First extragalactic detection of a phosphorus-bearing molecule with ALCHEMI: Phosphorus nitride (PN)
Journal article, 2022
Context. 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.
Aims. We searched for P-bearing molecules outside the Milky Way towards the nearby starburst Galaxy NGC 253.
Methods. Using observations from the ALMA Comprehensive High-resolution Extragalactic Molecular Inventory (ALCHEMI) project, we used the MAdrid Data CUBe Analysis package to model the emission of P-bearing molecules assuming local thermodynamic equilibrium (LTE) conditions. We also performed a non-LTE analysis using SpectralRadex.
Results. 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 +/- 0.09) x 10(13) cm(-2) and N = (6.5 +/- 1.6) x 10(12) cm(-2), which translate into abundances with respect to H-2 of chi = (8.0 +/- 1.0) x 10(-12) and chi = (4.4 +/- 1.2) x 10(-12). We derived a low excitation temperature of T-ex = (4.4 +/- 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 also searched for other P-bearing molecules (PO, PH3, 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. Conclusions. 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.
Aims. We searched for P-bearing molecules outside the Milky Way towards the nearby starburst Galaxy NGC 253.
Methods. Using observations from the ALMA Comprehensive High-resolution Extragalactic Molecular Inventory (ALCHEMI) project, we used the MAdrid Data CUBe Analysis package to model the emission of P-bearing molecules assuming local thermodynamic equilibrium (LTE) conditions. We also performed a non-LTE analysis using SpectralRadex.
Results. 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 +/- 0.09) x 10(13) cm(-2) and N = (6.5 +/- 1.6) x 10(12) cm(-2), which translate into abundances with respect to H-2 of chi = (8.0 +/- 1.0) x 10(-12) and chi = (4.4 +/- 1.2) x 10(-12). We derived a low excitation temperature of T-ex = (4.4 +/- 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 also searched for other P-bearing molecules (PO, PH3, 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. Conclusions. 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.
individual
galaxies
ISM
clouds
abundances
ISM
astrochemistry
NGC 253
molecules
ISM
Author
D. Haasler
Rey Juan Carlos University (URJC)
Spanish Astrobiology Center (INTA-CSIC)
V. M. Rivilla
Istituto nazionale di astrofisica (INAF)
Spanish Astrobiology Center (INTA-CSIC)
S. Martin
Atacama Large Millimeter-submillimeter Array (ALMA)
European Southern Observatory Santiago
J. Holdship
University College London (UCL)
Leiden University
S. Viti
University College London (UCL)
Leiden University
N. Harada
The Graduate University for Advanced Studies (SOKENDAI)
National Astronomical Observatory of Japan
Academia Sinica
J. Mangum
National Radio Astronomy Observatory
K. Sakamoto
Academia Sinica
Sebastien Muller
Chalmers, Space, Earth and Environment, Onsala Space Observatory
K. Tanaka
Keio University
Y. Yoshimura
University of Tokyo
K. Nakanishi
National Astronomical Observatory of Japan
The Graduate University for Advanced Studies (SOKENDAI)
L. Colzi
Istituto nazionale di astrofisica (INAF)
Spanish Astrobiology Center (INTA-CSIC)
L. Hunt
Istituto nazionale di astrofisica (INAF)
K. L. Emig
National Radio Astronomy Observatory
R. Aladro
Max Planck Society
P. Humire
Max Planck Society
C. Henkel
King Abdulaziz University
Max Planck Society
Chinese Academy of Sciences
P. van der Werf
Leiden University
Astronomy and Astrophysics
0004-6361 (ISSN) 1432-0746 (eISSN)
Vol. 659 A158Subject Categories
Astronomy, Astrophysics and Cosmology
DOI
10.1051/0004-6361/202142032