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Arxiv: MICONIC: JWST Unveils Shocked Hot Core Chemistry in the Western Nucleus of Arp 220 Published: 2/14/2025 6:29:43 PM Updated: 2/14/2025 6:29:43 PM

Paper abstract: We present full 3-28 micron JWST MIRI/MRS and NIRSpec/IFU spectra of thewestern nucleus of Arp 220, the nearest ultraluminous infrared galaxy. Thisnucleus has long been suggested to possibly host an embedded Compton-thick AGN.Millimeter observations of the dust continuum suggest the presence of adistinct 20 pc core with a dust temperature of T_\mathrm{d} \gtrsim500~\mathrm{K}, in addition to a 100 pc circumnuclear starburst disk. However,unambiguously identifying the nature of this core is challenging, due to theimmense obscuration, the nuclear starburst activity, and the nearby easternnucleus. With the JWST integral field spectrographs, we can, for the firsttime, separate the two nuclei across this full wavelength range, revealing awealth of molecular absorption features towards the western nucleus. We analysethe rovibrational bands detected at 4-22 micron, deriving column densities androtational temperatures for 10 distinct species. Optically thick features ofC_2H_2, HCN and HNC suggest that this molecular gas is hidden behind acurtain of cooler dust, and indicate that the column densities of C_2H_2and HCN are an order of magnitude higher than previously derived from Spitzerobservations. We identify a warm HCN component with rotational temperatureT_\mathrm{rot} = 330~\mathrm{K}, which we associate with radiative excitationby the hot inner nucleus. We propose a geometry where the detected moleculargas is located in the inner regions of the starburst disk, directly surroundingthe hot 20 parsec core. The chemical footprint of the western nucleus isreminiscent of that of hot cores, with additional evidence for shocks. Noevidence for the presence of an AGN in the form of X-ray-driven chemistry orextreme excitation is found.