We have mapped the distribution of continuum emission and of the OH absorption at 1667 MHz in the Seyfert galaxy NGC 5793 on parsec scales with VLBI. The observed structure of the OH absorption shows a velocity gradient across the continuum structure of C1(C), the velocity range symmetrically spanning the systemic velocity. The direction of the OH velocity gradient is inconsistent with that of rotation of the kiloparsec scale CO disk of the galaxy, though the P.A. of these two axes are nearly aligned: we have discovered a counter-rotating gaseous component in the inner region on scale of 10 parsecs. If circular rotation across C1(C) is invoked to explain the velocity gradient, based on the assumption that the H2O maser emission is from the same gas, the binding mass is . This value corresponds to nearly 5% of the molecular gas mass existing in the kiloparsec-scale galactic disk. The interpretation of the observed OH absorption distribution is not unique. The OH absorption could arise from dense molecular gas in the 10 parsec scale circumnuclear disk/torus or from spatially extended diffuse molecular gas on kiloparsec scales in the galactic disk, although we favour the former explanation. The structure and spectra of the continuum components are difficult to interpret. However, C1(C) is the strongest component and is that against which the strongest OH absorption occurs, suggesting that C1(C) is the core. More data are required to resolve this issue.
© European Southern Observatory (ESO) 2000
Online publication: July 27, 2000