2001ApJ...560L..37H

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<sub>LSR</sub>∼560 km.s<sup>–1</sup>), 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 H<sub>2</sub>O kilomaser with an active galactic nucleus. In early 2001, blueshifted maser emission (V<sub>LSR</sub>∼435 km.s<sup>–1</sup>) 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.