June
2026
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2026A&A...711A..16E
Authors
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Euclid Collaboration
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Roster, W.
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Salvato, M.
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Buchner, J.
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Shirley, R.
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Lusso, E.
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Landt, H.
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Zamorani, G.
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Siudek, M.
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Laloux, B.
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Matamoro Zatarain, T.
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Ricci, F.
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Fotopoulou, S.
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Ferré-Mateu, A.
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Lopez Lopez, X.
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Aghanim, N.
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Altieri, B.
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Amara, A.
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Andreon, S.
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Auricchio, N.
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Aussel, H.
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Baccigalupi, C.
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Baldi, M.
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Balestra, A.
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Bardelli, S.
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Battaglia, P.
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Biviano, A.
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Bonchi, A.
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Branchini, E.
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Brescia, M.
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Brinchmann, J.
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Camera, S.
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Cañas-Herrera, G.
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Capobianco, V.
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Carbone, C.
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Carretero, J.
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Casas, S.
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Castellano, M.
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Castignani, G.
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Cavuoti, S.
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Chambers, K. C.
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Cimatti, A.
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Colodro-Conde, C.
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Congedo, G.
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Conselice, C. J.
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Conversi, L.
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Copin, Y.
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Courbin, F.
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Courtois, H. M.
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Cropper, M.
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Da Silva, A.
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Degaudenzi, H.
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De Lucia, G.
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Di Giorgio, A. M.
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Dolding, C.
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Dole, H.
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Dubath, F.
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Duncan, C. A. J.
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Dupac, X.
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Dusini, S.
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Escoffier, S.
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Fabricius, M.
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Farina, M.
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Farinelli, R.
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Faustini, F.
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Ferriol, S.
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Finelli, F.
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Fosalba, P.
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Fourmanoit, N.
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Frailis, M.
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Franceschi, E.
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Galeotta, S.
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George, K.
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Gillis, B.
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Giocoli, C.
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Gracia-Carpio, J.
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Granett, B. R.
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Grazian, A.
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Grupp, F.
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Gwyn, S.
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Haugan, S. V. H.
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Holmes, W.
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Hook, I. M.
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Hormuth, F.
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Hornstrup, A.
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Hudelot, P.
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Jahnke, K.
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Jhabvala, M.
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Keihänen, E.
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Kermiche, S.
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Kiessling, A.
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Kubik, B.
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Kümmel, M.
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Kunz, M.
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Kurki-Suonio, H.
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Le Boulc'h, Q.
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Le Brun, A. M. C.
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Le Mignant, D.
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Ligori, S.
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Lilje, P. B.
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Lindholm, V.
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Lloro, I.
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Mainetti, G.
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Maino, D.
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Maiorano, E.
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Mansutti, O.
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Marcin, S.
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Marggraf, O.
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Martinelli, M.
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Martinet, N.
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Marulli, F.
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Massey, R.
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Masters, D. C.
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Medinaceli, E.
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Mei, S.
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Melchior, M.
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Mellier, Y.
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Meneghetti, M.
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Merlin, E.
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Meylan, G.
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Mora, A.
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Moresco, M.
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Moscardini, L.
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Nakajima, R.
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Neissner, C.
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Niemi, S.-M.
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Nightingale, J. W.
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Padilla, C.
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Paltani, S.
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Pasian, F.
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Pedersen, K.
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Percival, W. J.
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Pettorino, V.
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Pires, S.
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Polenta, G.
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Poncet, M.
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Popa, L. A.
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Pozzetti, L.
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Raison, F.
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Rebolo, R.
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Renzi, A.
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Rhodes, J.
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Riccio, G.
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Romelli, E.
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Roncarelli, M.
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Saglia, R.
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Sakr, Z.
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Sánchez, A. G.
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Sapone, D.
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Sartoris, B.
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Schewtschenko, J. A.
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Schirmer, M.
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Schneider, P.
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Schrabback, T.
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Secroun, A.
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Seidel, G.
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Seiffert, M.
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Serrano, S.
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Simon, P.
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Sirignano, C.
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Sirri, G.
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Stanco, L.
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Steinwagner, J.
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Tallada-Crespí, P.
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Tavagnacco, D.
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Taylor, A. N.
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Tereno, I.
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Toft, S.
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Toledo-Moreo, R.
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Torradeflot, F.
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Tutusaus, I.
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Valenziano, L.
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Valiviita, J.
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Vassallo, T.
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Verdoes Kleijn, G.
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Veropalumbo, A.
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Wang, Y.
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Weller, J.
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Zacchei, A.
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Zerbi, F. M.
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Zinchenko, I. A.
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Zucca, E.
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Allevato, V.
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Ballardini, M.
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Bolzonella, M.
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Bozzo, E.
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Burigana, C.
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Cabanac, R.
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Cappi, A.
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Di Ferdinando, D.
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Escartin Vigo, J. A.
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Gabarra, L.
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Huertas-Company, M.
