Letter to the Editor

Extended coronal line emission and new clues to a possible dual AGN in the merger J1356+1026

¹ Instituto de Astrofísica de Canarias, Calle Vía Láctea, s/n, E-38205 La Laguna, Tenerife, Spain
² Departamento de Astrofísica, Universidad de La Laguna, E-38206 La Laguna, Tenerife, Spain
³ Instituto de Radioastronomía and Astrofísica (IRyA-UNAM), 3-72 (Xangari), 8701 Morelia, Mexico
⁴ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50127 Firenze, Italy
⁵ Divisão de Astrofísica, INPE, Avenida dos Astronautas 1758, São José dos Campos, 12227-010, SP, Brazil
⁶ Instituto de Física Fundamental, CSIC, Calle Serrano 123, 28006 Madrid, Spain
⁷ Department of Physics & Astronomy, University of Sheffield, S3 7RH, Sheffield, UK
⁸ Centro de Astrobiología (CAB), CSIC-INTA, Camino Bajo del Castillo s/n, E-28692 Villanueva de la Cañada, Madrid, Spain
⁹ Department of Physics, University of Oxford, Oxford OX1 3RH, UK
¹⁰ School of Sciences, European University Cyprus, Diogenes Street, Engomi 1516, Nicosia, Cyprus
¹¹ Telespazio UK for the European Space Agency, ESAC, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Spain
¹² Observatorio Astronómico Nacional (OAN-IGN)-Observatorio de Madrid, Alfonso XII, 3, 28014 Madrid, Spain

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Received: 2 February 2026
Accepted: 13 April 2026

Abstract

Merging luminous galaxies are ideal laboratories for studying some of the most extreme astrophysical phenomena. The local (z = 0.1232) obscured quasar J1356+1026 has two nuclei, North and South (J1356N and J1356S), but despite numerous efforts, J1356S had not yet been confirmed as an AGN. Based on the superb sensitivity and spatial resolution of the MIRI/MRS instrument on board the JWST, we present new evidence suggesting that J1356S may indeed host an AGN with log L_bol = 43.4±0.08_0.5^0.6 erg s⁻¹. This is supported by the detection of strong coronal line emission at this location and by a spectral shape that differs from that of J1356N and those of the narrow-line region. Aided by the spatially resolved information of MIRI/MRS and VLT/SINFONI, we also found that the high-ionization gas, traced by the coronal lines [Ne V]14.3 μm and [Si VI]1.963 μm, extends from ∼13 − 15.5 kpc. This is likely a lower limit of the true extension, as suggested by the comparison with optical imaging from HST. The extended [Ne V] emission can be accounted for by photoionization from the quasar in J1356N in a relatively low-density environment, ranging from n_e ≤ 2000 − 3800 cm⁻³ in J1356N and n_e ≤ 600 − 1200 cm⁻³ in J1356S and the narrow-line region, as measured from the [Ne V]14.3 μm and 24.3 μm lines.