The Dust Mass of the Extremely Metal Poor Galaxy I Zw 18: Imaging at 100 and 160 microns

We will use PACS scan maps to measure the 100 and 160 micron flux of I Zw 18, the lowest metallicity galaxy within 20 Mpc. I Zw 18 offers a nearby window into to the processes which make stars in the primitive galaxies observed in the high redshift Universe. We will measure the dust mass of I Zw 18, and combine this with data already in hand to determine its dusttogas ratio, yielding *the* crucial measurement to establish how dusttogas ratio relates to metallicity in galaxies. Measuring the dust mas of I Zw 18 requires to accurately constraint of the long wavelength side of the dust infrared spectral energy distribution. Existing data has proven unsuccessful at detecting this galaxy longwards of 70 um. Our observations are designed to detect I Zw 18 at 100 um and either detect it or yield a strong constraint at 160 um. With a very modest time investment (under 5 hours) these measurements will determine whether the dusttogas ratio is linear or superlinear with metallicity. Because the dusttogas ratio is related to the formation and destruction of molecules and the heating of the ISM, this is a key measurement to understand the impact of metallicity on the evolution of galaxies. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]