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

Abstract

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 $T_{\mathrm{ex}}$=(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.

14 pages, 14 figures, 4 tables. Accepted for publication in A&A

References in corpus (13)

• The present-day star formation rate of the Milky-Way determined from Spitzer detected young stellar objects
• A 2 mm spectral line survey of the starburst galaxy NGC 253
• ALMA Reveals the Molecular Medium Fueling the Nearest Nuclear Starburst
• Confirmation of circumstellar phosphine
• ALMA and ROSINA detections of phosphorus-bearing molecules: the interstellar thread between star-forming regions and comets
• Phosphorus-bearing molecules in solar-type star forming regions: First PO detection
• Lambda = 3 mm line survey of nearby active galaxies
• Phosphorus-bearing molecules in the Galactic Center
• ALMA observations of 99 GHz free-free and H40$\alpha$ line emission from star formation in the centre of NGC 253
• Infrared H3+ and CO Studies of the Galactic Core: GCIRS 3 and GCIRS 1W
• The Distribution and Origin of C${}_2$H in NGC 253 from ALCHEMI
• Detection of phosphorus-bearing molecules towards a Solar-type protostar
• Diverse nuclear star-forming activities in the heart of NGC 253 resolved with ten-pc scale ALMA images

Cited by in corpus (6)

• Tracing Interstellar Heating: An ALCHEMI Measurement of the HCN Isomers in NGC 253
• ALCHEMI finds a "shocking" carbon footprint in the starburst galaxy NGC~253
• New routes for PN destruction and formation in the ISM via neutral-neutral gas-phase reactions and an extended database for reactions involving phosphorus
• Shock-induced HCNH+ abundance enhancement in the heart of the starburst galaxy NGC 253 unveiled by ALCHEMI
• Complex Organic Molecules towards the central molecular zone of NGC 253
• First Detections of PN, PO, and PO+ toward a Shocked Low-mass Starless Core