Tracing Interstellar Heating: An ALCHEMI Measurement of the HCN Isomers in NGC 253
Journal article, 2022
We analyze HCN and HNC emission in the nearby starburst galaxy NGC 253 to investigate its effectiveness in tracing heating processes associated with star formation. This study uses multiple HCN and HNC rotational transitions observed using the Atacama Large Millimeter/submillimeter Array via the ALCHEMI Large Program. To understand the conditions and associated heating mechanisms within NGC 253's dense gas, we employ Bayesian nested sampling techniques applied to chemical and radiative transfer models, which are constrained using our HCN and HNC measurements. We find that the volume density n H 2 and cosmic-ray ionization rate (CRIR) ζ are enhanced by about an order of magnitude in the galaxy’s central regions as compared to those further from the nucleus. In NGC 253's central giant molecular clouds (GMCs), where observed HCN/HNC abundance ratios are the lowest, n ∼ 105.5 cm−3 and ζ ∼ 10−12 s−1 (greater than 104 times the average Galactic rate). We find a positive correlation in the association of both density and CRIR with the number of star formation-related heating sources (supernova remnants, H ii regions, and super hot cores) located in each GMC, as well as a correlation between CRIRs and supernova rates. Additionally, we see an anticorrelation between the HCN/HNC ratio and CRIR, indicating that this ratio will be lower in regions where ζ is higher. Though previous studies suggested HCN and HNC may reveal strong mechanical heating processes in NGC 253's CMZ, we find cosmic-ray heating dominates the heating budget, and mechanical heating does not play a significant role in the HCN and HNC chemistry.
Author
E. Behrens
University of Virginia
J. G. Mangum
National Radio Astronomy Observatory
Jonathan Holdship
Leiden University
University College London (UCL)
Serena Viti
University College London (UCL)
Leiden University
N. Harada
The Graduate University for Advanced Studies (SOKENDAI)
National Astronomical Observatory of Japan
S. Martin
European Southern Observatory Santiago
Atacama Large Millimeter-submillimeter Array (ALMA)
K. 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
R. Herrero-Illana
European Southern Observatory Santiago
Institute of Space Sciences (ICE) - CSIC
Y. Yoshimura
University of Tokyo
Rebeca Aladro
Max Planck Society
L. Colzi
Centro de Astrobiologia (CAB)
K.L. Emig
National Radio Astronomy Observatory
C. Henkel
Max Planck Society
King Abdulaziz University
K. Y. Huang
Leiden University
Pedro Humire
Max Planck Society
D. S. Meier
National Radio Astronomy Observatory Socorro
New Mexico Institute of Mining and Technology
Víctor M. Rivilla
Centro de Astrobiologia (CAB)
P. van der Werf
Leiden University
Astrophysical Journal
0004-637X (ISSN) 1538-4357 (eISSN)
Vol. 939 2 119Subject Categories
Energy Engineering
Astronomy, Astrophysics and Cosmology
Atom and Molecular Physics and Optics
DOI
10.3847/1538-4357/ac91ce