The distribution of the molecular gas (as traced by CO) shows a maximum in the central region and a ring or spiral arm at . Further analysis of the major axis distribution reveals evidence for an inner ring-like structure at . The kinematics can be described by rigid rotation in the inner part, a turnover at , and differential rotation with a velocity of 230 km/s in the outer disk.
The observed continuum emission is mainly due to thermal radiation of cold dust with an average temperature of , with a small gradient from 20 K to 16 K from the center to the outer disk. This cold dust component is necessary to explain our results.
The dust emission closely follows the molecular gas in the central region, but is also detected at large radii where no CO can be seen (Fig. 5). In these regions the dust absorption cross section per H atom at is estimated to be , a value similar to that in the outer parts of other galaxies.
From the emission we estimated a molecular mass of NGC 5907 of , about smaller than from the CO emission. By combining the CO and continuum data we found that the CO-H -conversion ratio increases with galactocentric radius, from at the centre to at .
Accepted for publication in A&A
Figure: Contour map of the continuum emission of NGC 5907 at 245 GHz, overlaid onto an optical image extracted from the Digitized Sky Survey. The beam size of 15'' is indicated by the filled white circle in the lower right corner. The rms noise depends on the location in the map; on the galaxy it is about 1.5 mJy/beam area, contour levels are 4, 8, ..., 24 mJy/beam area