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 different regions of the Milky Way, but not yet towards an extragalactic environment. We searched nearby starburst Galaxy NGC 253. Using observations from ALMA Comprehensive High-resolution Extragalactic Molecular Inventory (ALCHEMI) project, we used MADCUBA package to model emission assuming Local Thermodynamic Equilibrium (LTE) conditions. also performed non-LTE analysis using SpectralRadex. report detection molecule, phosphorus nitride (PN), first time environment, two giant molecular clouds (GMCs) The LTE 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 H$_2$ $\chi$=(8.0$\pm$1.0)$\times$10$^{-12}$ $\chi$=(4.4$\pm$1.2)$\times$10$^{-12}$. derived low excitation temperature $T_{\rm ex}$=(4.4$\pm$1.3) K GMC brightest emission, indicates that sub-thermally excited. results consistent values. other (PO, PH$_{3}$, CP CCP), upper limits were derived. PO/PN ratios are $<$1.3 $<$1.7. abundance ratio between shock-tracer SiO 253 follows same trend previously found Galactic sources. Comparison chemical models can be explained by shock-driven chemistry followed cosmic-ray-driven photochemistry.