The CO luminosity and CO-H₂ conversion factor of diffuse ISM: does CO emission trace dense molecular gas?

Our understanding of the mass of molecular material in extragalactic systems is based on empirical determinations of the so-called X-factor, which relates the measured CO(1-0) intensity to the molecular gas mass. The paper by Liszt et al. highlighted in this issue shows that the X-factor for CO emission from diffuse clouds (thus where much of the hydrogen is atomic) is very similar to the values estimated for dense clouds where hydrogen is overwhelmingly molecular. Moreover, the diffuse cloud CO luminosity of the Milky Way is comparable to the corresponding value from dense clouds. Thus observations of ¹²CO in both the Milky Way and external galaxies need to account for the diffuse cloud contribution to the observed emission.