2021A&A...654A..55H

Context. Observations of chemical species can provide insights into the physical conditions of the emitting gas however it is important to understand how their abundances and excitation vary within different heating environments. C₂H is a molecule typically found in PDR regions of our own Galaxy but there is evidence to suggest it also traces other regions undergoing energetic processing in extragalactic environments.
Aims. As part of the ALCHEMI ALMA large program, we map the emission of C₂H in the central molecular zone of the nearby starburst galaxy NGC 253 at 1.6'' (28 pc) resolution and characterize it to understand its chemical origins.
Methods. We used spectral modeling of the N = 1-0 through N = 4-3 rotational transitions of C₂H to derive the C₂H column densities towards the dense clouds in NGC 253. We then use chemical modeling, including photodissociation region (PDR), dense cloud, and shock models to investigate the chemical processes and physical conditions that are producing the molecular emission.
Results. We find high C₂H column densities of ∼10¹⁵ cm^–2 detected towards the dense regions of NGC 253. We further find that these column densities cannot be reproduced if it is assumed that the emission arises from the PDR regions at the edge of the clouds. Instead, we find that the C₂H abundance remains high even in the high visual extinction interior of these clouds and that this is most likely caused by a high cosmic-ray ionization rate.