6. Summary and conclusions
We performed molecular line and bolometer observations of the edge-on galaxy NGC 5907. The important results which were concluded from that observations can be summarized as follows:
The distribution of the molecular gas (as traced by CO) shows a maximum in the central region and a ring or spiral arm structure at . Further analysis of the major axis distribution reveals a second ring at , near the turnover point of the rotation curve. 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. Further a high-velocity component is present in the central region which is most probably due to a central bar.
The observed 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 centre to the outer disk. This cold dust component is necessary to explain our results. The warmer dust detected by IRAS is not sufficient to account for the continuum emission at .
This dust emission follows closely the molecular gas in the central region, but is also detected at large radii, where no CO can be seen. In these regions the dust absorption cross section per H atom could be estimated to . Two outer maxima of the dust distribution, which correlate with similar features in the HI emission, may indicate the existence of outer spiral arms at a radius of about 11 kpc.
Because of its optical thinness the thermal continuum emission of cold dust provides an independent way to estimate hydrogen column densities. 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-H2 -conversion factor increases with galactocentric radius, from at the centre to at .
The comparison of our results concerning gas distribution, kinematics and dust properties lead to some similarities between NGC 5907 and other edge-on galaxies. NGC 5907 seems to be intermediate between the other galaxies in respect of some properties (central kinematics, intensities), probably because of its intermediate star formation.
© European Southern Observatory (ESO) 1997
Online publication: May 5, 1998