The Distribution and Origin of C$_2$H in NGC 253 from ALCHEMI
  Observations of chemical species can provide an insight into the physical
conditions of the emitting gas but it is important to understand how their
abundances and excitation vary within different heating environments. C$_2$H 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. As part of the ALCHEMI ALMA large
program, the emission of C$_2$H in the central molecular zone of the nearby
starburst galaxy NGC 253 was mapped at 1.6 " (28 pc) resolution and
characterized to understand its chemical origins. Spectral modelling of the
N=1-0 through N=4-3 rotational transitions of C$_2$H was used to derive the
C$_2$H column densities towards the dense clouds in NGC 253. Chemical
modelling, including PDR, dense cloud, and shock models were then used to
investigate the chemical processes and physical conditions that are producing
the molecular emission. We find high C$_2$H column densities of $\sim 10^{15}
cm^{-3}$ detected towards the dense regions of NGC 253. We further find that
these column densities cannot be reproduced by assuming that the emission
arises from the PDR regions at the edge of the clouds. Instead, we find that
the C$_2$H 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.
