The rich JWST spectrum of the western nucleus of Arp 220: Shocked hot core chemistry dominates the inner disc

We present full 3–28 μm JWST MIRI/MRS and NIRSpec/IFU spectra of the western nucleus of Arp 220, the nearest ultraluminous infrared galaxy. This nucleus has long been suggested to possibly host an embedded Compton-thick AGN. Millimetre observations of the dust continuum suggest the presence of a distinct 20 pc core with a dust temperature of T d ≳500 K, in addition to a 100 pc circumnuclear starburst disc. However, unambiguously identifying the nature of this core is challenging, due to the immense obscuration, the nuclear starburst activity, and the nearby eastern nucleus. With the JWST integral field spectrographs, for the first time we can separate the two nuclei across this full wavelength range, revealing a wealth of molecular absorption features towards the western nucleus. We analysed the rovibrational bands detected at 4–22 μm, deriving column densities and rotational temperatures for ten distinct species. Optically thick features of C 2 H 2 , HCN, and HNC suggest that this molecular gas is hidden behind a curtain of cooler dust and indicate that the column densities of C 2 H 2 and HCN are an order of magnitude higher than previously derived from Spitzer observations. We identified a warm HCN component with a rotational temperature of T rot = 330 K, which we associate with radiative excitation by the hot inner nucleus. We propose a geometry where the detected molecular gas is located in the inner regions of the starburst disc, directly surrounding the hot 20 pc core. The chemical footprint of the western nucleus is reminiscent of that of hot cores, with additional evidence of shocks. Despite the molecular material's close proximity to the central source, no evidence for the presence of an AGN in the form of X-ray-driven chemistry or extreme excitation was found.

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Publication Details

Journal
Astronomy and Astrophysics
Published
2025-06-11
DOI
https://doi.org/10.1051/0004-6361/202554141
Citations
3
Primary Topic
Astrophysics and Star Formation Studies
Type
article
Field-Weighted Citation Impact
3.46

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article

The rich JWST spectrum of the western nucleus of Arp 220: Shocked hot core chemistry dominates the inner disc

Fabian Walter, M. Guêdel, T. R. Greve, Michele Perna et al.
3 citations
Astronomy and Astrophysics
Astrophysics and Star Formation Studies
3.46
article

The rich JWST spectrum of the western nucleus of Arp 220: Shocked hot core chemistry dominates the inner disc

Fabian Walter, M. Guêdel, T. R. Greve, Michele Perna, Bernhard R. Brandl, Laura Hermosa Muñoz, A. Alonso‐Herrero, Alvaro Labiano, Victorine A. Buiten, L. Colina, M. Baes, P. Guillard, Göran Östlin, M. J. Ward, L. Pantoni, Torsten Boeker, T. P. Ray, S. Viti, M. García-Marín, P. van der Werf, Olivia Jones, Thomas Henning, Gillian Wright, Ewine F. van Dishoeck, Daniel Dicken
article en
3 citations

Abstract

We present full 3–28 μm JWST MIRI/MRS and NIRSpec/IFU spectra of the western nucleus of Arp 220, the nearest ultraluminous infrared galaxy. This nucleus has long been suggested to possibly host an embedded Compton-thick AGN. Millimetre observations of the dust continuum suggest the presence of a distinct 20 pc core with a dust temperature of T d ≳500 K, in addition to a 100 pc circumnuclear starburst disc. However, unambiguously identifying the nature of this core is challenging, due to the immense obscuration, the nuclear starburst activity, and the nearby eastern nucleus. With the JWST integral field spectrographs, for the first time we can separate the two nuclei across this full wavelength range, revealing a wealth of molecular absorption features towards the western nucleus. We analysed the rovibrational bands detected at 4–22 μm, deriving column densities and rotational temperatures for ten distinct species. Optically thick features of C 2 H 2 , HCN, and HNC suggest that this molecular gas is hidden behind a curtain of cooler dust and indicate that the column densities of C 2 H 2 and HCN are an order of magnitude higher than previously derived from Spitzer observations. We identified a warm HCN component with a rotational temperature of T rot = 330 K, which we associate with radiative excitation by the hot inner nucleus. We propose a geometry where the detected molecular gas is located in the inner regions of the starburst disc, directly surrounding the hot 20 pc core. The chemical footprint of the western nucleus is reminiscent of that of hot cores, with additional evidence of shocks. Despite the molecular material's close proximity to the central source, no evidence for the presence of an AGN in the form of X-ray-driven chemistry or extreme excitation was found.

Astronomy and AstrophysicsVol. 699
Leiden University (NL), Centre National de la Recherche Scientifique (FR), Space Telescope Science Institute (US), University of Vienna (AT), Dublin Institute For Advanced Studies (IE), University of Bonn (DE), Institut Universitaire de France (FR), Durham University (GB), Institut d'Astrophysique de Paris (FR), Ghent University (BE), ETH Zurich (CH), Sorbonne Université (FR), European Space Astronomy Centre (ES), Max Planck Institute for Astronomy (DE), Centro de Astrobiología (ES), Ministère de l'Enseignement Supérieur, de la Recherche et de l'Espace (FR), AlbaNova (SE), University College London (GB), UK Astronomy Technology Centre (GB), Technical University of Denmark (DK), Delft University of Technology (NL)
National Aeronautics and Space Administration, California Institute of Technology, Space Telescope Science Institute, National Research Foundation, Danmarks Grundforskningsfond, Swedish National Space Agency, Ministerio de Ciencia e Innovación, Jet Propulsion Laboratory, European Regional Development Fund, Agencia Estatal de Investigación
Openalex Percentile: Top 8%
Astrophysics and Star Formation Studies
3.46
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