The low CO content of the extremely metal-poor galaxy I Zw 18

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 pc(2) ( ICO < 1 K kms(-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, similar to 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.