Distribution of HCNH⁺ (2−1) in the CMZ of NGC 253. (a) HCNH⁺ (2−1) integrated intensity map (taken from Harada et al. 2024), clipped at the 3σ level. The contours start from 2.7 K km s⁻¹ (3σ) and increase by 2.7 K km s⁻¹. (b) HCNH⁺ (21) integrated intensity map that has been convolved to a circular beam of 4″ . The smoothed image manifests the large-scale structures traced by HCNH⁺ transitions. The contours start from 0.9 K km s⁻¹ (3σ) and increase by 0.9 K km s⁻¹. In each panel, the beam is shown in the lower-left corner, and the pluses indicate the positions of the nine most prominent GMCs.