ISO--LWS observations of the two nearby spiral galaxies: NGC6946 and   NGC1313
  (Abridged) We present the analysis of the main FIR fine structure lines emission in NGC1313 and NGC6946. We calculate that a component probably associated with the diffuse disks contributes <~40% in N6946 and ~30 % in N1313 to the total [CII] emission. The main PDR physical parameters responsible for the neutral atomic gas emission in N1313 and N6946 do not significantly differ from what Malhotra etal (2001) found by modelling the integrated emission of a sample of 60 normal galaxies,although there are evidences for a beam averaged contribution of a less active component inside NGC6946 higher than its contribution in the integrated emission of normal galaxies. CO and [CII] in N6946 are well correlated with a mean [CII]/CO ratio similar to that of the normal galaxies sample. In N1313 the [CII]/CO seems to systematically increase from the North to the south, along the S-shaped spiral arm, indicating much more inhomogeneous conditions than in N6946. HI and [CII] in N6946 are completely de-correlated, probably because they arise from different gas components. In N1313 we successfully detect two distinct gas components: a cirrus-like component where HI and [CII] are weakly correlated as observed in our Galaxy, and a component associated with dense PDRs completely de-correlated from HI as observed in N6946.Finally, we find that the HI residing in dense PDRs and presumably recently photo-dissociated, constitutes a few % of the total HI. In turn, this dense gas component produces most of the [CII] emission emitted by the atomic neutral medium. 
A. Contursi, M.J. Kaufman, G. Helou, D. J. Hollenbach, J. Brauher,   G.J. Stacey, D.A. Dale, S. Malhotra, M. Rubio, R.H. Rub...