Posterior α_CO PDFs for HATLAS114625 (left) and HATLAS121446 (right) starbursts. For each galaxy, we consider four dynamical mass models encompassing different underlying surface density mass distributions and hydrostatic equilibrium approximations. We also show the cumulative probability distribution in each panel with our α_CO upper limit defined as P(x < α_CO, uplim) ≈ 0.997 (i.e. 3-σ) and estimated by using the thick-disc plus Sérsic dynamical mass model. The coloured arrows indicate the median α_CO value for each PDF. We find median α_CO estimates consistent with the ULIRG-like value for both starburst galaxies.