Water Maser Emission from the Active Nucleus in M51

22 GHz water vapor `kilomaser' emission is reported from the central region of the Whirlpool galaxy M 51 (NGC 5194). The red-shifted spectral features (Vlsr ~ 560 km/s), flaring during most of the year 2000, originate from a spatially unresolved maser spot of size < 30 mas (< 1.5 pc), displaced by < 250 mas from the nucleus. The data provide the first direct evidence for the association of an H2O kilomaser with an active galactic nucleus (AGN). In early 2001, blue-shifted maser emission (Vlsr ~ 435 km/s) was also detected. Red- and blue-shifted features bracket the systemic velocity asymmetrically. Within the standard model of a rotating Keplerian torus, this may either suggest the presence of a highly eccentric circumnuclear cloud or red- and blue-shifted `high velocity' emission from a radially extended torus. Most consistent with the measured H2O position is, however, an association of the red-shifted H2O emission with the northern part of the bipolar radio jet. In this scenario, the (weaker) northern jet is receding while the blue-shifted H2O emission is associated with the approaching southern jet.Comment: 9 pages, 1 figur

The Discovery of Water Maser Emission from Eight Nearby Galaxies

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) and luminous (M_B < -19.5) 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 six months of monitoring, we detected accelerations among the systemic features ranging from 2 to 8 km/s/yr, 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.Comment: to appear in Astrophysical Journal, 1 May 200