α_CII (left), α_CI (middle), α_CO (right) versus metallicity for power law models. The shades, symbols, and legends are described in Fig. 4. We indicate the equations of linear regressions fitted to the 2D-PDF of the whole sample, and the associated coefficients of determination R². We also show the Galactic α_CO value from B13 and the range of values compatible with it within 30% (gray-shaded area), the α_CO vs. metallicity relations from M20, Schruba et al. (2012), Amorín et al. (2016), and Accurso et al. (2017), and the α_[CI] vs. metallicity relations from Glover & Clark (2016) and Heintz & Watson (2020). We use a Galactic conversion factor of 3.2 M_⊙ pc⁻²(K km s⁻¹)⁻¹ to correct for the helium mass (factor of 1.36) when comparing to the model prediction of M(H₂).