Abstract

We present sensitive CO (J = 1 -> 0) emission line observations of the three metal-poor dwarf irregular galaxies Leo P (Z similar to 3% Z(circle dot)), Sextans A (Z similar to 7.5% Z(circle dot)), and Sextans B (Z similar to 7.5% Z(circle dot)), 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 (4 sigma) 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(-1) pc(2) for Leo P, L-CO 12,400 K km s(-1) pc(2) for Sextans A, and L-CO 9700 K km s(-1) pc(2) 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-H-2 conversion factor increases sharply as metallicity decreases, or that stars are forming in these three galaxies very efficiently, requiring little molecular hydrogen.