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

Abstract

We present sensitive CO (J = 1 0) emission line observations of the three metal-poor dwarf irregular galaxies LeoP (Z3% Z e ), SextansA (Z7.5% Z e ), and SextansB (Z7.5% Z e ), 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) 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 LeoP, L CO <12,400 K km s -1 pc 2 for SextansA, and L CO <9700 K km s -1 pc 2 for SextansB. 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.