Comparison of the velocity profiles of the ¹³CO(4-3) (blue solid line) and ¹²CO(3-2) lines (green dotted line). The uncertainties on the ¹²CO spectrum are not plotted, since they are negligible compared with the ¹³CO ones. We normalized all the lines to have ∫S_ν dν = 500 Jy km s⁻¹. With this normalization, the peak flux of a line with a typical 500 km s⁻¹ width is unity. The red dashed line is the ¹³CO(3-2) profile measured by Zhang et al. (2018).