DUST-TO-GAS RATIO IN THE EXTREMELY METAL-POOR GALAXY I Zw 18

The blue compact dwarf galaxy I Zw 18 is one of the most metal-poor systems known in the local universe (12+log(O/H) = 7.17). In this work we study I Zw 18 using data from Spitzer , Herschel Space Telescope , and IRAM Plateau de Bure Interferometer. Our data set includes the most sensitive maps of I Zw 18, to date, in both the far-infrared and the CO J = 1 → 0 transition. We use dust emission models to derive a dust mass upper limit of only M dust ⩽ 1.1 × 10 4 M ☉ (3σ limit). This upper limit is driven by the non-detection at 160 μm, and it is a factor of 4–10 times smaller than previous estimates (depending on the model used). We also estimate an upper limit to the total dust-to-gas mass ratio of M Dust / M gas ⩽ 5.0 × 10 −5 . If a linear correlation between the dust-to-gas mass ratio and metallicity (measured as O/H) were to hold, we would expect a ratio of 3.9 × 10 −4 . We also show that the infrared spectral energy distribution is similar to that of starbursting systems.