2607.03432v1
Euclid: Discovery of 31 new quasars at $6.6 < z < 7.8$
First listed 2026-07-07 | Last updated 2026-07-03
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.
Short digest
This paper presents the first Euclid high-redshift quasar haul: 31 new quasars at 6.6 < z < 7.8 selected over about 3000 deg^2 from the first 1.5 years of the Euclid Wide Survey, using Euclid I_E, Y_E, J_E, and H_E imaging plus ancillary z-band data and spectroscopic confirmation with Keck, Magellan, and LBT. Twelve of the new objects lie at z >= 7, more than doubling the previously known quasar population in that regime, and the sample reaches J_E = 23.2 and M_1450 = -23.6, extending direct quasar work into the faint end of the z > 7 luminosity function. The headline source, EUCL J172902.75+641018.1 at z approximately 7.77, is reported as the most distant quasar yet found. More broadly, the result establishes Euclid as a discovery machine for the reionization-era quasar population and a major new route into early SMBH growth, quasar hosts, and the surrounding intergalactic medium.
Key figures to inspect
- Figure 1. Use this as the survey-context figure. It shows the Euclid Wide Survey area actually searched in the first 1.5 years, the eventual mission footprint, and where the newly discovered quasars lie on the sky, which immediately conveys the scale of the discovery space behind the 31-object sample.
- Figure 4. This is the key selection-definition figure. The color-color diagrams place the new quasars against contaminants, the synthetic high-redshift quasar track, and the empirical brown-dwarf track, making clear how Euclid photometry separates real z > 6.6 quasars from the main interloper population.
- Figure 5. This is the most important synthesis figure for the paper’s scientific claim. By plotting redshift against M_1450 for the new Euclid quasars alongside pre-Euclid quasars, SHELLQs objects, LBGs, and JWST faint-AGN candidates, it shows exactly how the sample expands the known population toward fainter luminosities at z roughly 7 and above.
- Figure 6. Include this because the paper’s headline object is EUCL J1729 at z approximately 7.77. The Euclid cutouts and LBT/LUCI spectrum provide the direct observational evidence behind the new redshift record and make the flagship discovery concrete for readers.
- Figure 9. This later diagnostic figure adds value beyond the discovery plots. The stacked rest-frame UV spectra test whether the new Euclid quasars look systematically different from comparison samples and support the paper’s conclusion that no significant spectral evolution is identified in the stacks.
Discussion
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