The JWST/NIRSpec view of the nuclear region in the prototypical merging galaxy NGC 6240

¹ Università di Firenze, Dipartimento di Fisica e Astronomia, via G. Sansone 1, 50019 Sesto F.no, Firenze, Italy
² INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125 Firenze, Italy
³ Centro de Astrobiología (CAB), CSIC-INTA, Ctra. de Ajalvir Km. 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁴ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁵ Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
⁶ Sorbonne Université, CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98 bis bd Arago, 75014 Paris, France
⁷ Department of Astrophysics, University of Vienna, Türkenschanzstraße 17, 1180 Wien, Austria
⁸ Max-Planck-Institut für Extraterrestrische Physik (MPE), Giessenbachstraße 1, D-85748 Garching, Germany
⁹ University of Trento, Via Sommarive 14, I-38123 Trento, Italy

^⋆ Corresponding author; matteo.ceci@unifi.it

Received: 11 September 2024
Accepted: 3 January 2025

Abstract

Merger events are thought to be an important phase in the assembly of massive galaxies. At the same time, active galactic nuclei (AGN) play a fundamental role in the evolution of their star formation histories. Both phenomena can be observed at work in NGC 6240, a local prototypical merger classified as an ultraluminous infrared galaxy (ULIRG) thanks to its elevated infrared luminosity. Interestingly, NGC 6240 hosts two AGN separated by 1.5″(∼735 pc), detected in both X-ray and radio band. Taking advantage of the unprecedented sensitivity and wavelength coverage provided by the integral field unit (IFU) of the NIRSpec instrument on board JWST, we observed the nuclear region of NGC 6240 in a field of view of 3.7″ × 3.7″(1.9 × 1.9 kpc²) in order to investigate gas kinematics and interstellar medium (ISM) properties with a high spatial resolution of ∼0.1″ (or ∼50 pc). We characterized the 2D stellar kinematics, separated the different gas kinematic components through multi-Gaussian fitting, and studied the excitation properties of the ISM from the near-infrared diagnostic diagram based on the H₂ 1–0 S(1)/Brγ and [Fe II]λ1.257 μm/Paβ lines ratios. We isolated the ionization cones of the two nuclei and detected coronal line emission from both of them. Using H₂ line ratios, we found that the molecular hydrogen gas is excited mostly by thermal processes. We computed a hot molecular gas mass of 1.3 × 10⁵ M_⊙ and an ionized gas mass in the range of 10⁵–10⁷ M_⊙, depending on assumptions. We studied with unprecedented spatial resolution and sensitivity the kinematics of the molecular and ionized gas phases, and we revealed the complex structure of the molecular gas and found a blueshifted outflow near the southern nucleus, together with filaments connecting a highly redshifted H₂ cloud with the two nuclei. We speculate on the possible nature of this H₂ cloud and propose two possible scenarios: outflowing gas or a tidal cloud falling onto the nuclei.