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

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
² Transdisciplinary Research Area (TRA) ‘Matter’/Argelander-Institut für Astronomie, University of Bonn, Bonn, Germany
³ Physics and Astronomy, University College London, London, UK
⁴ UK Astronomy Technology Centre, Royal Observatory, Blackford Hill Edinburgh, EH9 3HJ Scotland, UK
⁵ Centro de Astrobiología (CAB), CSIC-INTA, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
⁶ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9, 9000 Gent, Belgium
⁷ European Space Agency, c/o Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁸ Faculty of Aerospace Engineering, Delft University of Technology, Kluyverweg 1, 2629 HS Delft, The Netherlands
⁹ Centro de Astrobiología (CAB), CSIC–INTA, Cra. de Ajalvir Km. 4, 28850 Torrejón de Ardoz, Madrid, Spain
¹⁰ Cosmic Dawn Center (DAWN), Copenhagen, Denmark
¹¹ DTU Space, Technical University of Denmark, Elektrovej, Building 328, 2800 Kgs. Lyngby, Denmark
¹² Sorbonne Université, CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98bis bd Arago, 75014 Paris, France
¹³ Institut Universitaire de France, Ministère de l’Enseignement Supérieur et de la Recherche, 1 Rue Descartes, 75231 Paris Cedex 05, France
¹⁴ Telespazio UK for the European Space Agency (ESA), ESAC, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Spain
¹⁵ Department of Astronomy, Stockholm University, The Oskar Klein Centre, AlbaNova 106 91, Stockholm, Sweden
¹⁶ Max Planck Institute for Astronomy, Konigstuhl 17, 69117 Heidelberg, Germany
¹⁷ Centre for Extragalactic Astronomy, Durham University, South Road, Durham DH1 3LE, UK
¹⁸ Dept. of Astrophysics, University of Vienna, Türkenschanzstr. 17, 1180 Vienna, Austria
¹⁹ ETH Zürich, Institute for Particle Physics and Astrophysics, Wolfgang-Pauli-Str. 27, 8093 Zürich, Switzerland
²⁰ Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, D02 XF86 Dublin, Ireland

^⋆ Corresponding author.

Received: 14 February 2025
Accepted: 14 May 2025

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 C₂H₂, HCN, and HNC suggest that this molecular gas is hidden behind a curtain of cooler dust and indicate that the column densities of C₂H₂ 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.