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Phosphorus (P) is a crucial element for life given its central role inseveral biomolecules. P-bearing molecules have been discovered in differentregions of the Milky Way, but not yet towards an extragalactic environment. Wehave searched for P-bearing molecules towards the nearby starburst Galaxy NGC253. Using observations from the ALMA Comprehensive High-resolutionExtragalactic Molecular Inventory (ALCHEMI) project, we used the MADCUBApackage to model the emission of P-bearing molecules assuming LocalThermodynamic Equilibrium (LTE) conditions. We have also performed a non-LTEanalysis 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 yieldstotal 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 translateinto 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 excitationtemperature of $T_{\rm ex}$=(4.4$\pm$1.3) K towards the GMC with the brightestPN emission, which indicates that PN is sub-thermally excited. The non-LTEanalysis results in column densities consistent with the LTE values. We havealso searched for other P-bearing molecules (PO, PH$_{3}$, CP and CCP), andupper limits were derived. The derived PO/PN ratios are $<$1.3 and $<$1.7. Theabundance ratio between PN and the shock-tracer SiO derived towards NGC 253follows the same trend previously found towards Galactic sources. Comparison ofthe observations with chemical models indicates that the derived molecularabundances of PN in NGC 253 can be explained by shock-driven chemistry followedby cosmic-ray-driven photochemistry.

First extragalactic detection of a phosphorus-bearing molecule with ALCHEMI: phosphorus nitride (PN)