Euclid: Discovery of bright z ≃ 7 Lyman-break galaxies in UltraVISTA and Euclid COSMOS^★

¹ Department of Physics, Oxford University, Keble Road Oxford OX1 3RH, UK
² Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, University of Manchester, Oxford Road Manchester M13 9PL, UK
³ Department of Physics and Astronomy, University of the Western Cape, Bellville, Cape Town 7535, South Africa
⁴ MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge MA 02139, USA
⁵ Max-Planck-Institut für Astronomie, Königstuhl 17 69117 Heidelberg, Germany
⁶ Institute of Cosmology and Gravitation, University of Portsmouth, Portsmouth PO1 3FX, UK
⁷ Kapteyn Astronomical Institute, University of Groningen, PO Box 800 9700 AV Groningen, The Netherlands
⁸ Cosmic Dawn Center (DAWN)
⁹ Department of Physics & Astronomy, University of Sussex, Brighton BN1 9QH, UK
¹⁰ Leiden Observatory, Leiden University, Einsteinweg 55 2333 CC Leiden, The Netherlands
¹¹ Niels Bohr Institute, University of Copenhagen, Jagtvej 128 2200 Copenhagen, Denmark
¹² Department of Astronomy, University of Geneva, ch. d’Ecogia 16 1290 Versoix, Switzerland
¹³ School of Mathematics and Physics, University of Surrey, Guildford, Surrey GU2 7XH, UK
¹⁴ INAF-Osservatorio Astronomico di Brera, Via Brera 28 20122 Milano, Italy
¹⁵ INAF-Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Piero Gobetti 93/3 40129 Bologna, Italy
¹⁶ IFPU, Institute for Fundamental Physics of the Universe, Via Beirut 2 34151 Trieste, Italy
¹⁷ INAF-Osservatorio Astronomico di Trieste, Via G. B. Tiepolo 11 34143 Trieste, Italy
¹⁸ INFN, Sezione di Trieste, Via Valerio 2 34127 Trieste TS, Italy
¹⁹ SISSA, International School for Advanced Studies, Via Bonomea 265 34136 Trieste TS, Italy
²⁰ Dipartimento di Fisica e Astronomia, Università di Bologna, Via Gobetti 93/2 40129 Bologna, Italy
²¹ INFN-Sezione di Bologna, Viale Berti Pichat 6/2 40127 Bologna, Italy
²² Dipartimento di Fisica, Università di Genova, Via Dodecaneso 33 16146 Genova, Italy
²³ INFN-Sezione di Genova, Via Dodecaneso 33 16146 Genova, Italy
²⁴ Department of Physics “E. Pancini”, University Federico II, Via Cinthia 6 80126 Napoli, Italy
²⁵ INAF-Osservatorio Astronomico di Capodimonte, Via Moiariello 16 80131 Napoli, Italy
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 European Space Agency/ESTEC, Keplerlaan 1 2201 AZ Noordwijk, The Netherlands
³⁰ INAF-IASF Milano, Via Alfonso Corti 12 20133 Milano, Italy
³¹ Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT), Avenida Complutense 40 28040 Madrid, Spain
³² Port d’Informació Científica, Campus UAB, C. Albareda s/n 08193 Bellaterra (Barcelona), Spain
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 Instituto de Astrofísica de Canarias, Vía Láctea 38205 La Laguna Tenerife, Spain
³⁸ Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
³⁹ European Space Agency/ESRIN, Largo Galileo Galilei 1 00044 Frascati Roma, Italy
⁴⁰ ESAC/ESA, Camino Bajo del Castillo, s/n. Urb. Villafranca del Castillo 28692 Villanueva de la Cañada Madrid, Spain
⁴¹ Université Claude Bernard Lyon 1, CNRS/IN2P3, IP2I Lyon, UMR 5822, Villeurbanne F-69100, France
⁴² Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (IEEC-UB), Martí i Franquès 1 08028 Barcelona, Spain
⁴³ Institució Catalana de Recerca i Estudis Avançats (ICREA), Passeig de Lluís Companys 23 08010 Barcelona, Spain
⁴⁴ Institut de Ciencies de l’Espai (IEEC-CSIC), Campus UAB, Carrer de Can Magrans s/n Cerdanyola del Vallés 08193 Barcelona, Spain
⁴⁵ UCB Lyon 1, CNRS/IN2P3, IUF, IP2I Lyon, 4 rue Enrico Fermi 69622 Villeurbanne, France
⁴⁶ Mullard Space Science Laboratory, University College London, Holmbury St Mary Dorking Surrey RH5 6NT, UK
⁴⁷ Departamento de Física, Faculdade de Ciências, Universidade de Lisboa, Edifício C8 Campo Grande PT1749-016 Lisboa, Portugal
⁴⁸ Instituto de Astrofísica e Ciências do Espaço, Faculdade de Ciências, Universidade de Lisboa, Campo Grande 1749-016 Lisboa, Portugal
⁴⁹ Université Paris-Saclay, CNRS, Institut d’astrophysique spatiale, 91405 Orsay, France
