Starburst Energy Feedback Seen Through HCO$^+$/HOC$^+$ Emission in NGC 253 from ALCHEMI. (arXiv:2109.06476v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Harada_N/0/1/0/all/0/1">Nanase Harada</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Martin_S/0/1/0/all/0/1">Sergio Mart&#xed;n</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Mangum_J/0/1/0/all/0/1">Jeffrey G. Mangum</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sakamoto_K/0/1/0/all/0/1">Kazushi Sakamoto</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Muller_S/0/1/0/all/0/1">Sebastien Muller</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tanaka_K/0/1/0/all/0/1">Kunihiko Tanaka</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Nakanishi_K/0/1/0/all/0/1">Kouichiro Nakanishi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Herrero_Illana_R/0/1/0/all/0/1">Rub&#xe9;n Herrero-Illana</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Yoshimura_Y/0/1/0/all/0/1">Yuki Yoshimura</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Muhle_S/0/1/0/all/0/1">Stefanie M&#xfc;hle</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Aladro_R/0/1/0/all/0/1">Rebeca Aladro</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Colzi_L/0/1/0/all/0/1">Laura Colzi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Rivilla_V/0/1/0/all/0/1">V&#xed;ctor M. Rivilla</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Aalto_S/0/1/0/all/0/1">Susanne Aalto</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Behrens_E/0/1/0/all/0/1">Erica Behrens</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Henkel_C/0/1/0/all/0/1">Christian Henkel</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Holdship_J/0/1/0/all/0/1">Jonathan Holdship</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Humire_P/0/1/0/all/0/1">P. K. Humire</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Meier_D/0/1/0/all/0/1">David S. Meier</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Nishimura_Y/0/1/0/all/0/1">Yuri Nishimura</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Werf_P/0/1/0/all/0/1">Paul P. van der Werf</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Viti_S/0/1/0/all/0/1">Serena Viti</a>

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 $sim 1.5times 10^{-11} – 6times 10^{-10}$, 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 $G_0gtrsim 10^3$ 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.

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 $sim 1.5times 10^{-11} – 6times 10^{-10}$, 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 $G_0gtrsim 10^3$ 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.

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