VLBI imaging of OH absorption: The puzzle of the nuclear region of NGC 3079

Abstract

Broad hydroxyl (OH) absorption lines in the 1667- and 1665-MHz transitions towards the central region of NGC 3079 have been observed at high resolution with the European VLBI Network (EVN). Velocity fields of two OH absorption components have been resolved across the unresolved nuclear radio continuum source of extention ∼10 pc. The velocity field of the OH absorption close to the systemic velocity shows rotation in nearly the same sense as that of the edge-on galactic-scale molecular disc probed by CO(1-0) emission. The velocity field of the blueshifted OH absorption displays a gradient in almost the opposite direction. The blueshifted velocity field represents a non-rotational component, which may trace an outflow from the nucleus, or material driven and shocked by the kiloparsec-scale superbubble. This OH absorption component traces a structure that does not support a counter-rotating disc suggested on the basis of the neutral hydrogen absorption.