ISM properties in low-metallicity environments
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abstract
We present new 450 and 850 micron SCUBA data and 1.3 mm MAMBO data of the
dwarf galaxies II Zw 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), 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 (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 II Zw 40, where we are unable to accurately
constrain the 2175 angstrom bump carrier.
