Different α_CO models across varying metallicites. In this work, we adopt the model proposed by Genzel et al. (2015), shown in the blue points, which corresponds to the geometric mean of the prescriptions of Bolatto et al. (2013) and Genzel et al. (2012), as indicated in Eq. (3). As a reference, we also include the model proposed by Accurso et al. (2017), which would lead to higher values for α_CO for the galaxies in our sample. To prevent extrapolation into poorly constrained regions of parameter space, we impose an upper limit on α_CO, set to the lowest emission metallicity measured in our sample, which leads to an α_CO ∼  15.