Top: histograms showing the difference between the polarization angles of dust and CO (J = 2 → 1) maps. The grey histogram is a result of comparing the polarization map in Fig. 3 (left) with that of Fig. 7. In the red histogram the Stokes V-corrected CO (J = 2 → 1) polarization map of Fig. 9 is compared to the dust polarization map. The clear alignment of the polarization vectors of the two different tracers in the red histogram is strong evidence for a conversion of linear to circular polarization. Bottom: similar histograms for the comparison of the polarization vectors of dust and CO (J = 1 → 0) maps.