2001A&A...366..811T

We present ¹²CO J = 1->0 and J = 2->1 observations of the low metallicity (12 + log(O/H) = 7.74) Local Group dwarf irregular galaxy WLM made with the 15 m SEST and 14 m FCRAO telescopes. Despite the presence a number of HII regions, we find no CO emission. We obtain low upper limits on the integrated intensity (I_CO≤0.18K.km/s for CO (1->0)). The non-detection is consistent with the result of Taylor, Kobulnicky & Skillman (1998AJ....116.2746T), that dwarf galaxies below a metallicity of ∼7.9 are not detected in CO emission. WLM shows that this trend continues for low metallicity galaxies even as their metallicities approach 7.9. These results are consistent with the models of the metal poor ISM by Norman & Spaans (1997ApJ...480..145N). By comparing our CO data with observations of star formation in WLM, we find evidence for a high CO to H₂ conversion factor.