Ratio between [CI]-based molecular gas mass estimates and CO(1–0) line luminosity, computed for the ULIRGs in our sample that have both lines available, and plotted as a function of the infrared luminosity. This ratio is interpreted as the α_CO factor (see Eq. (10)). The right part of the plot shows the resulting distribution of [CI]-based α_CO values. The dotted line indicates the CO-to-H₂ conversion factor for the Milky Way, and the dashed line corresponds to the value commonly used in the literature for (U)LIRGs (Downes & Solomon 1998). The mean and median values obtained for our sample (reported at the top-right corner) are α_CO = 1.9 ± 0.4 M_⊙ and 1.7 ± 0.5 M_⊙ (K km s⁻¹ pc²)⁻¹, respectively.