1997A&A...319..450G

We have studied with the IRAM interferometer the ¹²CO(1-0) emission 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 an accurate determination of NGC5907's kinematical center (α(J1950.0)= 15h14m35.5s, δ(J1950.0)=56d30'43.3"). Within ±2" from the center, we detect molecular gas with velocities of ±160km/s (relative to the source systemic velocity V_sys=677km/s(LSR)), and, within ±10" from the center, velocities of up to ±180km/s. Part of the gas exhibits non-circular motions. We explain the gas kinematics by the presence of a stellar bar with an angular velocity ({OMEGA}_p=70km/s/kpc and of an Outer Inner Linblad Resonance (oILR) at r≃14". The dynamical mass inside the R<300pc region, estimated from the steepness of the apparent rotation curve, is ≃5x10⁹M_☉. It is comparable to the stellar mass derived from near IR photometry and >10 times larger than the mass of gas in the same region. The CO brightness distribution and, presumably, the molecular gas distribution are highly asymmetrical. The interferometer, which filters out the smooth disk emission detected by the 30-m telescope, reveals two narrow structures parallel to the major axis. We identify those with spiral arms. From their position and velocities, we determine an inclination i=86.5° of the plane of the galaxy. The thickness of the arms, perpendicularly to this plane, is found to be ≤3" (≤150pc). The large mass concentration near the nucleus and the flat rotation curve of NGC 5907 are more typical of early or intermediate-type spirals than of late type spirals. Yet, the large CO and HI luminosties, the conspicuous molecular arms, and the absence of visible bulge on optical photographs are more characteristic of Sc-Sd galaxies. NGC 5907 may thus be at a critical stage of galactic evolution.