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 similar to km s(-1)), flaring during most of the year 2000, originate from a spatially unresolved maser spot of size less than or similar to 30 mas (less than or similar to1.5 pc), displaced by less than or similar to 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 similar to 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.