Energizing Star Formation: The Cosmic-Ray Ionization Rate in NGC 253 Derived from ALCHEMI Measurements of H3O+ and SO
Journal article, 2022
The cosmic-ray ionization rate (CRIR) is a key parameter in understanding the physical and chemical processes in the interstellar medium. Cosmic rays are a significant source of energy in star formation regions, impacting the physical and chemical processes that drive the formation of stars. Previous studies of the circum-molecular zone of the starburst galaxy NGC 253 have found evidence for a high CRIR value: 10(3)-10(6) times the average CRIR within the Milky Way. This is a broad constraint, and one goal of this study is to determine this value with much higher precision. We exploit ALMA observations toward the central molecular zone of NGC 253 to measure the CRIR. We first demonstrate that the abundance ratio of H3O+ and SO is strongly sensitive to the CRIR. We then combine chemical and radiative transfer models with nested sampling to infer the gas properties and CRIR of several star-forming regions in NGC 253 from emission from their transitions. We find that each of the four regions modeled has a CRIR in the range (1-80) x 10(-14) s(-1) and that this result adequately fits the abundances of other species that are believed to be sensitive to cosmic rays, including C2H, HCO+, HOC+, and CO. From shock and photon-dominated/X-ray dominated region models, we further find that neither UV-/X-ray-driven nor shock-dominated chemistry is a viable single alternative as none of these processes can adequately fit the abundances of all of these species.
Author
Jonathan Holdship
University College London (UCL)
Leiden University
Jeffrey G. Mangum
National Radio Astronomy Observatory
Serena Viti
Leiden University
University College London (UCL)
Erica Behrens
University of Virginia
Nanase Harada
National Astronomical Observatory of Japan
The Graduate University for Advanced Studies (SOKENDAI)
Sergio Martin
European Southern Observatory Santiago
Atacama Large Millimeter-submillimeter Array (ALMA)
Kazushi Sakamoto
National Astronomical Observatory of Japan
Sebastien Muller
Chalmers, Space, Earth and Environment, Onsala Space Observatory
Kunihiko Tanaka
Keio University
Kouichiro Nakanishi
National Astronomical Observatory of Japan
The Graduate University for Advanced Studies (SOKENDAI)
Ruben Herrero-Illana
Spanish National Research Council (CSIC)
European Southern Observatory Santiago
Yuki Yoshimura
University of Tokyo
Rebeca Aladro
Max Planck Society
Laura Colzi
Spanish National Research Council (CSIC)
Istituto nazionale di astrofisica (INAF)
Kimberly L. Emig
National Radio Astronomy Observatory
Christian Henkel
Max Planck Society
King Abdulaziz University
Yuri Nishimura
National Astronomical Observatory of Japan
University of Tokyo
Victor M. Rivilla
Spanish National Research Council (CSIC)
Istituto nazionale di astrofisica (INAF)
Paul P. van der Werf
Leiden University
Astrophysical Journal
0004-637X (ISSN) 1538-4357 (eISSN)
Vol. 931 2 89Subject Categories
Astronomy, Astrophysics and Cosmology
Atom and Molecular Physics and Optics
Other Physics Topics
DOI
10.3847/1538-4357/ac6753