MICONIC: Dual active galactic nuclei, star formation, and ionised gas outflows in NGC 6240 seen with MIRI/JWST

¹ Centro de Astrobiología (CAB) CSIC-INTA, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
² Telespazio UK for the European Space Agency (ESA), ESAC, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Spain
³ Sorbonne Université, CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98bis Bd Arago, 75014 Paris, France
⁴ Institut Universitaire de France, Ministère de l’Enseignement Supérieur et de la Recherche, 1 Rue Descartes, 75231 Paris Cedex 05, France
⁵ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9, B-9000 Gent, Belgium
⁶ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
⁷ UK Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ Scotland, UK
⁸ European Space Agency, c/o Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁹ Centro de Astrobiología (CAB) CSIC-INTA, Ctra. de Ajalvir km 4, Torrejón de Ardoz, 28850 Madrid, Spain
¹⁰ Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
¹¹ Department of Astronomy, Stockholm University, The Oskar Klein Centre, AlbaNova, SE-106 91 Stockholm, Sweden
¹² Centre for Extragalactic Astronomy, Durham University, South Road, Durham DH1 3LE, UK
¹³ Faculty of Aerospace Engineering, Delft University of Technology, Kluyverweg 1, 2629 HS Delft, The Netherlands
¹⁴ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
¹⁵ Dept. of Astrophysics, University of Vienna, Türkenschanzstr. 17, A-1180 Vienna, Austria
¹⁶ ETH Zürich, Institute for Particle Physics and Astrophysics, Wolfgang-Pauli-Str. 27, 8093 Zürich, Switzerland
¹⁷ Max Planck Institute for Astronomy, Konigstuhl 17, 69117 Heidelberg, Germany
¹⁸ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, F-91191 Gif-sur-Yvette, France
¹⁹ School of Cosmic Physics, Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin D02 XF86, Ireland

^⋆ Corresponding author; lhermosa@cab.inta-csic.es

Received: 30 September 2024
Accepted: 26 November 2024

Abstract

Context. Galaxy mergers are an important and complex phase during the evolution of galaxies. They may trigger nuclear activity and/or strong star forming episodes in galaxy centres that potentially alter the evolution of the system.

Aims. As part of the guaranteed time observations program Mid-Infrared Characterization Of Nearby Iconic galaxy Centers (MICONIC), we used the medium-resolution spectrometer (MRS) of the Mid-Infrared Instrument on board the James Webb Space Telescope (JWST) to study NGC 6240. We aim to characterise the dual active galactic nuclei (AGN), the ionised gas outflows, and the main properties of the interstellar medium over a mapped area of 6.6″ × 7.7″.

Aims. We obtained integral field spectroscopic mid-infrared data (wavelength from 4.9 to 28 μm) of NGC 6240. We modelled the emission lines through a kinematic decomposition that accounts for the possible existence of various components.

Methods. We have resolved both nuclei of NGC 6240 for the first time in the full 5−28 μm spectral range. The fine structure lines in the southern (S) nucleus are broader than for the northern (N) nucleus (full width at half maximum of ≥1500 versus ∼700 km s⁻¹ on average, respectively). High excitation lines, such as [Ne V], [Ne VI], and [Mg V], are clearly detected in the N nucleus. In the S nucleus, the same lines can be detected but only after a decomposition of the polycyclic aromatic hydrocarbon features in the integrated spectrum, due to a combination of a strong mid-IR continuum, broad emission lines, and intense star formation (SF). The SF is distributed all over the mapped field of view of 3.5 kpc × 4.1 kpc (projected), with the maximum located around the S nucleus. Both nuclear regions appear to be connected by a bridge region that is detected with all the emission lines. Based on the observed MRS line ratios and the high velocity dispersion (σ ∼ 600 km s⁻¹), shocks also dominate the emission in this system. We detected the presence of outflows as a bubble north-west from the N nucleus and at the S nucleus. We estimated an ionised mass outflow rate of 1.4 ± 0.3 M_⊙ yr⁻¹ and 1.8 ± 0.2 M_⊙ yr⁻¹, respectively. Given the derived kinetic power of these outflows, both the AGN and the starburst could have triggered them.