Spatial distribution of phosphorus carriers in galactic and extragalactic star-forming regions

Phosphorus (P) is a crucial element for life given its central role in several biomolecules, although the formation mechanism of P-bearing molecules in the interstellar medium still remains poorly understood. Using observations from the ALMA Comprehensive High-resolution Extragalactic Molecular Inventory (ALCHEMI) project, we carried out an LTE and a non-LTE analysis to model the emission of P-bearing molecules towards the nearby starburt galaxy NGC 253. 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 of NGC 253. Using complementary observations carried out with ALMA, we targeted the low mass star-forming region IRAS 16293-2422 to map the spatial distribution of P-bearing molecules at a high spatial resolution (50 au). The observations target three sets of rotational transitions of PN and phosphorus monoxide (PO), allowing us to perform a multi-transition analysis to study the excitation of these molecules. The observations show that the emission of PN and PO arises from shocked material traced by the shock tracers sulfur monoxide (SO), and sulfur dioxide (SO2). Comparison of the observations with chemical models supports the formation of P-bearing molecules in shocked regions where a yet unidentified P-carrier is desorbed from the dust grain surface and gas phase reactions lead to the formation of PN and PO.