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

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
² Istituto di Radioastronomia, CNR, Via Gobetti 101, 40129 Bologna, Italy
³ Osservatorio Astronomico di Cagliari, Loc. Poggio dei Pini, Strada 54, 09012 Capoterra (CA), Italy
⁴ SAO/SMA Project, PO Box 824, Hilo, HI 96721, USA

Corresponding author: C. Henkel, chenkel@mpifr-bonn.mpg.de

Received: 18 June 2003
Accepted: 16 October 2003

Abstract

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´´ from the putative nuclear continuum source. The bulk of the maser emission is spread over an area not larger than  mas². 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 ~5´´ to the northeast, is likely related to an optically obscured site of massive star formation. Another maser component, situated within the innermost few 10´´  of the galaxy, is also identified.