2024A&A...690A.171P

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.