Neural Network Constraints on the Cosmic-Ray Ionization Rate and Other Physical Conditions in NGC 253 with ALCHEMI Measurements of HCN and HNC

Abstract We use a neural network model and Atacama Large Millimeter/submillimeter Array (ALMA) observations of HCN and HNC to constrain the physical conditions, most notably the cosmic-ray ionization rate (CRIR, ζ ), in the Central Molecular Zone (CMZ) of the starburst galaxy NGC 253. Using output from the chemical code UCLCHEM , we train a neural network model to emulate UCLCHEM and derive HCN and HNC molecular abundances from a given set of physical conditions. We combine the neural network with radiative transfer modeling to generate modeled integrated intensities, which we compare to measurements of HCN and HNC from the ALMA Large Program ALCHEMI. Using a Bayesian nested sampling framework, we constrain the CRIR, molecular gas volume and column densities, kinetic temperature, and beam-filling factor across NGC 253's CMZ. The neural network model successfully recovers UCLCHEM molecular abundances with ∼3% error and, when used with our Bayesian inference algorithm, increases the parameter-inference speed tenfold. We create images of these physical parameters across NGC 253's CMZ at 50 pc resolution and find that the CRIR, in addition to the other gas parameters, is spatially variable with ζ ∼ a few ×10 −14 s −1 at r ≳ 100 pc from the nucleus, increasing to ζ > 10 −13 s −1 at its center. These inferred CRIRs are consistent within 1 dex with theoretical predictions based on nonthermal emission. Additionally, the high CRIRs estimated in NGC 253's CMZ can be explained by the large number of cosmic-ray-producing sources as well as a potential suppression of cosmic-ray diffusion near their injection sites.

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Publication Details

Journal
The Astrophysical Journal
Published
2024-11-29
DOI
https://doi.org/10.3847/1538-4357/ad85db
Citations
7
Primary Topic
Stellar, planetary, and galactic studies
Type
article
Field-Weighted Citation Impact
2.32
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article

Neural Network Constraints on the Cosmic-Ray Ionization Rate and Other Physical Conditions in NGC 253 with ALCHEMI Measurements of HCN and HNC

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7 citations
The Astrophysical Journal
Stellar, planetary, and galactic studies
2.32
article

Neural Network Constraints on the Cosmic-Ray Ionization Rate and Other Physical Conditions in NGC 253 with ALCHEMI Measurements of HCN and HNC

David S. Meier, Laura Colzi, S. Müller, Kunihiko Tanaka, V. M. Rivilla, Ko-Yun Huang, Mathilde Bouvier, S. Martín, Kazushi Sakamoto, Cosima Eibensteiner, Nanase Harada, Joshua Butterworth, S. Viti, P. van der Werf, Erica Behrens, C. Henkel, Jonathan Holdship, J. G. Mangum
article en
7 citations

Abstract

Abstract We use a neural network model and Atacama Large Millimeter/submillimeter Array (ALMA) observations of HCN and HNC to constrain the physical conditions, most notably the cosmic-ray ionization rate (CRIR, ζ ), in the Central Molecular Zone (CMZ) of the starburst galaxy NGC 253. Using output from the chemical code UCLCHEM , we train a neural network model to emulate UCLCHEM and derive HCN and HNC molecular abundances from a given set of physical conditions. We combine the neural network with radiative transfer modeling to generate modeled integrated intensities, which we compare to measurements of HCN and HNC from the ALMA Large Program ALCHEMI. Using a Bayesian nested sampling framework, we constrain the CRIR, molecular gas volume and column densities, kinetic temperature, and beam-filling factor across NGC 253's CMZ. The neural network model successfully recovers UCLCHEM molecular abundances with ∼3% error and, when used with our Bayesian inference algorithm, increases the parameter-inference speed tenfold. We create images of these physical parameters across NGC 253's CMZ at 50 pc resolution and find that the CRIR, in addition to the other gas parameters, is spatially variable with ζ ∼ a few ×10 −14 s −1 at r ≳ 100 pc from the nucleus, increasing to ζ > 10 −13 s −1 at its center. These inferred CRIRs are consistent within 1 dex with theoretical predictions based on nonthermal emission. Additionally, the high CRIRs estimated in NGC 253's CMZ can be explained by the large number of cosmic-ray-producing sources as well as a potential suppression of cosmic-ray diffusion near their injection sites.

The Astrophysical JournalVol. 977(1)
Leiden University (NL), Institute of Astronomy and Astrophysics, Academia Sinica (TW), University of Bonn (DE), Chinese Academy of Sciences (CN), The Graduate University for Advanced Studies, SOKENDAI (JP), Keio University (JP), New Mexico Institute of Mining and Technology (US), National Radio Astronomy Observatory (US), Atacama Large Millimeter Submillimeter Array (CL), Xinjiang Astronomical Observatory (CN), McCormick (United States) (US), Centro de Astrobiología (ES), European Southern Observatory (CL), Max Planck Institute for Radio Astronomy (DE), National Astronomical Observatory of Japan (JP), University College London (GB), University of Virginia (US), Chalmers University of Technology (SE)
Openalex Percentile: Top 12%
Stellar, planetary, and galactic studies
2.32
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