3.2. Infrared emission from within dwarf galaxies
The distribution of cold dust within dwarf galaxies could be studied in only one case, namely in the resolved (1.5' resolution) 170 µm ISOPHOT map of the Small Magellanic Cloud (Wilke et al. , ). The 170 µm ISO map of the SMC reveals a wealth of structure, not only consisting of filamentary FIR emitting regions, but also of numerous (243 in total) bright sources which trace the bar along its major axis as well as the bridge which connects the SMC to the LMC. Most of the brighter sources have cold components, associated with molecular clouds. The discrete sources were found to contribute 28%, 29% and 36% to the integrated flux densities at 60, 100 and 170 µm, respectively. The SED was modelled by the superposition of 45K, 20.5K and 10K blackbody components with emissivity index = 2. The average dust colour temperature (averaged over all pixels of the 170/100 colour map) was found to be TD = 20.3 K.
Bot et al.  have compared the FIR map of the SMC with an HI map of similar resolution. This reveals a good spatial correlation of the two emissions in the diffuse regions of the maps (regions that fall outside of the correlation are either hot star-forming regions, or cold molecular clouds with no associated HI). Adding the IRAS data allows them to compute the FIR emissivity per unit H atom. Bot et al.  found that this emissivity is lower than in the Milky Way, and in fact it is even lower than the lower metallicity (Z / 10) of the SMC would imply, suggesting that depletion mechanisms at work in the ISM have more than a linear dependence on metallicity.
Although FIR emission from dwarf galaxies has been associated only with gas-rich dwarfs, in one particular case such emission has been detected in a dwarf elliptical galaxy, as well. This is the case of NGC 205, one of the companions of M 31, classified as a peculiar dE5. This galaxy shows signatures of recent star-formation (Hodge ) and of extended HI emission (Young & Lo ), and was detected by IRAS (Rice et al. , Knapp et al. , with a SED steeply rising between 60 and 100 µm. Based on ISOPHOT observations, Haas  showed that the FIR emission is resolved and similar to that seen in HI. He also presented evidence for a very cold dust component, of 10 K, coming from the center of the galaxy.
Galliano et al.  modelled the UV-optical/MIR-FIR-submm SED of the low metallicity nearby dwarf galaxy NGC 1569. (Fig. 10). This study is noteworthy in that it constrains the grain size distribution through the MIR-FIR ISOCAM and ISOPHOT observations and therefore gives more specific information about grain properties in dwarf galaxies. The results indicate a paucity of Polycyclic Aromatic Hydrocarbons (PAHs) due to an enhanced destruction in the intense ambient UV radiation field, as well as an overabundance (compared to Milky Way type dust) of small grains of size ~ 3nm, possibly indicative of a redistribution of grain sizes through the effect of shocks.
Figure 10. NGC 1569 observations and modeled SED from Galliano et al. . The data are indicated by crosses: vertical bars are the errors on the flux density values and the horizontal bars indicate the widths of the broadbands. The lines show the predictions of the dust model with its different components. Diamonds indicate the model predictions integrated over the observational broadbands and colour-corrected.