Starburst Energy Feedback Seen Through HCO${}^{+}$/HOC${}^{+}$ Emission in NGC 253 from ALCHEMI

Abstract

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 $\hat{I}\P \gtrsim 10^{-14}$ 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.

57 pages, 32 figures, accepted for publication in ApJ