The Rich JWST Spectrum of the Western Nucleus of Arp 220: Shocked Hot Core Chemistry Dominates the Inner Disk

Victorine Buiten, Paul van der Werf, Serena Viti, Daniel Dicken, Almudena Alonso Herrero, Gillian Wright, Torsten Böker, Bernhard Brandl, Luis Colina, Macarena García Maríin, Thomas Greve, Pierre Guillard, Olivia Jones, Laura Hermosa Muñoz, Álvaro Labiano, Göran Östlin, Lara Pantoni, Martin Ward, Michele Perna, Ewine van Dishoeck, Thomas Henning, Manuel Güdel, Thomas Ray

First listed 2025-02-14 | Last updated 2025-05-16

Abstract

We present full 3-28 $\mathrm{μm}$ 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. Millimeter observations of the dust continuum suggest the presence of a distinct 20 pc core with a dust temperature of $T_\mathrm{d} \gtrsim 500~\mathrm{K}$, in addition to a 100 pc circumnuclear starburst disk. 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, we can, for the first time, separate the two nuclei across this full wavelength range, revealing a wealth of molecular absorption features towards the western nucleus. We analyse the rovibrational bands detected at 4-22 $\mathrm{μm}$, deriving column densities and rotational temperatures for 10 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 identify a warm HCN component with rotational temperature $T_\mathrm{rot} = 330~\mathrm{K}$, 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 disk, directly surrounding the hot 20 $\mathrm{pc}$ core. The chemical footprint of the western nucleus is reminiscent of that of hot cores, with additional evidence for 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 is found.

Short digest

JWST MIRI/MRS and NIRSpec/IFU 3–28 μm spectroscopy isolates the western nucleus of Arp 220 and reveals a forest of rovibrational absorptions, enabling column densities and rotational temperatures for 10 species, with optically thick C2H2, HCN, and HNC and columns ≈10× higher than Spitzer. A warm HCN component with Trot ≈ 330 K points to radiative excitation by a hot, ~20 pc inner core veiled by cooler dust. The chemistry resembles hot cores with added shocks, placing the absorbing gas in the inner starburst disk directly encasing the core. No X-ray–driven chemistry or extreme excitation is found, arguing against an energetically dominant buried AGN in this band.

Key figures to inspect

• Figure 1: Use the MIRI false-colour map and 0.435″ apertures to verify clean separation of the western vs. eastern nucleus and how the MRS channel fields cover the WN region used for spectral extraction.
• Figure 2: Scan the combined 3–28 μm spectrum to see the breadth and depth of the molecular bands; note the saturated C2H2/HCN/HNC absorption and where the strongest rovibrational branches fall across the IFU coverage.
• Figure 3: Inspect the C2H2+HCN modeling with two HCN components (plus N2H+ and CO2) to understand why a warm HCN component is required—watch the asymmetric HCN Q-branch fit and the optically thick regime implied by the residuals.
• Figure 4: Examine the H2O band fit and line identifications to gauge the water excitation and column, and check the residuals/fit window to assess robustness and possible contamination by nearby C2H features.