First extragalactic detection of a phosphorus-bearing molecule with
  ALCHEMI: phosphorus nitride (PN)
  Phosphorus (P) is a crucial element for life given its central role in
several biomolecules. P-bearing molecules have been discovered in different
regions of the Milky Way, but not yet towards an extragalactic environment. We
have searched for P-bearing molecules towards the nearby starburst Galaxy NGC
253. Using observations from the ALMA Comprehensive High-resolution
Extragalactic Molecular Inventory (ALCHEMI) project, we used the MADCUBA
package to model the emission of P-bearing molecules assuming Local
Thermodynamic Equilibrium (LTE) conditions. We have also performed a non-LTE
analysis using SpectralRadex. We report the detection of a P-bearing molecule,
phosphorus nitride (PN), for the first time in an extragalactic environment,
towards two giant molecular clouds (GMCs) of NGC 253. The LTE analysis yields
total PN beam-averaged column densities $N$=(1.20$\pm$0.09)$\times$10$^{13}$
cm$^{-2}$ and $N$=(6.5$\pm$1.6)$\times$10$^{12}$ cm$^{-2}$, which translate
into abundances with respect to H$_2$ of $\chi$=(8.0$\pm$1.0)$\times$10$^{-12}$
and $\chi$=(4.4$\pm$1.2)$\times$10$^{-12}$. We derived a low excitation
temperature of $T_{\rm ex}$=(4.4$\pm$1.3) K towards the GMC with the brightest
PN emission, which indicates that PN is sub-thermally excited. The non-LTE
analysis results in column densities consistent with the LTE values. We have
also searched for other P-bearing molecules (PO, PH$_{3}$, CP and CCP), and
upper limits were derived. The derived PO/PN ratios are $<$1.3 and $<$1.7. The
abundance ratio between PN and the shock-tracer SiO derived towards NGC 253
follows the same trend previously found towards Galactic sources. Comparison of
the observations with chemical models indicates that the derived molecular
abundances of PN in NGC 253 can be explained by shock-driven chemistry followed
by cosmic-ray-driven photochemistry.
