THE INTERSTELLAR-MEDIUM IN THE EDGE-ON GALAXY NGC-5907 - COLD DUST AND MOLECULAR LINE EMISSION

In this paper we present new observations of the interstellar medium i n the non-interacting edge-on galaxy NGC 5907. We have observed the J = 2 - 1 and J = 1 - 0 lines of the (CO)-C-12 molecule and radio contin uum emission at lambda 1.2 mm. The distribution of the molecular gas ( as traced by CO) shows a maximum in the central region and a ring or s piral arm at r similar to 7 kpc. Further analysis of the major axis di stribution reveals evidence for an inner ring-like structure at r simi lar to 3.5 kpc. The kinematics can be described by rigid rotation in t he inner part, a turnover at similar to 3 kpc, and differential rotati on with a velocity of 230 km/s in the outer disk. The observed continu um emission is mainly due to thermal radiation of cold dust with an av erage temperature of T-d = 18 K, with a small gradient from 20 K to 16 K from the centre to the outer disk. This cold dust component is nece ssary to explain our results. The dust emission closely follows the mo lecular 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 s ection per FT atom at lambda 1.2 mm is estimated to be sigma(lambda)(H I) similar to 4.5 10(-27) cm(2), a value similar to that in the outer parts of other galaxies. From the lambda 1.2mm emission we estimated a molecular mass of NGC 5907 of 0.9 10(9) M., about 50 % smaller than f rom the CO emission. By combining the CO and continuum data we found t hat the CO-H-2-conversion ratio increases with galactocentric radius, from similar to 0.7 10(20) at the centre to similar to 1.6 10(20) cm(- 2)/K km s(-1) at r = 7.5 kpc. A comparison of NGC 5907 and other edge- on galaxies concerning gas distribution, central kinematics and dust p roperties is presented.