ISM properties in low-metallicity environments III. The dust spectral energy distributions of II Zw40, He 2-10 and NGC1140

We present new 450 and 850 mu m SCUBA data and 1.3 mm MAMBO data of the dwarf galaxies IIZw 40, He 2-10 and NGC 1140. Additional ISOCAM, IRAS as well as ground based data are used to construct the observed mid-infrared to millimeter spectral energy distribution of these galaxies. These spectral energy distributions are modeled in a self-consistent way, as was achieved with NGC 1569 (Galliano et al. 2003, A&A, 407, 159), synthesizing both the global stellar radiation field and the dust emission, with further constraints provided by the photoionisation of the gas. Our study shows that low-metallicity galaxies have very different dust properties compared to the Galaxy. Our main results are: (i) a paucity of PAHs which are likely destroyed by the hard penetrating radiation field; (ii) a very small (similar to 3-4 nm) average size of grains, consistent with the fragmentation and erosion of dust particles by the numerous shocks; (iii) a significant millimetre excess in the dust spectral energy distribution which can be explained by the presence of ubiquitous very cold dust (T = 5-9 K) accounting for 40 to 80% of the total dust mass, probably distributed in small clumps. We derive a range of gas-to-dust mass ratios between 300 and 2000, larger than the Galactic values and dust-to-metals ratios of 1/30 to 1/2. The modeled dust size distributions are used to synthesize an extinction curve for each galaxy. The UV slopes of the extinction curves resemble that observed in some regions in the Large Magellanic Cloud. The 2175 angstrom bumps of the modeled extinction curves are weaker than that of the Galaxy, except in the case of IIZw 40 where we are unable to accurately constrain the 2175 angstrom bump carrier.