Water vapor in the starburst galaxy NGC 253: A new nuclear maser?

22 GHz water vapor emission was observed toward the central region of the spiral starburst galaxy NGC 253. Monitoring observations with the 100-m telescope at Effelsberg and measurements with the BnC array of the VLA reveal three distinct velocity components, all of them blueshifted with respect to the systemic velocity. The main component arises from a region close to the dynamical center and is displaced by <1&DPRIME; from the putative nuclear continuum source. The bulk of the maser emission is spread over an area not larger than 70 x 50 mas(2). Its radial velocity may be explained by masing gas that is part of a nuclear accretion disk or of a counterrotating kinematical subsystem or by gas that is entrained by the nuclear superwind or by an expanding supernova shell. A weaker feature, located &SIM;5&DPRIME; to the northeast, is likely related to an optically obscured site of massive star formation. Another maser component, situated within the innermost few 10&DPRIME; of the galaxy, is also identified.