Outflowing shocked gas dominates the NIR H₂ emission from the dual AGN NGC 6240

¹ Institute of Theoretical Astrophysics, University of Oslo, P.O. Box 1029 Blindern 0315, Oslo, Norway
² European Southern Observatory (ESO), Karl-Schwarzschild-Strasse 2, D-85748 Garching, Germany
³ Section of Astrophysics, Astronomy & Mechanics, Department of Physics, National and Kapodistrian University of Athens, University Campus Zografos, GR 15784 Athens, Greece
⁴ INAF – Osservatorio Astronomico di Brera, Via Brera 28, 20121 Milano, Italy
⁵ Dipartimento di Fisica e Astronomia Augusto Righi, Università degli Studi di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
⁶ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 101, 40129 Bologna, Italy
⁷ ASTRON, The Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD, Dwingeloo, The Netherlands
⁸ Kapteyn Astronomical Institute, P.O. Box 800, 9700 AV, Groningen, The Netherlands
⁹ Department of Physics & Astronomy, University of South Carolina, Columbia, SC 29208, USA

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Received: 17 June 2025
Accepted: 13 March 2026

Abstract

We present a multi-line study of the kinematics of the molecular and ionised gas phases in the central ∼2 kpc of the luminous infrared galaxy and dual active galactic nucleus (AGN) NGC 6240, based on archival JWST/NIRSpec and ALMA observations. Since a primary goal of our analysis is to study outflows, we devised a new spectral-line fitting approach to de-blend rotating and non-rotating gas components in the observed near-infrared (NIR) emission lines. Our method is more physically motivated than previous approaches and better tailored to the extreme feedback mechanisms at work in NGC 6240. We find that ∼65% of the Paα, H₂, and [FeII] line fluxes within the NIRSpec field of view arise from gas components that are kinematically decoupled from the stars. In particular, the NIR H₂ lines show the most deviation from the stars, with peak emission between the two rotating stellar structures. The polycyclic aromatic hydrocarbon (PAH) emission feature at 3.3 μm does not follow the NIR H₂ morphology, indicating that the latter does not trace photon-dominated regions. In the non-rotating gas components, we identified a bi-conical wind that launched from the northern AGN, expanding along the minor axis of stellar rotation. This wind is dominated by ionised gas and, although it entrains some H₂ gas, it does not show a H₂/PAH enhancement, suggesting either high UV irradiation or expansion along a relatively gas-free path. Furthermore, we find bright non-rotating gas emission between the two AGNs and around the southern AGN, which we interpret as due to an outflow that launched from the southern nucleus, coinciding with the massive molecular outflow previously studied in cold (sub-)millimetre tracers. The strong H₂/PAH enhancement measured in this region, coextensive with high velocity redshifted gas (v ∼ 900 km s⁻¹), suggests that the shocks responsible for the high H₂/PAH ratios are due to the outflow rather than to the collision of media during the merger. Our results show that most of the NIR line emission in NGC 6240 is decoupled from the stars, and that most of the warm H₂ is shock-excited and embedded in a powerful outflow, where it coexists with colder molecular gas.