No evidence of active galactic nucleus features in the nuclei of Arp 220 from JWST/NIRSpec IFS

¹ Centro de Astrobiología (CAB), CSIC–INTA, Cra. de Ajalvir Km. 4, 28850 Torrejón de Ardoz, Madrid, Spain
² Instituto de Física Fundamental, CSIC, Calle Serrano 123, 28006 Madrid, Spain
³ Università di Firenze, Dipartimento di Fisica e Astronomia, Via G. Sansone 1, 50019 Sesto Fiorentino, Firenze, Italy
⁴ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125 Firenze, Italy
⁵ University of Trento, Via Sommarive 14, I-38123 Trento, Italy
⁶ European Space Agency, c/o STScI, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁷ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁸ Cavendish Laboratory – Astrophysics Group, University of Cambridge, 19 JJ Thomson Avenue, Cambridge CB3 0HE, UK
⁹ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
¹⁰ Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
¹¹ Sorbonne Université, CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98 bis bd Arago, 75014 Paris, France
¹² European Space Agency, European Space Research and Technology Centre, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands

Received: 22 March 2024
Accepted: 17 July 2024

Abstract

Context. Arp 220 is the nearest ultra-luminous infrared galaxy. It shows evidence of 100 pc scale molecular outflows that are likely connected with galaxy-scale outflows traced by ionised and neutral gas. The two highly obscured nuclei of Arp 220 are sites of intense star formation, with extreme (far-infrared based) star formation rate surface densities, Σ_SFR ≳ 10³ M_⊙ yr⁻¹ kpc⁻². Despite extensive investigations that searched for active galactic nucleus (AGN) activity in the Arp 220 nuclei, direct evidence remains elusive.

Aims. We present JWST/NIRSpec integral field spectroscopy (IFS) observations covering the 0.9 − 5.1 μm wavelength range of the innermost (5″ × 4″, i.e. 1.8 × 1.5 kpc) regions of Arp 220. The primary goal is to investigate the potential presence of AGN signatures in the nuclear regions by analysing the spectra extracted from circular apertures with a radius of 55 pc (0.15″) around each of the two nuclei.

Methods. The analysis aims to identify highly ionised gas emission lines (with ionisation potential > 54 eV) and other spectral features indicative of AGN activity. Atomic and molecular gas kinematics were also taken into account to study the outflow signatures at < 60 pc scales.

Results. We identify ∼70 atomic and ∼50 molecular emission lines in the nuclear spectra of Arp 220. We used recombination line ratios to measure optical extinctions in the range A_V ∼ 11 − 14 mag. High-ionisation lines are not detected, except for the [Mg IV] line at 4.49 μm, which we interpret as due to shocks rather than to AGN ionisation. We identify broadening and multiple kinematic components in the H I and H₂ lines caused by outflows and shocks, with velocities up to ∼550 km s⁻¹. Significantly higher velocities (up to ∼900 km s⁻¹) are detected in the off-nuclear regions, but they do not conclusively represent direct evidence for AGN activity. Broad-line region components are not detected in any permitted emission line within the NIRSpec wavelength range.

Conclusions. Even with the unprecedented sensitivity of JWST/NIRSpec IFS, achieving an unambiguous identification or exclusion of the presence of an AGN in the Arp 220 system remains challenging because of its extreme dust obscuration.