A HIGH-RESOLUTION (CO)-C-12(1-0) STUDY OF THE NUCLEUS OF NGC-5907 - OBSERVATIONS AND MODELING

We have studied with the IRAM interferometer the (CO)-C-12(1-0) emissi on around the nucleus of NGC 5907, an edge-on spiral at the center of which we had previously detected fast rotating molecular gas. The high angular resolution (3.1''>, or 160 pc along the major axis) allowed a n accurate determination of NGC 5907's kinematical center (alpha(J1950 .0) = 15(h)14(m)35.5(s), delta(J1950.0)= 56 degrees 30' 43.3''). Withi n +/-2'' from the center, we detect molecular gas with velocities of /-160 kms(-1) (relative to the source systemic velocity V-sys = 677 km s(-1) (LSR)), and, within +/-10'' from the center, velocities of up to +/-180 km/s. Part of the gas exhibits non-circular motions. We explai n the gas kinematics by the presence of a stellar bar with an angular velocity (Ohm(p) = 70 kmskpc(-1)) and of an Outer Inner Linblad Resona nce (oILR) at r similar or equal to 14''. The dynamical mass inside th e R < 300 pc region, estimated from the steepness of the apparent rota tion curve, is similar or equal to 5 10(9)M.. It is comparable to the stellar mass derived from near IR photometry and > 10 times larger tha n the mass of gas in the same region. The CO brightness distribution a nd, presumably, the molecular gas distribution are highly asymmetrical . The interferometer, which filters out the smooth disk emission detec ted by the 30-m telescope, reveals two narrow structures parallel to t he major axis. We identify those with spiral arms. From their position and velocities, we determine an inclination i = 86.5 degrees of the p lane of the galaxy. The thickness of the arms, perpendicularly to this plane, is found to be less than or equal to 3'' (less than or equal t o 150 pc). The large mass concentration near the nucleus and the flat rotation curve of NGC 5907 are more typical of early or intermediate-t ype spirals than of late type spirals. Yet, the large CO and HI lumino sties, the conspicuous molecular arms, and the absence of visible bulg e on optical photographs are more characteristic of Sc-Sd galaxies. NG C5907 may thus be at a critical stage of galactic evolution.