CN (N = 1 − 0)/C¹⁷O (J = 1 − 0) (left) and CN (N = 2 − 1)/C¹⁷O (J = 2 − 1) (right) line ratios obtained from ALCHEMI data (Martín et al. 2021), indicating the presence of photodissociation regions. A 3σ clipping was applied. Red contours indicate the H_α emission in logarithmic scale from MUSE archival data (ID:0102.B-0078(A), PI:Laura Zschaechner). Regions where we detect Class I maser emission in the J₋₁ → (J − 1)₀ − E series (see Sect. 4.1.2) are labeled in red and with slightly larger numbers. The center and edges of the inner Lindblad resonance from Iodice et al. (2014) are denoted with a black ellipse and dash-dotted gray ellipses, respectively. Higher ratios indicate a higher photodissociation level. A square root stretch has been applied for an easy visualization.