Se p 20 01 Water Maser Emission from the Active Nucleus in M 51

22GHz water vapor ‘kilomaser’ emission is reported from the central region of the Whirlpool galaxy M51 (NGC5194). 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 < ∼ 30mas (< ∼ 1.5 pc), displaced by < ∼ 250mas 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. Redand 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 redand 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. Subject headings: galaxies: nuclei — galaxies: individual (M51 (NGC5194)) — masers — radio lines: galaxies