CO emission in low-luminosity, H I-rich galaxies

We present ¹²CO 1 → 0 observations of 11 low-luminosity (M_B > -18), H I-rich dwarf galaxies. Only the three most metal-rich galaxies, with 12 + log (O/H) ≈ 8.2, are detected. Very deep CO spectra of six extremely metal-poor systems [12 + log (O/H) ≤ 7.5] yield only low upper limits on the CO surface brightness, I_CO < 0.1 K km s^-1. Three of these six have never before been observed in a CO line, while the others now have much more stringent upper limits. For the very low metallicity galaxy Leo A, we do not confirm a previously reported detection in CO, and the limits are consistent with another recent nondetection. We combine these new observations with data from the literature to form a sample of dwarf galaxies that all have CO observations and measured oxygen abundances. No known galaxies with 12 + log (O/H) < 7.9 (Z < 0.1 Z_⊙) have been detected in CO. Most of the star-forming galaxies with higher [12 + log (O/H) > 8.1] metallicities are detected at similar or higher I_CO surface brightnesses. The data are consistent with a strong dependence of the I_CO/M_H2 ≡ X_CO conversion factor on ambient metallicity. The strikingly low upper limits on some metal-poor galaxies lead us to predict that the conversion factor is nonlinear, increasing sharply below ∼1/10 of the solar metallicity [12 + log (O/H) ≤ 7.9].