Euclid: Star clusters in IC 342, NGC 2403, and Holmberg II^⋆

¹ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010 6500 GL, Nijmegen, The Netherlands
² Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
³ INAF-Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Piero Gobetti 93/3, 40129 Bologna, Italy
⁴ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
⁵ INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁶ Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550, 67000 Strasbourg, France
⁷ Instituto de Astrofísica de Canarias, Vía Láctea, 38205 La Laguna, Tenerife, Spain
⁸ Universidad de La Laguna, Departamento de Astrofísica, 38206 La Laguna, Tenerife, Spain
⁹ Université Paris-Saclay, CNRS, Institut d’astrophysique spatiale, 91405 Orsay, France
¹⁰ ESAC/ESA, Camino Bajo del Castillo, s/n., Urb. Villafranca del Castillo, 28692 Villanueva de la Cañada, Madrid, Spain
¹¹ School of Mathematics and Physics, University of Surrey, Guildford, Surrey GU2 7XH, UK
¹² INAF-Osservatorio Astronomico di Brera, Via Brera 28, 20122 Milano, Italy
¹³ IFPU, Institute for Fundamental Physics of the Universe, via Beirut 2, 34151 Trieste, Italy
¹⁴ INAF-Osservatorio Astronomico di Trieste, Via G. B. Tiepolo 11, 34143 Trieste, Italy
¹⁵ INFN, Sezione di Trieste, Via Valerio 2, 34127 Trieste TS, Italy
¹⁶ SISSA, International School for Advanced Studies, Via Bonomea 265, 34136 Trieste TS, Italy
¹⁷ Dipartimento di Fisica e Astronomia, Università di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
¹⁸ INFN-Sezione di Bologna, Viale Berti Pichat 6/2, 40127 Bologna, Italy
¹⁹ INAF-Osservatorio Astronomico di Padova, Via dell’Osservatorio 5, 35122 Padova, Italy
²⁰ Dipartimento di Fisica, Università di Genova, Via Dodecaneso 33, 16146 Genova, Italy
²¹ INFN-Sezione di Genova, Via Dodecaneso 33, 16146 Genova, Italy
²² Department of Physics "E. Pancini", University Federico II, Via Cinthia 6, 80126 Napoli, Italy
²³ INAF-Osservatorio Astronomico di Capodimonte, Via Moiariello 16, 80131 Napoli, Italy
²⁴ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, PT4150-762 Porto, Portugal
²⁵ Faculdade de Ciências da Universidade do Porto, Rua do Campo de Alegre, 4150-007 Porto, Portugal
²⁶ Dipartimento di Fisica, Università degli Studi di Torino, Via P. Giuria 1, 10125 Torino, Italy
²⁷ INFN-Sezione di Torino, Via P. Giuria 1, 10125 Torino, Italy
²⁸ INAF-Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese (TO), Italy
²⁹ INAF-IASF Milano, Via Alfonso Corti 12, 20133 Milano, Italy
³⁰ Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT), Avenida Complutense 40, 28040 Madrid, Spain
³¹ Port d’Informació Científica, Campus UAB, C. Albareda s/n, 08193 Bellaterra (Barcelona), Spain
³² Institute for Theoretical Particle Physics and Cosmology (TTK), RWTH Aachen University, 52056 Aachen, Germany
³³ INAF-Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monteporzio Catone, Italy
³⁴ INFN section of Naples, Via Cinthia 6, 80126 Napoli, Italy
³⁵ Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
³⁶ Dipartimento di Fisica e Astronomia “Augusto Righi" – Alma Mater Studiorum Università di Bologna, Viale Berti Pichat 6/2, 40127 Bologna, Italy
³⁷ Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, UK
³⁸ European Space Agency/ESRIN, Largo Galileo Galilei 1, 00044 Frascati, Roma, Italy
³⁹ Université Claude Bernard Lyon 1, CNRS/IN2P3, IP2I Lyon, UMR 5822, Villeurbanne F-69100, France
⁴⁰ Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (IEEC-UB), Martí i Franquès 1, 08028 Barcelona, Spain
⁴¹ Institució Catalana de Recerca i Estudis Avançats (ICREA), Passeig de Lluís Companys 23, 08010 Barcelona, Spain
⁴² UCB Lyon 1, CNRS/IN2P3, IUF, IP2I Lyon, 4 rue Enrico Fermi, 69622 Villeurbanne, France
⁴³ Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK
⁴⁴ Departamento de Física, Faculdade de Ciências, Universidade de Lisboa, Edifício C8, Campo Grande, PT1749-016 Lisboa, Portugal
⁴⁵ Instituto de Astrofísica e Ciências do Espaço, Faculdade de Ciências, Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal
⁴⁶ Department of Astronomy, University of Geneva, ch. d’Ecogia 16, 1290 Versoix, Switzerland
⁴⁷ INFN-Padova, Via Marzolo 8, 35131 Padova, Italy
⁴⁸ Aix-Marseille Université, CNRS/IN2P3, CPPM, Marseille, France
⁴⁹ Max Planck Institute for Extraterrestrial Physics, Giessenbachstr. 1, 85748 Garching, Germany
⁵⁰ Universitäts-Sternwarte München, Fakultät für Physik, Ludwig-Maximilians-Universität München, Scheinerstrasse 1, 81679 München, Germany
⁵¹ INAF-Istituto di Astrofisica e Planetologia Spaziali, via del Fosso del Cavaliere, 100, 00100 Roma, Italy
⁵² Space Science Data Center, Italian Space Agency, via del Politecnico snc, 00133 Roma, Italy
⁵³ School of Physics, HH Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK
⁵⁴ FRACTAL S.L.N.E., calle Tulipán 2, Portal 13 1A, 28231 Las Rozas de Madrid, Spain
⁵⁵ Institute of Theoretical Astrophysics, University of Oslo, P.O. Box 1029 Blindern, 0315 Oslo, Norway
