Water Maser Emission from the Active Nucleus in M51

At 22 GHz, water vapor "kilomaser" emission is reported from the central region of the Whirlpool galaxy M51 (NGC 5194). The redshifted spectral features (V_LSR ~ 560 km s^-1), 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. In early 2001, blueshifted maser emission (V_LSR ~ 435 km s^-1) was also detected. Red- and blueshifted features bracket the systemic velocity asymmetrically. Within the standard model of a rotating Keplerian torus, this may suggest the presence of either a highly eccentric circumnuclear cloud or red- and blueshifted "high-velocity" emission from a radially extended torus. Most consistent with the measured H₂O position, however, is an association of the redshifted H₂O emission with the northern part of the bipolar radio jet. In this scenario, the (weaker) northern jet is receding while the blueshifted H₂O emission is associated with the approaching southern jet.