Review

Euclid: Quick Data Release (Q1) -- Optical hotspots in powerful radio galaxies

M. Hardcastle L. Bisigello M. Bondi M. Magliocchetti L. Morabito J. Petley F. Ricci H. Rottgering K. Rubinur D. Smith D. Stern B. Altieri S. Andreon N. Auricchio C. Baccigalupi M. Baldi A. Balestra S. Bardelli P. Battaglia A. Biviano E. Branchini M. Brescia S. Camera V. Capobianco C. Carbone J. Carretero S. Casas M. Castellano G. Castignani S. Cavuoti K. Chambers A. Cimatti C. Colodro-Conde G. Congedo C. Conselice L. Conversi Y. Copin A. Costille F. Courbin H. Courtois M. Cropper J. Cuby A. Da Silva H. Degaudenzi G. de Lucia C. Dolding H. Dole F. Dubath X. Dupac M. Farina R. Farinelli S. Ferriol P. Fosalba S. Fotopoulou M. Frailis E. Franceschi M. Fumana S. Galeotta K. George B. Gillis C. Giocoli J. Graciá-Carpio A. Grazian F. Grupp S. Haugan J. Hoar W. Holmes I. Hook F. Hormuth A. Hornstrup K. Jahnke M. Jhabvala B. Joachimi S. Kermiche A. Kiessling B. Kubik M. Kümmel M. Kunz H. Kurki-Suonio A. L. Le Brun S. Ligori P. Lilje V. Lindholm I. Lloro G. Mainetti O. Mansutti O. Marggraf M. Martinelli N. Martinet F. Marulli R. Massey E. Medinaceli S. Mei M. Meneghetti E. Merlin G. Meylan A. Mora M. Moresco L. Moscardini R. Nakajima C. Neissner S. Niemi J. Nightingale C. Padilla S. Paltani F. Pasian K. Pedersen W. Percival V. Pettorino S. Pires G. Polenta L. Popa F. Raison A. Renzi J. Rhodes G. Riccio E. Romelli M. Roncarelli B. Rusholme R. Saglia Z. Sakr D. Sapone B. Sartoris M. Schirmer P. Schneider A. Secroun G. Seidel E. Sihvola P. Simon C. Sirignano G. Sirri J. Skottfelt L. Stanco P. Tallada-Crespí A. Taylor H. Teplitz I. Tereno N. Tessore S. Toft R. Toledo-Moreo F. Torradeflot I. Tutusaus J. Valiviita T. Vassallo G. Kleijn A. Veropalumbo Y. Wang J. Weller O. Williams A. Zacchei G. Zamorani F. Zerbi E. Zucca J. García-Bellido V. Scottez F. Tarsitano J. L. D. O. Physics Astronomy Mathematics U. O. Hertfordshire C. Lane Hatfield AL10 9AB Uk INAF-Osservatorio Astronomico di Padova 5. Viadell'Osservatorio 35131 Padova Italy Inaf Istituto di Radioastronomia V. Gobetti 40129 Bologna I. D. A. E. P. Spaziali Via del Fosso del Cavaliere 100 00133 Roma D. Physics Centre for Extragalactic Astronomy Durham University S. Road Durham DH1 3LE Institute for Computational Cosmology Leiden Observatory Leiden University 55 Einsteinweg 2333 CC Leiden T. Netherlands D. O. Mathematics Physics Roma Tre University 84 ViadellaVascaNavale 00133 Rome INAF-Osservatorio Astronomico di Roma 33 viaFrascati 00078 Monteporzio Catone Institute of Theoretical Astrophysics U. Oslo P. O. B. 1. Blindern 0315 Oslo Norway Jet Propulsion Laboratory California institute of Technology 4800 Oak Grove Drive Pasadena Ca 91109 Usa Esacesa C. D. del Castillo Sn. Urb. Villafranca del Castillo 28692 Villanueva de la Cañada Madrid Spain INAF-Osservatorio Astronomico di Brera 28 ViaBrera 20133 Milano INAF-Osservatorio di Astrofisica e Scienza Dello Spazio di Bologna V. Gobetti Ifpu Institute for Fundamental Physics of the Universe 2. viaBeirut 34151 Trieste INAF-Osservatorio Astronomico di Trieste 11 ViaG.B.Tiepolo 34151 Trieste Infn Sezione di Trieste 2. ViaValerio TS 34127Trieste Sissa International School For Advanced Studies Via Bonomea TS 34136Trieste D. Astronomia U. Bologna V. Gobetti I. di Bologna 62 VialeBertiPichat 40129 Bologna D. Fisica U. Genova 33 viaDodecaneso 16146 Genova INFN-Sezione di Genova D. Pancini University Federico 6. ViaCinthia 80126 Napoli INAF-Osservatorio Astronomico di Capodimonte 16 ViaMoiariello 80131 Napoli U. Torino 1. viaP.Giuria 10125 Torino INFN-Sezione di Torino INAF-Osservatorio Astrofisico di Torino 20 viaOsservatorio 1. P. Torinese Inaf-Iasf Milano 12 ViaAlfonsoCorti 20133 Milano Centro de Investigaciones Energ'eticas Medioambientales Y. Tecnol'ogicas 40 AvenidaComplutense 28692 Madrid Port d'Informaci'O Cient'ifica C. Uab C. Sn 08193 Bellaterra Institute for Particle Physics Cosmology Rwth Aachen University 52056 Aachen Germany V. DeutschesZentrumfürLuft-undRaumfahrte. Linder Höhe 51147 Köln INFN-Sezione di Napoli I. F. Astronomy U. Hawai'i 2680 Woodlawn Drive Honolulu Hi D. Bologna Instituto de Astrofísica de Canarias E. Laguna Tenerife U. Edinburgh Royal Observatory B. Hill Edinburgh EH9 3HJ J. B. C. F. Astrophysics U. Manchester Oxford Road M13 9PL European Space AgencyESRIN 1. LargoGalileoGalilei 00044 Frascati Roma 1. Universit'eClaudeBernardLyon CNRS-IN2P3 Ipn Lyon Umr Villeurbanne F-69100 France A. Université Cnrs Cnes Lam Marseille Institut de Ciències del Cosmos U. Barcelona 1. Mart'iiFranques 08193 Barcelona I. C. de Recerca I Estudis Avançats 23 PasseigdeLluı́sCompanys 08193 Barcelona Institut de Ciències de l'Espai Carrer de Can Magrans Sn Cerdanyola del Vall'es 08193 Barcelona 1. UCBLyon Iuf 4. R. E. Fermi 69622 Villeurbanne Mullard Space Science Laboratory U. London Holmbury St Mary Dorking Surrey RH5 6NT Canada-France-Hawaii Telescope 65-1238 Mamalahoa Hwy Kamuela Hi Departamento de F'isica F. D. de Ciências Universidade de Lisboa C. Edif'icio C. Grande PT1749-016 Lisboa Portugal Instituto de Astrofísica e Ciências do Espaço 1749-016 Lisboa Department of Physics Astronomy U. Geneva 16 ch.d'Ecogia 1290 Versoix Switzerland Universit'e Paris-Saclay Institut d'Astrophysique Spatiale 91405 Orsay I. D. E. D. Catalunya Edifici Rdit C. Upc 08860 Castelldefels Barcelona Institute of Space Sciences Sn. School Of Physics HH Wills Physics Laboratory U. Bristol Tyndall Avenue Bristol BS8 1TL University Observatory Lmu Faculty of Physics 1. Scheinerstr. 81679 Munich Max Planck Institute for Extraterrestrial Physics 1. Giessenbachstr. 85748 Garching U. München F. Physik Ludwig-Maximilians-Universität München 8. 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Gobetti Universit'e Paris Cit'e Astroparticule Et Cosmologie 75005 Paris CNRS-UCB International Research Laboratory Centre Pierre Bin'etruy Irl CPB-IN2P3 Berkeley I. Physics Laboratory of Astrophysics Ecole Polytechnique F'ed'erale de Lausanne Observatoire de Sauverny T. U. F. T. E. S. Agency C. D. del Castillo Urb. Villafranca del Castillo Villanueva de la Cañada 28692 Madrid Institut de F'isica d'Altes Energies The Barcelona Institute of Science Technology European Space AgencyESTEC 1. Keplerlaan 2201 AZ Noordwijk S. O. Mathematics Statistics N. University Herschel Building Newcastle-upon-Tyne NE1 7RU Dark Niels Bohr Institute U. Copenhagen Jagtvej 2200 Copenhagen Waterloo Centre for Astrophysics U. Waterloo Waterloo Ontario N2L 3G1 Canada Perimeter Institute for Theoretical Physics Ontario N2L 2Y5 Cea Aim 91191 Gif-sur-yvette S. Center Italian Space Agency Via del Politecnico Snc 00133 Roma Institute of Space Science Str. Atomistilor nr. 409 Măgurele Ilfov 077125 România D. Galilei Universita' di Padova 8. ViaMarzolo 35131 Padova INFN-Padova CaltechIPAC 1. E. C. Blvd. Ca Instituto de Física Teórica UAM-CSIC Campus de Cantoblanco 28692 Madrid Institut de Recherche en Astrophysique et Plan'etologie U. Toulouse

