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. 2003, Wilke et al. 2004). 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 T_D = 20.3 K.

Bot et al. 2004 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. 2004 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 1973) and of extended HI emission (Young & Lo 1997), and was detected by IRAS (Rice et al. 1988, Knapp et al. 1989), with a SED steeply rising between 60 and 100 µm. Based on ISOPHOT observations, Haas (1998) 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.

The properties of gas-rich dwarf galaxies were found to markedly differ from those of spiral galaxies at shorter IR wavelengths. Thus, when comparing the MIR emission of five IBm galaxies to their H or FIR properties, Hunter et al. 2001 discovered a noticeable deficit of the 6.75 µm component in their galaxies. This deficit is not observed at 15 µm, i.e. the 6.75 / 15 µm flux ratio is very different in IBm galaxies than in spirals, which is interpreted as tracing both a deficit of PAH, possibly destroyed by the radiation field, and an increase of the mean temperature of the very small grains, again linked to the radiation field (see also O'Halloran et al. 2000). This interpretation of the observations is confirmed by detailed spectroscopic measurements of similar objects. In NGC 5253 the MIR SED is dominated by a "hot" continuum from which the PAH features are absent Crowther et al. 1999. The blue compact galaxy II Zw 40 presents a similar spectrum, i.e. a strongly rising continuum with no bands Madden 2000. In both cases this is interpreted as being the result of very hard radiation fields that both destroy the PAH and shift the continuum emission from very small grains toward shorter wavelengths. These galaxies are essentially small starburst regions and their IR spectrum thus reflects the IR properties of starburst galaxies discussed elsewhere in this volume. That we are indeed seeing a gradual destruction of PAHs, and not an effect of metallicity that would prevent the formation of these species is demonstrated by the fact that we find galaxies of similar metallicity (NGC 5253 and NGC 1569 for instance at Z/4 - Z/5) without or with PAH emission (see Galliano et al. 2003 for NGC 1569). In fact, even the SMC at Z / 10 has a MIR spectrum with pronounced PAH features Reach et al. 2000.

Many of the qualitative indications about grain and PAH abundance indicated above have been quantitatively estimated by Galliano et al. (2003) in their detailed modelling of the UV-optical/MIR-FIR-submm SED of the low metallicity nearby dwarf galaxy NGC 1569. (Fig. 15). 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 confirm the paucity of 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 15. NGC 1569 observations and modeled SED from Galliano et al. (2003). 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.