⁵⁶ Leiden Observatory, Leiden University, Einsteinweg 55, 2333 CC, Leiden, The Netherlands
⁵⁷ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
⁵⁸ Felix Hormuth Engineering, Goethestr. 17, 69181 Leimen, Germany
⁵⁹ Technical University of Denmark, Elektrovej 327, 2800 Kgs. Lyngby, Denmark
⁶⁰ Cosmic Dawn Center (DAWN), Denmark
⁶¹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁶² NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁶³ Department of Physics and Helsinki Institute of Physics, Gustaf Hällströmin katu 2, 00014 University of Helsinki, Finland
⁶⁴ Université de Genève, Département de Physique Théorique and Centre for Astroparticle Physics, 24 quai Ernest-Ansermet, CH-1211 Genève 4, Switzerland
⁶⁵ Department of Physics, P.O. Box 64 00014 University of Helsinki, Finland
⁶⁶ Helsinki Institute of Physics, Gustaf Hällströmin katu 2, University of Helsinki, Helsinki, Finland
⁶⁷ Laboratoire d’etude de l’Univers et des phenomenes eXtremes, Observatoire de Paris, Université PSL, Sorbonne Université, CNRS, 92190 Meudon, France
⁶⁸ SKA Observatory, Jodrell Bank, Lower Withington, Macclesfield, Cheshire SK11 9FT, UK
⁶⁹ Dipartimento di Fisica “Aldo Pontremoli", Università degli Studi di Milano, Via Celoria 16, 20133 Milano, Italy
⁷⁰ INFN-Sezione di Milano, Via Celoria 16, 20133 Milano, Italy
⁷¹ Universität Bonn, Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
⁷² INFN-Sezione di Roma, Piazzale Aldo Moro, 2 – c/o Dipartimento di Fisica, Edificio G. Marconi, 00185 Roma, Italy
⁷³ Aix-Marseille Université, CNRS, CNES, LAM, Marseille, France
⁷⁴ Dipartimento di Fisica e Astronomia “Augusto Righi" – Alma Mater Studiorum Università di Bologna, via Piero Gobetti 93/2, 40129 Bologna, Italy
⁷⁵ Department of Physics, Institute for Computational Cosmology, Durham University, South Road, Durham DH1 3LE, UK
⁷⁶ Université Paris Cité, CNRS, Astroparticule et Cosmologie, 75013 Paris, France
⁷⁷ CNRS-UCB International Research Laboratory, Centre Pierre Binetruy, IRL2007, CPB-IN2P3 Berkeley, USA
⁷⁸ Institut d’Astrophysique de Paris, 98bis Boulevard Arago, 75014 Paris, France
⁷⁹ Institut d’Astrophysique de Paris, UMR 7095, CNRS, and Sorbonne Université, 98 bis boulevard Arago, 75014 Paris, France
⁸⁰ Institute of Physics, Laboratory of Astrophysics, Ecole Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland
⁸¹ Aurora Technology for European Space Agency (ESA), Camino bajo del Castillo, s/n, Urbanizacion Villafranca del Castillo, Villanueva de la Cañada, 28692 Madrid, Spain
⁸² Institut de Física d’Altes Energies (IFAE), The Barcelona Institute of Science and Technology, Campus UAB, 08193 Bellaterra (Barcelona), Spain
⁸³ European Space Agency/ESTEC, Keplerlaan 1, 2201 AZ, Noordwijk, The Netherlands
⁸⁴ DARK, Niels Bohr Institute, University of Copenhagen, Jagtvej 155, 2200 Copenhagen, Denmark
⁸⁵ Waterloo Centre for Astrophysics, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada
⁸⁶ Department of Physics and Astronomy, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada
⁸⁷ Perimeter Institute for Theoretical Physics, Waterloo, Ontario N2L 2Y5, Canada
⁸⁸ Centre National d’Etudes Spatiales – Centre spatial de Toulouse, 18 avenue Edouard Belin, 31401 Toulouse Cedex 9, France
⁸⁹ Institute of Space Science, Str. Atomistilor, nr. 409 Măgurele, Ilfov 077125, Romania
⁹⁰ Dipartimento di Fisica e Astronomia “G. Galilei", Università di Padova, Via Marzolo 8, 35131 Padova, Italy
⁹¹ Institut für Theoretische Physik, University of Heidelberg, Philosophenweg 16, 69120 Heidelberg, Germany
⁹² Institut de Recherche en Astrophysique et Planétologie (IRAP), Université de Toulouse, CNRS, UPS, CNES, 14 Av. Edouard Belin, 31400 Toulouse, France
⁹³ Université St Joseph; Faculty of Sciences, Beirut, Lebanon
⁹⁴ Departamento de Física, FCFM, Universidad de Chile, Blanco Encalada 2008, Santiago, Chile
⁹⁵ Institut d’Estudis Espacials de Catalunya (IEEC), Edifici RDIT, Campus UPC, 08860 Castelldefels, Barcelona, Spain
⁹⁶ Satlantis, University Science Park, Sede Bld 48940, Leioa-Bilbao, Spain
⁹⁷ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans, s/n, 08193 Barcelona, Spain
⁹⁸ Centre for Electronic Imaging, Open University, Walton Hall, Milton Keynes MK7 6AA, UK
⁹⁹ Instituto de Astrofísica e Ciências do Espaço, Faculdade de Ciências, Universidade de Lisboa, Tapada da Ajuda, 1349-018 Lisboa, Portugal
¹⁰⁰ Universidad Politécnica de Cartagena, Departamento de Electrónica y Tecnología de Computadoras, Plaza del Hospital 1, 30202 Cartagena, Spain
¹⁰¹ Centre for Information Technology, University of Groningen, P.O. Box 11044 9700 CA, Groningen, The Netherlands
¹⁰² INFN-Bologna, Via Irnerio 46, 40126 Bologna, Italy
¹⁰³ Kapteyn Astronomical Institute, University of Groningen, PO Box 800 9700 AV, Groningen, The Netherlands
¹⁰⁴ Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, CA 91125, USA
¹⁰⁵ ICL, Junia, Université Catholique de Lille, LITL, 59000 Lille, France

