2015ApJ...814...30W

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 (L_CO) in these metal-poor galaxies. We find the CO luminosities to be L_CO< 2900 K km/s pc² for Leo P, L_CO< 12,400 K km/s pc² for Sextans A, and L_CO< 9700 K km/s pc² for Sextans B. Comparison of our results with recent observational estimates of the factor for converting between L_CO and the mass of molecular hydrogen, as well as theoretical models, provides further evidence that either the CO-to-H₂conversion factor increases sharply as metallicity decreases, or that stars are forming in these three galaxies very efficiently, requiring little molecular hydrogen.