H.E.S.S. observations of composite Seyfert–starburst galaxies

¹ University of Southern Denmark, Campusvej 55, DK-5230, Odense M, Denmark
² Astronomy & Astrophysics Section, School of Cosmic Physics, Dublin Institute for Advanced Studies, DIAS Dunsink Observatory, Dublin, D15 XR2R, Ireland
³ Max-Planck-Institut für Kernphysik, PO Box 103980, D 69029, Heidelberg, Germany
⁴ Laboratoire Leprince-Ringuet, École Polytechnique, CNRS, Institut Polytechnique de Paris, F-91128, Palaiseau, France
⁵ University of Namibia, Department of Physics, Private Bag 13301, Windhoek, 10005, Namibia
⁶ Centre for Space Research, North-West University, Potchefstroom, 2520, South Africa
⁷ Deutsches Elektronen-Synchrotron DESY, Platanenallee 6, 15738, Zeuthen, Germany
⁸ Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Strasse 24/25, D 14476, Potsdam, Germany
⁹ Université Paris Cité, CNRS, Astroparticule et Cosmologie, F-75013, Paris, France
¹⁰ Department of Physics and Electrical Engineering, Linnaeus University, 351 95, Växjö, Sweden
¹¹ Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, D 12489, Berlin, Germany
¹² LUX, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, 5 Pl. Jules Janssen, 92190, Meudon, France
¹³ Sorbonne Université, CNRS/IN2P3, Laboratoire de Physique Nucléaire, et de Hautes Energies, LPNHE, 4 place Jussieu, 75005, Paris, France
¹⁴ IRFU, CEA, Université Paris-Saclay, F-91191, Gif-sur-Yvette, France
¹⁵ Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen Centre for Astroparticle Physics, Nikolaus-Fiebiger-Str. 2, 91058, Erlangen, Germany
¹⁶ Instytut Fizyki Jadrowej PAN, ul. Radzikowskiego 152, ul. Radzikowskiego 152, 31-342, Kraków, Poland
¹⁷ School of Physics, University of the Witwatersrand, 1 Jan Smuts Avenue, Braamfontein, Johannesburg, 2050, South Africa
¹⁸ School of Physical Sciences and Centre for Astrophysics & Relativity, Dublin City University, Glasnevin, D09 W6Y4, Ireland
¹⁹ University of Oxford, Department of Physics, Denys Wilkinson Building, Keble Road, Oxford, OX1 3RH, UK, UK
²⁰ Aix Marseille Université, CNRS/IN2P3, CPPM, Marseille, France
²¹ School of Science, Western Sydney University, Locked Bag 1797, Penrith South DC, NSW, 2751, Australia
²² Landessternwarte, Universität Heidelberg, Königstuhl, D 69117, Heidelberg, Germany
²³ Université Bordeaux, CNRS, LP2I Bordeaux, UMR 5797, F-33170, Gradignan, France
²⁴ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, D 72076, Tübingen, Germany
²⁵ Universität Innsbruck, Institut für Astro- und Teilchenphysik, Technikerstraße 25, 6020, Innsbruck, Austria
²⁶ Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, D 22761, Hamburg, Germany
²⁷ Obserwatorium Astronomiczne, Uniwersytet Jagielloński, ul. Orla 171, 30-244, Kraków, Poland
²⁸ Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziadzka 5, 87-100, Torun, Poland
²⁹ Laboratoire Univers et Particules de Montpellier, Université Montpellier, CNRS/IN2P3, CC 72, Place Eugène Bataillon, F-34095, Montpellier Cedex 5, France
³⁰ INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, I-09047, Selargius, CA, Italy
³¹ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, 00-716, Warsaw, Poland
³² University of Leicester, School of Physics and Astronomy, University Road, Leicester, LE1 7RH, UK
³³ Fakultät für Physik und Astronomie, Julius-Maximilians-Universität Würzburg, Emil-Fischer-Str. 31, 97074, Würzburg, Germany
³⁴ Yerevan Physics Institute, 2 Alikhanian Brothers St., 0036, Yerevan, Armenia
³⁵ Department of Physics, Konan University, 8-9-1 Okamoto, Higashinada, Kobe, Hyogo, 658-8501, Japan
³⁶ Kapteyn Astronomical Institute, University of Groningen, Landleven 12, 9747 AD, Groningen, The Netherlands
³⁷ GRAPPA, Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH, Amsterdam, The Netherlands

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Received: 5 June 2025
Accepted: 1 February 2026

Abstract

Context. Composite galaxies that contain both Seyfert and starburst components may produce very high-energy (VHE; > 100 GeV) γ-ray emission at a wide range of spatial scales, from a few Schwarzschild radii of a supermassive black hole (SMBH; R_S = 10⁻⁶ pc for M_SMBH = 10⁷ M_⊙) to dimensions of kiloparsec-size jet-driven outflows. In addition to supernova remnants, various sources have been suggested to explain data collected on composite galaxies, including multi-messenger neutrino and ultra-high-energy cosmic-ray data.

Aims. The closest composite Seyfert–starburst galaxies (NGC 1068, the Circinus galaxy, and NGC 4945) are observed with the High Energy Stereoscopic System (H.E.S.S.) to provide constraints on cosmic-ray populations in these systems.

Methods. Data obtained in H.E.S.S. observations have been analyzed to search for VHE γ-ray counterparts to the GeV γ-ray signals detected with Fermi-LAT and for potential spectral components in the VHE range.

Results. No significant signals have been found in these H.E.S.S. data. Upper limits on the VHE γ-ray fluxes were applied to constrain theoretical models involving different spectral components.