^⋆⋆ Corresponding author: s.larsen@astro.ru.nl

Received: 20 March 2025
Accepted: 28 July 2025

Abstract

We examine the star cluster populations in the three nearby (3.20–3.45 Mpc) galaxies IC 342, NGC 2403, and Holmberg II, observed as part of the Euclid Early Release Observations programme. Our main focus in this paper is old globular clusters (GCs), for which the wide field of view and excellent image quality of Euclid offer substantial advantages over previous works. With respect to IC 342, in particular, this is the first study of stellar clusters that goes beyond its nuclear cluster. After a selection process based on size and magnitude criteria, followed by visual inspection, we identified 111 old (≳1 Gyr) GC candidates in IC 342, 49 in NGC 2403 (of which 15 were previously known), along with 7 in Holmberg II. In addition, a number of younger and/or intermediate-age candidates were identified. The colour distributions of GC candidates in the two larger galaxies show hints of bimodality, with peaks at (I_E−{H_E)₀ = 0.36 and 0.79 in both IC 342 and NGC 2403, corresponding to metallicities of [Fe/H] ≈ −1.5 and [Fe/H] ≈ −0.5. These results are similar to those reported for the metal-poor and metal-rich GC subpopulations in the Milky Way. The luminosity functions of our GC candidates exhibit an excess of relatively faint objects, relative to a canonical, approximately Gaussian GC luminosity function (GCLF). Although some contamination from background galaxies and younger clusters might be present in our GC samples, we argue that the excess could be at least partially real. In particular, we find this could be the case for IC 342, where the excess objects could be similar to those previously identified, for example, in M101 and NGC 6946. The specific frequency of old GCs in IC 342, as determined on the basis of the brighter half of the GCLF, appears to be unusually low with S_N ≈ 0.2–0.3. The combined luminosity function of young and intermediate-age clusters in all three galaxies is consistent with a power-law distribution of dN/dL ∝ L^{−2.3 ± 0.1}. The total numbers of young clusters brighter than M(I_E) = −8 in NGC 2403 and Holmberg II are comparable with those found in their Local Group counterparts, namely, M33 and the Small Magellanic Cloud, respectively.