... factor

Units: molecules cm^-2/ (K km s^-1).

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... Galaxy

The values given by Israel (1997, 2000) include the effects of the radiation field, implicitly containing the radiation field/metallicity correlation of his galaxy sample. $X_{\rm CO}$ is positively and almost linearly correlated with radiation field, so the dependence of $X_{\rm CO}$ for constant radiation field is even larger: $\log$  $X_{\rm CO}$ $\propto$ -4 [O/H] (Israel 2000). By adopting the coefficient -2.5 we implicitly assume the same radiation/metallicity correlation within the Galaxy as over his galaxy sample.

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