⁵⁰ INFN-Padova, Via Marzolo 8 35131 Padova, Italy
⁵¹ Aix-Marseille Université, CNRS/IN2P3, CPPM, Marseille, France
⁵² INAF-Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere 100 00100 Roma, Italy
⁵³ Space Science Data Center, Italian Space Agency, Via del Politecnico snc 00133 Roma, Italy
⁵⁴ INFN-Bologna, Via Irnerio 46 40126 Bologna, Italy
⁵⁵ Institut d’Estudis Espacials de Catalunya (IEEC), Edifici RDIT, Campus UPC, 08860 Castelldefels Barcelona, Spain
⁵⁶ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n 08193 Barcelona, Spain
⁵⁷ University Observatory, LMU Faculty of Physics, Scheinerstrasse 1 81679 Munich, Germany
⁵⁸ Max Planck Institute for Extraterrestrial Physics, Giessenbachstr. 1 85748 Garching, Germany
⁵⁹ INAF-Osservatorio Astronomico di Padova, Via dell’Osservatorio 5 35122 Padova, Italy
⁶⁰ Universitäts-Sternwarte München, Fakultät für Physik, Ludwig-Maximilians-Universität München, Scheinerstrasse 1 81679 München, Germany
⁶¹ Dipartimento di Fisica “Aldo Pontremoli”, Università degli Studi di Milano, Via Celoria 16 20133 Milano, Italy
⁶² INFN-Sezione di Milano, Via Celoria 16 20133 Milano, Italy
⁶³ Institute of Theoretical Astrophysics, University of Oslo, P.O. Box 1029 Blindern 0315 Oslo, Norway
⁶⁴ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive Pasadena CA 91109, USA
⁶⁵ Department of Physics, Lancaster University, Lancaster LA1 4YB, UK
⁶⁶ Felix Hormuth Engineering, Goethestr. 17 69181 Leimen, Germany
⁶⁷ Technical University of Denmark, Elektrovej 327 2800 Kgs. Lyngby, Denmark
⁶⁸ Cosmic Dawn Center (DAWN), Denmark
⁶⁹ NASA Goddard Space Flight Center, Greenbelt MD 20771, USA
⁷⁰ Department of Physics and Astronomy, University College London, Gower Street London WC1E 6BT, UK
⁷¹ Department of Physics and Helsinki Institute of Physics, Gustaf Hällströmin katu 2, University of Helsinki, 00014 Helsinki, Finland
⁷² Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
⁷³ Université de Genève, Département de Physique Théorique and Centre for Astroparticle Physics, 24 quai Ernest-Ansermet CH-1211 Genève 4, Switzerland
⁷⁴ Department of Physics, P.O. Box 64 University of Helsinki 00014 Helsinki, Finland
⁷⁵ Helsinki Institute of Physics, Gustaf Hällströmin katu 2 University of Helsinki 00014 Helsinki, Finland
⁷⁶ Laboratoire d’etude de l’Univers et des phenomenes eXtremes, Observatoire de Paris, Université PSL, Sorbonne Université, CNRS, 92190 Meudon, France
⁷⁷ SKAO, Jodrell Bank, Lower Withington, Macclesfield SK11 9FT, United Kingdom
⁷⁸ Centre de Calcul de l’IN2P3/CNRS, 21 avenue Pierre de Coubertin 69627 Villeurbanne Cedex, France
⁷⁹ Universität Bonn, Argelander-Institut für Astronomie, Auf dem Hügel 71 53121 Bonn, Germany
⁸⁰ INFN-Sezione di Roma, Piazzale Aldo Moro, 2 – c/o Dipartimento di Fisica Edificio G. Marconi 00185 Roma, Italy
⁸¹ Aix-Marseille Université, CNRS, CNES, LAM, Marseille, France
⁸² Dipartimento di Fisica e Astronomia “Augusto Righi” – Alma Mater Studiorum Università di Bologna, Via Piero Gobetti 93/2 40129 Bologna, Italy
⁸³ Department of Physics, Institute for Computational Cosmology, Durham University, South Road Durham DH1 3LE, UK
⁸⁴ Université Paris Cité, CNRS, Astroparticule et Cosmologie, 75013 Paris, France
⁸⁵ CNRS-UCB International Research Laboratory, Centre Pierre Binétruy, IRL2007, CPB-IN2P3 Berkeley, USA
⁸⁶ University of Applied Sciences and Arts of Northwestern Switzerland, School of Engineering, 5210 Windisch, Switzerland
⁸⁷ Institut d’Astrophysique de Paris, 98bis Boulevard Arago 75014 Paris, France
⁸⁸ Institut d’Astrophysique de Paris, UMR 7095, CNRS, and Sorbonne Université, 98 bis boulevard Arago 75014 Paris, France
⁸⁹ Institute of Physics, Laboratory of Astrophysics, Ecole Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland
⁹⁰ Telespazio UK S.L. for European Space Agency (ESA), Camino bajo del Castillo, s/n Urbanizacion Villafranca del Castillo Villanueva de la Cañada 28692 Madrid, Spain
⁹¹ Institut de Física d’Altes Energies (IFAE), The Barcelona Institute of Science and Technology, Campus UAB, 08193 Bellaterra (Barcelona), Spain
