Distributions of HCNH⁺ (2−1) (a), HCNH⁺ (3−2) (b), HCNH⁺ (4−3) (c), and HCNH⁺ (5−4) (d) emission in the CMZ of NGC 253. All intensity maps are integrated from 100 to 400 km s⁻¹ and share a common integrated-intensity scale indicated by the color bar. In each panel, the overlaid contours represent the integrated intensities of respective HCNH⁺ transitions, starting at 1.2 K km s⁻¹ and increasing by 1.2 K km s⁻¹. The positions of the nine GMCs are marked with plus symbols. In each panel, the beam is shown in the lower-left corner.