Sep 2026 · 0 citations · 70 references

Physics

Abstract

The hotspots of powerful radio galaxies are the sites of high-energy particle acceleration, but radio observations give us limited information on the electron energy distributions that they generate, which affects our overall understanding of their physics and energetics. Optical synchrotron counterparts to radio galaxy hotspots, which probe high electron energies, have been known for many years, but have only been studied in a small number of objects selected as being very bright in the radio. By combining Low-Frequency Array (LOFAR) observations with the Euclid Q1 data release in the Euclid Deep Field North, we identify 77 powerful radio galaxies with hotspots and show that a small but significant fraction of them, 7--15\%, may have detected infrared and optical counterparts to their hotspots. The hotspot spectral energy distributions in our clearest cases are consistent with particle acceleration up to (but not beyond) 1\,TeV in energy on the assumption of field strengths close to equipartition, whereas non-detected hotspots either do not reach such high electron energies or would have to have very different electron energy spectra or significantly sub-equipartition magnetic field strengths. Two new candidate optical hotspots are clearly detected matching high-resolution LOFAR data; one is remarkable as both the highest-redshift optical hotspot known to date ($z \approx 1.6$) and also the first optical detection of the hotspot of the inner lobe of a restarting source. We discuss approaches to performing a large-scale study of hotspots with the full wide-area Euclid surveys, particularly when combined with the international baselines of LOFAR, enabling for the first time a statistical analysis of high-energy particle acceleration in hotspots.

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