ABSTRACT. 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.

Subject headings: galaxies: individual (M51); galaxies: nuclei; masers; radio lines: galaxies

Print publication: Issue 1 (2001 October 10)
Received 2001 July 11, accepted for publication 2001 August 31
Published 2001 September 21