4.3. The ISO-IRAS Color Diagram
A large number of extragalactic broad-band measurements was collected by ISOCAM at wavelengths between 4 and 18 µm, most frequently using the ``LW2'' band centered at 6.75 µm, and the ``LW3'' band centered at 15 µm. The LW2 filter was designed to capture mostly AFE emission, and LW3 was aimed at the continuum range beyond the bulk of the AFE, though it ends up with a small contribution from the 12.5 µm feature. The 6.75-to-15 µm color ratio has emerged as an interesting diagnostic of the radiation environment. It remains relatively constant and near unity as the ISM of galaxies proceeds from quiescent to mildly active. As dust heating increases further, the 15 µm flux increases steeply compared to 6.75 µm, pointing to a significant contribution by dust at color temperature 100 K < TMIR < 200 K, typical of a heating intensity up to 104 times that of the diffuse interstellar radiation field in the local Milky Way (Figure 6; Helou et al. 1997; Helou 1999). While such a temperature could result from classical dust heated within or just outside HII regions, there is no decisive evidence as to the size of grains involved. It is simpler at this time to associate this component empirically with the observed emission spectrum of HII regions and their immediate surroundings (Tran 1998; Contursi 1998). This emission has severely depressed AFE, and is dominated by a steeply rising though not quite a blackbody continuum, consistent with mild fluctuations in grain temperatures, T/T ~ 0.5. This HII region hot dust component at TMIR becomes detectable in systems where the color temperature from the 60-to-100 µm ratio is only TMIR / 2, demonstrating the broad distribution of dust temperatures within any galaxy. The combined data from ISO and IRAS on these systems are consistent with an extension of the ``two-component model'' of infrared emission (see Section 3.4 above) and demonstrate the fallacy of modeling the infrared spectra of galaxies as single temperature dust emission. The low 6.75-to-15 µm color ratio is associated with the active component, and combines in a variable proportion with a component with a 6.75-to-15 µm near unity. This color behavior was observed in the sample of galaxies used for the Key Project on normal galaxies (Helou et al. 1996, Silbermann et al. 1999), and confirmed in the sample of galaxies observed under ISOCAM Guaranteed Time (see Figure 1 in Vigroux et al. 1999). See also Section 4.4 below.
Figure 6. The ISO-IRAS color-color diagram for normal star-forming galaxies. The 7 µ and 15 µ bands do not show any sign of the increased heating signified by the rise of R(60, 100) until this latter ratio exceeds about 0.6, after which the 15 µ band starts detecting the rising continuum from warm ``Very Small Grains''. This plot shows the data for the sample used in the ISO Key Project on Normal Galaxies (cf. Section 4.1).