2021ApJ...923...24H

Molecular abundances are sensitive to the UV photon flux and cosmic-ray ionization rate. In starburst environments, the effects of high-energy photons and particles are expected to be stronger. We examine these astrochemical signatures through multiple transitions of HCO⁺ and its metastable isomer HOC⁺ in the center of the starburst galaxy NGC 253 using data from the Atacama Large Millimeter/submillimeter Array large program ALMA Comprehensive High-resolution Extragalactic Molecular inventory. The distribution of the HOC⁺(1-0) integrated intensity shows its association with "superbubbles," cavities created either by supernovae or expanding H II regions. The observed HCO⁺/HOC⁺ abundance ratios are ∼10-150, and the fractional abundance of HOC⁺ relative to H₂ is ∼1.5 x 10^–11-6 x 10^–10, which implies that the HOC⁺ abundance in the center of NGC 253 is significantly higher than in quiescent spiral arm dark clouds in the Galaxy and the Galactic center clouds. Comparison with chemical models implies either an interstellar radiation field of G₀ >= 10³ if the maximum visual extinction is >=5, or a cosmic-ray ionization rate of ζ >= 10^–14 s^–1 (3-4 orders of magnitude higher than that within clouds in the Galactic spiral arms) to reproduce the observed results. From the difference in formation routes of HOC⁺, we propose that a low-excitation line of HOC⁺ traces cosmic-ray dominated regions, while high-excitation lines trace photodissociation regions. Our results suggest that the interstellar medium in the center of NGC 253 is significantly affected by energy input from UV photons and cosmic rays, sources of energy feedback.