Starburst Energy Feedback Seen through HCO+/HOC+ Emission in NGC 253 from ALCHEMI
Journal article, 2021
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 similar to 10-150, and the fractional abundance of HOC+ relative to H-2 is similar to 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 G (0) greater than or similar to 10(3) if the maximum visual extinction is greater than or similar to 5, or a cosmic-ray ionization rate of zeta greater than or similar to 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.
Author
Nanase Harada
The Graduate University for Advanced Studies (SOKENDAI)
Academia Sinica
National Astronomical Observatory of Japan
Sergio Martin
European Southern Observatory Santiago
Atacama Large Millimeter-submillimeter Array (ALMA)
Jeffrey G. Mangum
National Radio Astronomy Observatory
Kazushi Sakamoto
Academia Sinica
Sebastien Muller
Chalmers, Space, Earth and Environment, Onsala Space Observatory
Kunihiko Tanaka
Keio University
Kouichiro Nakanishi
The Graduate University for Advanced Studies (SOKENDAI)
National Astronomical Observatory of Japan
Ruben Herrero-Illana
Spanish National Research Council (CSIC)
European Southern Observatory Santiago
Yuki Yoshimura
University of Tokyo
Stefanie Muhle
University of Bonn
Rebeca Aladro
Max Planck Society
Laura Colzi
Istituto nazionale di astrofisica (INAF)
Spanish National Research Council (CSIC)
Victor M. Rivilla
Spanish National Research Council (CSIC)
Istituto nazionale di astrofisica (INAF)
Susanne Aalto
Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics
Erica Behrens
University of Virginia
Christian Henkel
King Abdulaziz University
Max Planck Society
Chinese Academy of Sciences
Jonathan Holdship
Leiden University
University College London (UCL)
P. K. Humire
Max Planck Society
David S. Meier
New Mexico Institute of Mining and Technology
National Radio Astronomy Observatory
Yuri Nishimura
National Astronomical Observatory of Japan
University of Tokyo
Paul P. van der Werf
Leiden University
Serena Viti
Leiden University
University College London (UCL)
Astrophysical Journal
0004-637X (ISSN) 1538-4357 (eISSN)
Vol. 923 1 24Subject Categories (SSIF 2011)
Meteorology and Atmospheric Sciences
Astronomy, Astrophysics and Cosmology
Atom and Molecular Physics and Optics
DOI
10.3847/1538-4357/ac26b8