The Low CO Content of the Extremely Metal‐poor Galaxy I Zw 18

Abstract

We present sensitive molecular line observations of the metal-poor blue compact dwarf I Zw 18 obtained with the IRAM Plateau de Bure interferometer. These data constrain the CO J = 1 → 0 luminosity within our 300 pc (FWHM) beam to be LCO < 1 × 105 K km s-1 pc2 (ICO < 1 K km s-1), an order of magnitude lower than previous limits. Although I Zw 18 is starbursting, it has a CO luminosity similar to or less than nearby low-mass irregulars (e.g., NGC 1569, the SMC, and NGC 6822). There is less CO in I Zw 18 relative to its B-band luminosity, H I mass, or star formation rate than in spiral or dwarf starburst galaxies (including the nearby dwarf starburst IC 10). Comparing the star formation rate to our CO upper limit reveals that unless molecular gas forms stars much more efficiently in I Zw 18 than in our own Galaxy, it must have a very low CO to H2 ratio, ~10-2 times the Galactic value. We detect 3 mm continuum emission, presumably due to thermal dust and free-free emission, toward the radio peak.