Radially stacked CO spectra for r_gal ≤ 4 kpc. We stack over the central 4 kpc. Furthermore, the predicted C¹⁸O(1–0) emission line is shown, based on the ¹³CO(1–0) emission line and assuming a line ratio of 0.2 > R_18/13 > 0.1 (Jiménez-Donaire et al. 2017). The C¹⁸O(1–0) in M101 seems to be fainter than we would expect based on values from EMPIRE. The blue-shaded background indicates the line mask over which we integrate the spectrum.