2source 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.

No abstract

No abstract

Using the Green Bank Telescope, we conducted a ''snapshot'' survey for water maser emission toward the nuclei of 611 galaxies and detected eight new sources. The sample consisted of nearby (v < 5000 km s À1 ) and luminous (M B < À19:5) galaxies, some with known nuclear activity but most not previously known to host AGNs. Our detections include both megamasers associated with AGNs and relatively low luminosity masers probably associated with star formation. The detection in UGC 3789 is particularly intriguing because the spectrum shows both systemic and high-velocity lines indicative of emission from an AGN accretion disk seen edge-on. Based on 6 months of monitoring, we detected accelerations among the systemic features ranging from 2 to 8 km s À1 yr À1 , the larger values belonging to the most redshifted systemic components. High-velocity maser lines in UGC 3789 show no detectable drift over the same period. Although UGC 3789 was not known to be an AGN prior to this survey, the presence of a disk maser is strong evidence for nuclear activity, and an optical spectrum obtained later has confirmed it. With follow-up observations, it may be possible to measure a geometric distance to UGC 3789.