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

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 $z\ge 7$, more than doubling the number of previously known quasars at $z\ge 7$. 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.

Accepted for publication in A&A