CARMA CO OBSERVATIONS OF THREE EXTREMELY METAL-POOR, STAR-FORMING GALAXIES

We present sensitive CO (J = 1 → ?> 0) emission line observations of the three metal-poor dwarf irregular galaxies Leo P (Z ∼ 3% Z⊙), Sextans A (Z ∼ 7.5% Z⊙), and Sextans B (Z ∼ 7.5% Z⊙), all obtained with the Combined Array for Millimeter-wave Astronomy interferometer. While no CO emission was detected, the proximity of the three systems allows us to place very stringent (4σ) upper limits on the CO luminosity (LCO) in these metal-poor galaxies. We find the CO luminosities to be LCO < 2900 K km s−1 pc2 for Leo P, LCO < 12,400 K km s−1 pc2 for Sextans A, and LCO < 9700 K km s−1 pc2 for Sextans B. Comparison of our results with recent observational estimates of the factor for converting between LCO and the mass of molecular hydrogen, as well as theoretical models, provides further evidence that either the CO-to-H2 conversion factor increases sharply as metallicity decreases, or that stars are forming in these three galaxies very efficiently, requiring little molecular hydrogen.