CO Emission in Low-Luminosity, H [CSC]i[/CSC]–rich Galaxies

Abstract

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 ≡ 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].