⁹² DARK, Niels Bohr Institute, University of Copenhagen, Jagtvej 155 2200 Copenhagen, Denmark
⁹³ Waterloo Centre for Astrophysics, University of Waterloo, Waterloo Ontario N2L 3G1, Canada
⁹⁴ Department of Physics and Astronomy, University of Waterloo, Waterloo Ontario N2L 3G1, Canada
⁹⁵ Perimeter Institute for Theoretical Physics, Waterloo Ontario N2L 2Y5, Canada
⁹⁶ Centre National d’Etudes Spatiales – Centre spatial de Toulouse, 18 avenue Edouard Belin 31401 Toulouse Cedex 9, France
⁹⁷ Institute of Space Science, Str. Atomistilor nr. 409 Măgurele Ilfov 077125, Romania
⁹⁸ Dipartimento di Fisica e Astronomia “G. Galilei”, Università di Padova, Via Marzolo 8 35131 Padova, Italy
⁹⁹ Institut für Theoretische Physik, University of Heidelberg, Philosophenweg 16 69120 Heidelberg, Germany
¹⁰⁰ Institut de Recherche en Astrophysique et Planétologie (IRAP), Université de Toulouse, CNRS, UPS, CNES, 14 Av. Edouard Belin 31400 Toulouse, France
¹⁰¹ Université St Joseph; Faculty of Sciences, Beirut, Lebanon
¹⁰² Departamento de Física, FCFM, Universidad de Chile, Blanco Encalada 2008 Santiago, Chile
¹⁰³ Universität Innsbruck, Institut für Astro- und Teilchenphysik, Technikerstr. 25/8 6020 Innsbruck, Austria
¹⁰⁴ Satlantis, University Science Park, Sede Bld 48940 Leioa-Bilbao, Spain
¹⁰⁵ Infrared Processing and Analysis Center, California Institute of Technology, Pasadena CA 91125, USA
¹⁰⁶ Instituto de Astrofísica e Ciências do Espaço, Faculdade de Ciências, Universidade de Lisboa, Tapada da Ajuda 1349-018 Lisboa, Portugal
¹⁰⁷ Universidad Politécnica de Cartagena, Departamento de Electrónica y Tecnología de Computadoras, Plaza del Hospital 1 30202 Cartagena, Spain
¹⁰⁸ Aurora Technology for European Space Agency (ESA), Camino bajo del Castillo, s/n Urbanizacion Villafranca del Castillo Villanueva de la Cañada 28692 Madrid, Spain
¹⁰⁹ ICL, Junia, Université Catholique de Lille, LITL, 59000 Lille, France
¹¹⁰ ICSC – Centro Nazionale di Ricerca in High Performance Computing, Big Data e Quantum Computing, Via Magnanelli 2 Bologna, Italy

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Received: 1 October 2025
Accepted: 12 January 2026

Abstract

We present a search for z ≃ 7 Lyman-break galaxies using the 1.72 deg² near-infrared (NIR) UltraVISTA survey in the COSMOS field, reaching 5σ depths in Y of 26.2. We incorporated deep Euclid optical and Euclid + Spitzer NIR imaging for a full spectral energy distribution (SED) fitting analysis. We found 289 candidate galaxies at 6.5 ≤ z ≤ 7.5 covering −22.6 ≤ M_UV ≤ −20.2, faint enough to overlap with Hubble Space Telescope studies. We conducted a separate selection by including complementary Euclid performance verification imaging (reaching 5σ depths of 26.3), yielding 140 galaxies in 0.65 deg², with 38 sources unique to this sample. We computed the rest-frame UV luminosity function (UV LF) from our samples, extending below the knee (M^∗ = 21.14^+0.28_−0.25). We find that the shape of the UV LF is consistent with both a Schechter function and a double power law (DPL) at the magnitudes probed by this sample, with a DPL preferred at M_UV < −22.5 when bright-end results are included. The UltraVISTA + Euclid sample provides a clean measurement of the LF due to the overlapping NIR filters identifying molecular absorption features in the SEDs of ultra-cool dwarf interlopers, and additional faint galaxies were recovered. A comparison with JWST LFs at z > 7 suggests a gentle evolution in the bright-end slope, although this is limited by a lack of robust bright-end measurements at z > 9. We forecast that in the Euclid Deep Fields, the removal of contaminant ultra-cool dwarfs as point sources will be possible at J_E < 24.5. Finally, we present a high-equivalent-width Lyman-α emitter candidate identified by combining HSC, VISTA, and Euclid broadband photometry, highlighting the synergistic power these instruments will have in the Euclid Auxiliary Fields for identifying extreme sources in the epoch of reionisation.