ISO represented a big improvement compared with IRAS in angular resolution, spectral grasp and sensitivity. Thus, in terms of imaging, the ISOPHOT instrument was able to make a better distinction between diffuse and discrete sources and their colours. Another point was the extension in wavelength coverage to 200µm. In fact observations of nearby galaxies displayed a wide range of FIR colours between various morphological components, implying UV/optical/ near-infrared (NIR) interstellar radiation fields with a wide range of intensities and/or colours:
These features appear to be general to nearby spirals mapped by ISOPHOT. They are perhaps best illustrated by the ISOPHOT map of M 31 (Haas et al. 1998), reproduced in Fig. 2. A ring of 10kpc radius and a diffuse disk component are clearly seen on this image at 170µm. The diffuse emission can be traced out to a radius of 22kpc, so the galaxy has a similar overall size in the FIR as seen in the optical bands. In addition, there is a faint nuclear source, which is seen more prominently in HIRES IRAS 60µm maps at similar resolution and in H. The overall SED, which is also constrained by a 240µm COBE/DIRBE point, can be well described as a a superposition of two modified (m=2) Planck curves, with 16 and 45K. The cold dust component at 16K arises from both the ring structure (30%) and the diffuse disk (70%), illustrating the importance of the diffuse emission at least for this example. The 45K component matches up well with HII region complexes in the ring structure. Associated with each HII region complex are also compact, cold emission sources (see Fig. 3 of Schmidtobreick, Haas & Lemke 2000) with dust temperatures in the 15 to 20K range. These could well represent the parent molecular clouds in the star formation complexes which gave rise to the HII regions. Detailed examination of the morphology of the ring shows a smooth component of cold dust emission as well as the discrete cold dust sources.
There are also beautiful datasets on normal galaxies which are more active in star formation than M 31, notably the Small Magellanic Cloud (type Sm), which was described by Bot et al. (2002) and by Wilke et al. (2002), and M 33 (type Scd; Hippelein et al. 2002). These galaxies also show a prominent diffuse cold dust component, upon which a warm dust component associated with HII region complexes is superimposed. The statistics of the HII region complexes are superior to those in M 31, and, in the case of M 33, the HII region complexes exhibit a trend for the 60:100 µm colour temperature to become colder with increasing galactocentric distance, as observed in our galaxy.
Preliminary results from ISOPHOT maps of nearby spiral galaxies were obtained by Tuffs et al. (1996) for NGC 6946, by Hippelein et al. (1996a, b) for M 51 and M 101 and for a sample of nearby galaxies by Alton et al. (1998b) and Davies et al. (1999). Claims by the latter two references that the FIR disks are more extended than the optical disks are not supported by the higher linear resolution maps of M 31 and M 33 nor by the deep map of NGC 891 (Popescu et al. 2001). However, the maps presented by Alton et al. (1998b) and Davies et al. (1999) were not corrected for transient effects of the detector, and therefore both the calibration and the derived scale lengths are uncertain.
In general there is a wealth of imaging data in the ISO archive still to be exploited, particularly from observations using the dedicated mapping "P32" astronomical observation template. Excellent photometric images are now becoming available for this mode, reduced using new software techniques (see for example the map of M 101 in Tuffs & Gabriel 2002a, b).