The sensitivity of ISO and its spectral grasp extending to 200 µm made it the first observatory capable of routinely measuring the infrared emission corresponding to the bulk of starlight absorbed by interstellar dust in "normal" (1) galaxies. Here we review ISO's view of the morphological and spectral energy distribution (SED) characteristics of the dust continuum emission (emitted in the 40-200 µm spectral range) from normal galaxies, and its interpretation. In this review we only discuss the results from the ISOPHOT instrument (Lemke et al. 1996) on board ISO. For the spectral observations of these systems we refer to the review at this meeting by Helou.
Although technically more demanding than observations in the Mid-Infrared (MIR) regime, only observations in the Far-Infrared (FIR) directly probe the role played by dust in the energy budget of star-forming galaxies. All star-forming galaxies are at least in part optically thick in the ultraviolet (UV)-optical regime, and the absorbed energy is predominantly re-radiated in the FIR. But the real investigative power of FIR astronomy lies in the fact that even for optically thin components of the interstellar medium, the large grains which dominate the FIR emission are in (or near to) equilibrium with the ambient interstellar radiation field (ISRF). Therefore, the grains act as test particles with FIR colours characteristic of the intensity and colour of the ISRF. This is illustrated in Fig. 1, which shows the predicted variation of infrared colours with radiation field intensities, for standard filter combinations of the ISOPHOT and ISOCAM instruments (on board ISO), and of the IRAS survey.
Figure 1. Predicted colour ratios for standard filter combinations 170/100 µm (solid line); 100/60 µm (dotted line); 60/25 µm (dashed line) and 25/12 µm (dot-dashed line) as a function of the strength of the local ISRF , where = 1 near the sun. The calculations were made for spherical grains of astrophysical silicate, with the optical properties given by Laor & Draine (1993). A grain size distribution n(a) da a-3.5 was assumed, where a is the grain radius ( 0.001 a 0.25 µm). The colour of the radiation field illuminating the grains is fixed to that determined for the solar neighbourhood by Mezger, Mathis & Panagia (1982).
In particular, a filter set covering the range 60 to 170 µm probes intensities in the ISRF ranging from those expected for HII regions to those expected in the outskirts of disks of normal galaxies. By contrast, the MIR colour ratios are almost independent of the intensity of the ISRF, since they are determined by the relative abundance of small, impulsively heated grains.
1 We use the term "normal" to denote star-forming systems not undergoing a starburst, and not dominated by AGN activity. Back.