Starburst Energy Feedback Seen Through HCO$^+$/HOC$^+$ Emission in NGC
  253 from ALCHEMI
  Molecular abundances are sensitive to 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 ALMA
large program ALCHEMI. The distribution of the HOC$^+$(1-0) integrated
intensity shows its association with "superbubbles", cavities created either by
supernovae or expanding HII regions. The observed HCO$^+$/HOC$^+$ abundance
ratios are $\sim 10-150$, and the fractional abundance of HOC$^+$ relative to
H$_2$ is $\sim 1.5\times 10^{-11} - 6\times 10^{-10}$, 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 $G_0\gtrsim 10^3$ if the maximum visual extinction is $\gtrsim 5$, or a
cosmic-ray ionization rate of $\zeta \gtrsim 10^{-14}$ 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.
