Radially stacked ¹²CO(1–0) (orange) and ¹³CO(1–0) (purple) spectra. For a better comparison, we scale the ¹³CO(1–0) brightness temperature up by a factor 5. The S/N of the ¹³CO(1–0) is indicated in each panel (green indicates spectra where S/N₁₃ > 5. We stack in radial bins of size 1.25 kpc. The shaded region indicates the spectral range over which we integrate the spectra. We detect significant ¹³CO(1–0) emission out to r_gal ∼ 8 kpc.