The emerging result of all ISOPHOT statistical studies was that the SEDs of normal galaxies in the 40 - 200 µm spectral range require both warm and cold dust emission components to be fitted. First indications of this result were given by the multi-filter ISOPHOT photometry obtained for 8 inactive spiral galaxies (Krügel et al. 1998; Siebenmorgen, Krügel & Chini 1999). Stickel et al. (2000) and Contursi et al. (2001) found a cold dust component for most of the galaxies in their samples (The Serendipity and the Coma/A1367 samples, respectively), for which upper limits for the cold dust temperatures were inferred. The cold dust component is most prominent in the most "quiescent" galaxies, like those contained in the ISOPHOT Virgo Cluster Deep sample, where the cold dust temperatures were found to be broadly distributed, with a median of 18K (Popescu et al. 2002), some 8 - 10K lower than would have been predicted by IRAS. The corresponding dust masses were correspondingly found to be increased by factors of typically 2 - 10 (Stickel et al. 2000) for the Serendipity Sample and by factors 6 - 13 (Popescu et al. 2002) for the Virgo Sample, with respect with previous IRAS determinations. As a consequence, the derived gas-to-dust ratios are much closer to the canonical value of ~ 160 for the Milky Way (Stickel et al. 2000, Contursi et al. 2001), but with a broad distribution of values (Popescu et al. 2002). The FIR properties of the analysed galaxies do not seem to be affected by the environment (Contursi et al. 2001).
Of particular interest are the results concerning the trends with Hubble type found by Popescu et al. (2002) for the ISOPHOT Virgo Cluster Deep sample. A tendency was found for the temperatures of the cold dust component to become colder, and for the cold dust surface densities (normalised to optical area) to increase with increasing lateness in the Hubble type (Figs. 5a,b). A particularly surprising result was the low dust temperatures (ranging down to less than 10K) and large dust masses associated with the Virgo Im and Blue Compact Dwarf (BCD) galaxies. Another important trend is the increase of the normalised (to K' band magnitude) FIR luminosity as we progress from the early to the later Hubble types (Fig. 5c). This result was later confirmed by Bendo et al. (2002b) for the RSA sample. A related result was also obtained by Pierini et al. (1999) for the LWS data on Virgo galaxies, where a strong correlation of normalised [CII] emission with H equivalent widths was interpreted as a trend of increasing star-formation rate along the Hubble sequence.
Figure 5. Trends with Hubble type for the ISOPHOT Virgo Cluster Deep sample (Popescu et al. 2002). The distribution of a) cold dust temperatures TDcold; b) cold dust mass surface densities MDcold / D2; c) normalised FIR luminosity (to the K' band magnitude) for different Hubble types. The hatched histograms represent the distributions for the galaxies with SEDs fitted by only one dust component. The filled histograms represent the distributions for the galaxies with detections only at 100 and 170µm.
Finally, Popescu & Tuffs (2002b) also found an increase of the ratio of the dust emission to the total stellar emitted output along the Hubble sequence. This correlation is quite strong, ranging from typical values of ~ 15% for early spirals to up to ~ 50% for some late spirals (Popescu & Tuffs 2002b). This, together with the trend for a decrease in the temperature of the cold dust, would suggest a trend of increasing opacities with increasing star-formation activity. The extreme BCDs can have even higher percentages of their bolometric output re-radiated in the thermal infrared. This correlation can be also interpreted as a sequence from normal to dwarf gas rich galaxies, with the dwarfs having an increased contribution of the FIR output to the total bolometric output. These findings could be important for our perception of the distant Universe, where, according to the hierarchical galaxy formation scenarios, gas rich dwarf galaxies should prevail. We would then expect these galaxies to make a higher contribution to the total FIR output in the early Universe than previously expected.