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 arrow right 0 transition. We use dust emission models to derive a dust mass upper limit of only M sub(dust) [< or =, slant] 1.1 x 10 super(4) M sub([middot in circle]) (3[sigma] limit). This upper limit is driven by the non-detection at 160 mu m, 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 sub(Dust)/M sub(gas) [< or =, slant] 5.0 x 10 super(-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 x 10 super(-4). We also show that the infrared spectral energy distribution is similar to that of starbursting systems.