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arXiv · 2607.03432

Euclid: Discovery of 31 new quasars at $6.6 < z < 7.8$

D. Yang·J. F. Hennawi·F. Guarneri·J. Wolf·S. Belladitta·J. -T. Schindler·A. C. N. Hughes·E. Bañados·D. J. Mortlock·J. Yang·F. Wang·X. Fan·K. Jahnke·D. Stern·C. J. Willott·A. J. Barth·H. J. A. Rottgering·R. G. Varadaraj·R. Decarli·A. -C. Eilers·M. Ezziati·Y. Fu·J. Huang·X. Jin·Y. Kang·L. N. Martinez-Ramirez·Y. Matsuoka·M. Onoue·R. Pello·R. P. Remigio·W. L. Tee·B. Venemans·G. Vietri·B. Wang·L. J. Abbo·H. Atek·S. Bisogni·S. E. I. Bosman·R. A. A. Bowler·C. J. Conselice·F. B. Davies·C. M. Gutierrez·Y. Harikane·K. Rubinur·C. C. Lovell·M. Magliocchetti·J. Matthee·F. Ricci·M. Scialpi·D. Scott·L. Spinoglio·F. Tarsitano·Y. Toba·F. Walter·J. R. Weaver·G. Zamorani·B. Altieri·A. Amara·S. Andreon·H. Aussel·C. Baccigalupi·M. Baldi·A. Balestra·S. Bardelli·P. Battaglia·A. Biviano·E. Branchini·M. Brescia·S. Camera·G. Cañas-Herrera·V. Capobianco·C. Carbone·J. Carretero·M. Castellano·G. Castignani·S. Cavuoti·K. C. Chambers·A. Cimatti·C. Colodro-Conde·G. Congedo·L. Conversi·Y. Copin·F. Courbin·H. M. Courtois·M. Cropper·J. -C. Cuillandre·H. Degaudenzi·G. De Lucia·C. Dolding·H. Dole·M. Douspis·F. Dubath·X. Dupac·S. Dusini·S. Escoffier·M. Farina·R. Farinelli·S. Ferriol·F. Finelli·N. Fourmanoit
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Abstract

We report the discovery of 31 new high-$z$ quasars in the redshift range $6.6 < z < 7.8$. These quasars were selected from approximately 3000 deg$^2$ of sky covered during the first 1.5 years of the Euclid Wide Survey, representing the initial results of the Euclid high-$z$ quasar search. Our candidate selection employed multiple machine-learning and probabilistic techniques applied to the Euclid $I_E$, $Y_E$, $J_E$, and $H_E$ images, supplemented by ancillary $z$-band data when available. Spectroscopic follow-up observations were carried out with Keck, Magellan, and the Large Binocular Telescope (LBT). Among the new discoveries, there are 12 quasars at $z \geq 7$, more than doubling the number of previously known quasars at $z \geq 7$. The newly discovered quasars exhibit $21.2 < J_E < 23.2$ ($-25.5 < M_{1450} < -23.6$), extending quasar studies to the faint end of the quasar luminosity function (QLF) at $z \gtrsim 7$. The quasar with the highest-$z$, EUCL J172902.75+641018.1 at $z \approx 7.77$, sets the new redshift record for the most distant quasar ever reported. These discoveries demonstrate Euclid's transformative role in high-$z$ quasar discovery and set the stage for future follow-up studies of the early galaxies hosting quasars, supermassive black hole growth, and the intergalactic medium in the epoch of reionisation.

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D. Yang, J. F. Hennawi, F. Guarneri, J. Wolf, S. Belladitta, J. -T. Schindler, A. C. N. Hughes, E. Bañados, D. J. Mortlock, J. Yang, F. Wang, X. Fan, K. Jahnke, D. Stern, C. J. Willott, A. J. Barth, H. J. A. Rottgering, R. G. Varadaraj, R. Decarli, A. -C. Eilers, M. Ezziati, Y. Fu, J. Huang, X. Jin, Y. Kang, L. N. Martinez-Ramirez, Y. Matsuoka, M. Onoue, R. Pello, R. P. Remigio, W. L. Tee, B. Venemans, G. Vietri, B. Wang, L. J. Abbo, H. Atek, S. Bisogni, S. E. I. Bosman, R. A. A. Bowler, C. J. Conselice, F. B. Davies, C. M. Gutierrez, Y. Harikane, K. Rubinur, C. C. Lovell, M. Magliocchetti, J. Matthee, F. Ricci, M. Scialpi, D. Scott, L. Spinoglio, F. Tarsitano, Y. Toba, F. Walter, J. R. Weaver, G. Zamorani, B. Altieri, A. Amara, S. Andreon, H. Aussel, C. Baccigalupi, M. Baldi, A. Balestra, S. Bardelli, P. Battaglia, A. Biviano, E. Branchini, M. Brescia, S. Camera, G. Cañas-Herrera, V. Capobianco, C. Carbone, J. Carretero, M. Castellano, G. Castignani, S. Cavuoti, K. C. Chambers, A. Cimatti, C. Colodro-Conde, G. Congedo, L. Conversi, Y. Copin, F. Courbin, H. M. Courtois, M. Cropper, J. -C. Cuillandre, H. Degaudenzi, G. De Lucia, C. Dolding, H. Dole, M. Douspis, F. Dubath, X. Dupac, S. Dusini, S. Escoffier, M. Farina, R. Farinelli, S. Ferriol, F. Finelli, N. Fourmanoit. 2026-07-03. Euclid: Discovery of 31 new quasars at $6.6 < z < 7.8$. https://doi.org/10.1051/0004-6361/202658883

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