2.2. Spatial distribution of the IR emission
Although all infrared maps show similar features, i.e. enhanced emission in the spiral arms and nucleus, a number of properties are emerging. Alton et al. (1998b) and Davies et al. (1999) have shown that ISOPHOT 200 µm maps of normal spirals have a scale-length which is as large, or even larger than that of the stars. As the dust temperature probably decreases outwards, this implies that the scale-length of the cold dust column density is even larger. Comparison of radial profiles between cold dust traced at 850 µm with SCUBA and the atomic and molecular components show that while the cold dust is more extended than the molecular gas, it is still less extended than the HI gas ([Davies et al. 1999]). Furthermore the radial distributions of dust and molecular gas agree well in the central part of the disk ([Israel et al. 1999]). This suggests that this cold dust is mostly associated with the molecular gas but that a non-negligible fraction also resides in the HI gas. As we progress outward in the galaxy, the ISM becomes mostly atomic and the dust phase associated to HI becomes more apparent.
Warmer dust, such as that detected by IRAS or ISOCAM, is clearly less extended than the cold phase and has a scale length smaller than that of the stars. In NGC 6946, Malhotra et al. (1996) have shown that the ISOCAM profiles at 6.75 and 15 µm have a scale length similar to that of the molecular gas and H emission. The 60 µm scale length is intermediate between that of molecular gas and stars. In fact it is close to that of the HI + H2 profiles, likely showing a transition between the warm and cold component. In a thorough ISOCAM study of the SW ring in M31, Pagani et al. (1999) show a clear spatial correlation of the MIR emission with HI and H2, while the correlation is much poorer with H. This effect is also seen in NGC 891 ([Mattila et al. 1999]) and NGC 7331 ([Smith et al. 1998]).
The association of the MIR radiation with the HI + H2 gas in the inner part of the galaxies confirms that the emission originates mainly from the PDRs at the outer layers of interstellar clouds. Instead, the larger scale-length of the FIR/mm component points, for the colder dust, to an origin in the inner parts of interstellar clouds.