Comparison between the H₂CO distribution with that of τ₃₅₃, N_HI, and ¹³CO. The spatial distribution of τ₃₅₃ is derived from the Planck measurements (top), the HI column density from EBHIS (Winkel et al. 2016), and the¹³ CO (1−0) integrated intensity from Schneider et al. (2011) at an angular resolution of 10.8. In all panels, the contours correspond to the smoothed integrated optical depth map of H₂CO (1_1,0−1_1,1) integrated from −10 km s⁻¹ to 20 km s⁻¹, and they start from 0.09 km s⁻¹ and increase by 0.09 km s⁻¹. The SSI (see Sect. 6.3) of the two corresponding tracers is indicated in the lower left corner of each panel.