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Martín-Fleitas, J.
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Matthew, S.
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Mauri, N.
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Metcalf, R. B.
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Pezzotta, A.
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Pöntinen, M.
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Porciani, C.
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Risso, I.
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Scottez, V.
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Sereno, M.
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Tenti, M.
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Viel, M.
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Wiesmann, M.
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Akrami, Y.
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Andika, I. T.
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Anselmi, S.
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Archidiacono, M.
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Atrio-Barandela, F.
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Benoist, C.
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Benson, K.
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Bertacca, D.
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Bethermin, M.
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Bisigello, L.
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Blanchard, A.
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Blot, L.
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Böhringer, H.
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Brown, M. L.
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Bruton, S.
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Calabro, A.
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Caro, F.
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Carvalho, C. S.
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Castro, T.
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Cogato, F.
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Cooray, A. R.
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Cucciati, O.
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Davini, S.
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De Paolis, F.
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Desprez, G.
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Díaz-Sánchez, A.
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Diaz, J. J.
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Di Domizio, S.
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Diego, J. M.
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Enia, A.
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Fang, Y.
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Ferrari, A. G.
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Finoguenov, A.
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Fontana, A.
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Franco, A.
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Ganga, K.
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García-Bellido, J.
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Gasparetto, T.
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Gautard, V.
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Gaztanaga, E.
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Giacomini, F.
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Gianotti, F.
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Gozaliasl, G.
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Guidi, M.
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Gutierrez, C. M.
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Hall, A.
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Hartley, W. G.
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Hemmati, S.
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Hernández-Monteagudo, C.
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Hildebrandt, H.
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Hjorth, J.
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Kajava, J. J. E.
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Kang, Y.
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Kansal, V.
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Karagiannis, D.
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Kiiveri, K.
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Kirkpatrick, C. C.
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Kruk, S.
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Le Graet, J.
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Legrand, L.
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Lembo, M.
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Lepori, F.
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Leroy, G.
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Lesci, G. F.
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Lesgourgues, J.
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Leuzzi, L.
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Liaudat, T. I.
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Loureiro, A.
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Macias-Perez, J.
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Maggio, G.
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Magliocchetti, M.
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Mannucci, F.
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Maoli, R.
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Martins, C. J. A. P.
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Maurin, L.
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Miluzio, M.
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Monaco, P.
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Moretti, C.
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Morgante, G.
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Naidoo, K.
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Navarro-Alsina, A.
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Nesseris, S.
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Passalacqua, F.
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Paterson, K.
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Patrizii, L.
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Pisani, A.
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Potter, D.
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Quai, S.
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Radovich, M.
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Sacquegna, S.
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Sahlén, M.
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Sanders, D. B.
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Sarpa, E.
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Schneider, A.
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Sciotti, D.
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Sellentin, E.
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Shankar, F.
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Smith, L. C.
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Tanidis, K.
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Testera, G.
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Teyssier, R.
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Tosi, S.
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Troja, A.
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Tucci, M.
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Valieri, C.
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Venhola, A.
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Vergani, D.
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Verza, G.
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Vielzeuf, P.
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Viitanen, A.
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Walton, N. A.
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Soubrie, E.
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Scott, D.
Abstract
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To better understand the role of active galactic nuclei (AGN) in galaxy evolution, it is crucial to work with a complete and pure AGN sample. X-ray surveys are key to doing so, but their larger positional uncertainties complicate counterpart (CTP) association, further compounded by the limited availability of deep, uniform multi-wavelength ancillary data. Euclid is revolutionising this identification effort, offering extensive coverage of nearly the entire extragalactic sky, particularly in the near-infrared bands, where AGN are more easily detected. Using the first Euclid Quick Data Release (Q1), we validated the methods for identifying and classifying Euclid CTPs of known point-like sources from major X-ray surveys, including XMM-Newton, Chandra, and eROSITA. Using Bayesian statistics, combined with machine learning (ML), as incorporated in the algorithm NWAY, we identified the CTPs of 11 286 X-ray sources from the three X-ray telescopes. For the large majority of 10 194 sources, the association is unique, with the remaining ∼10% of multi-CTP cases equally split between XMM-Newton and eROSITA. Six percent of the Euclid CTPs are detected in more than one X-ray survey. We then used ML to distinguish between Galactic (8%) and extragalactic (92%) sources. We computed photo-zs using deep learning for the 9259 sources detected in the tenth data release of the DESI Legacy Survey, reaching an accuracy and a fraction of outliers of roughly 5%. Based on their X-ray luminosities, all CTPs identified as extragalactic are classified as AGN, most of which appear as type I AGN according to their hardness ratios. With this paper, we release our catalogue, which includes identifiers, basic X-ray properties, the reliability of the associations, and additional property extensions, such as Galactic- or extragalactic classifications and photometric/spectroscopic redshifts. We also provide probabilities for sub-selecting the sample based on purity and completeness, in order to allow users to tailor the sample according to their specific needs.
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