Probing circumnuclear molecular gas in NGC 5793 with OH absorption

We have observed the nuclear region of an edge-on spiral galaxy NGC 5793 wi th VLBI in order to determine the morphology of the central continuum sourc e at 6 and 18 cm and the distribution of OH molecular gas at 18 cm on parse c scales. NGC 5793, with a type 2 Seyfert nucleus, is known to have peculia r H2O maser features in its center, indicating the presence of a rotating d isk/torus (Hagiwara et al. 1997). Previous interferometric observations cou ld not clarify both the continuum and OH gas structures due to low angular resolution (Gardner & Whiteoak 1986, Gardner et al. 1992). We find multiple continuum peaks forming a bent jet, though the observed peculiar morpholog y of the continuum source has no straightforward interpretation. We have al so imaged the distribution of OH absorption toward the central continuum so urce. Due to the relative weakness of the continuum emission only the centr al continuum peak shows evidence of detectable opacity of the OH absorbing gas. The OH velocity field contour map reveals a velocity gradient of 8.7 k m s(-1) pc(-1) nearly along the galaxy's optical axis. If we suppose that t he observed OH velocity gradient is caused by simple solid rotation around the nucleus of the galaxy, the central binding mass is 4.2 x 10(8) Mg. The direction of the OH velocity field shows a reversal of a sense of rotation with respect to that of the outer galactic disk observed by CO (1 - 0), imp lying the existence of an independent kinematical system in the central 10 parsec-scale region of the galaxy. Alternatively, the observed OH molecular absorption might possibly arise in the parsec;scale circumnuclear region o f NGC 5793.