REVIEW 5 minor 119 references
Euclid has found 31 new quasars at 6.6 < z < 7.8, including a record-holder at z ≈ 7.77 that more than doubles the known sample beyond redshift 7.
Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →
T0 review · grok-4.5
2026-07-12 02:30 UTC pith:XXZ6YO6U
load-bearing objection Solid spectroscopic census: 31 new Euclid quasars, 12 at z≥7, new record at z≈7.77, and a real faint-end sample.
Euclid: Discovery of 31 new quasars at 6.6 < z < 7.8
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Spectroscopic follow-up of machine-learning and probabilistic candidates drawn from the first 3000 deg² of the Euclid Wide Survey has confirmed 31 new quasars at 6.6 < z < 7.8, of which 12 sit at z ≥ 7 (more than doubling the pre-Euclid tally) and one reaches z ≈ 7.77, the current redshift record. Their J_E magnitudes (21.2–23.2) place them on the faint side of the high-z luminosity function.
What carries the argument
Multi-algorithm photometric selection (extreme deconvolution, XGBoost, template SED fitting, Bayesian model comparison) on Euclid I_E Y_E J_E H_E (plus ancillary z-band) that isolates Lyman-break dropouts, followed by optical/near-IR spectroscopy on 10-m and 6.5-m telescopes that confirms the break and emission lines.
Load-bearing premise
Redshifts and quasar classifications rest on visual inspection of the Lyman-alpha break or line; some of the faintest objects with atypical profiles could still be luminous galaxies or heavily absorbed systems rather than type-I quasars.
What would settle it
Deeper rest-frame UV or optical spectra (or high-resolution size measurements) that show the faintest ‘quasars’ lack broad high-ionisation lines or are spatially extended would reclassify them as galaxies and shrink the claimed z ≥ 7 sample.
If this is right
- The known census of z ≥ 7 quasars is more than doubled, giving a usable sample for early black-hole growth studies.
- Faint (M1450 ~ −24) quasars at z ≳ 7 become accessible for IGM damping-wing and proximity-zone measurements.
- Two radio-loud objects already demonstrate Euclid–LOFAR synergy for jet-powered systems at z ~ 7.
- Full-survey forecasts of >100 quasars at 7 < z < 7.5 become plausible once selection completeness is quantified.
Where Pith is reading between the lines
- If the faint-end space density continues to match or exceed pre-Euclid extrapolations, existing quasar luminosity functions at z ≳ 7 will need upward revision.
- The same selection pipeline applied to the remaining ~11 000 deg² should produce the first secure z > 8 quasars within a few years.
- Objects that sit near the quasar–galaxy luminosity-function crossover will force a practical distinction between type-I AGN and extreme star-forming galaxies at cosmic dawn.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper reports the spectroscopic confirmation of 31 new quasars at 6.6 < z < 7.8 selected from ~3000 deg^{2} of early Euclid Wide Survey imaging. Candidate selection used multiple machine-learning and probabilistic methods on Euclid I_E, Y_E, J_E, H_E photometry (plus ancillary z-band when available); follow-up was obtained with Keck (LRIS, KCWI, MOSFIRE), Magellan/FIRE, and LBT (MODS/LUCI). Twelve of the objects lie at z ≥ 7, more than doubling the pre-Euclid census at these redshifts, and EUCL J172902.75+641018.1 at z ≈ 7.77 is presented as the new redshift record. The sample reaches 21.2 < J_E < 23.2 (−25.5 < M_1450 < −23.6), thereby extending the known population to the faint end of the QLF at z ≳ 7. Discovery spectra, multi-band cutouts, photometry, and observation logs are provided in the main text and appendices.
Significance. If the spectroscopic classifications hold, this is a major observational advance: it more than doubles the z ≥ 7 quasar sample, establishes a new redshift record, and populates the previously sparse faint end (M_1450 ∼ −24) at z ≳ 7. The work demonstrates Euclid’s practical capability for high-z quasar discovery and supplies a concrete target list for JWST, ALMA/NOEMA, and IGM studies. Strengths include multi-instrument confirmation, public cutouts, and transparent discussion of residual redshift and classification uncertainties for the weakest-Lyα objects. The result is an observational census rather than a model-dependent inference, so the central claim is robust to the usual free parameters of photometric selection.
minor comments (5)
- Redshifts are visual (typical Δz ∼ 0.05–0.1, up to ∼0.2 for weak/absorbed Lyα; Sect. 5 and Table A.3 notes). A short quantitative statement of how many objects fall into the higher-uncertainty bin, and whether any of the z ≥ 7 claims rest solely on those, would help readers assess the record-holder and the doubling claim.
- Sect. 5.1 and the notes on atypical Lyα profiles correctly flag that a subset of the faintest sources could be luminous galaxies or strong damping-wing systems. Fig. 10 (point-source consistency) is reassuring; a one-sentence cross-reference in the abstract or summary would make the residual ambiguity more visible to non-specialists.
- Southern follow-up is incomplete (Sect. 2.1, 7). The provisional comparison with the Euclid Collaboration: Barnett et al. (2019) forecast is appropriately caveated, but a clearer statement of the confirmed fraction of the high-priority list would strengthen the discussion of possible QLF tension.
- Table A.3 and the M_1450 conversion assume a fixed α = −1.7. A brief note on the sensitivity of the faintest M_1450 values to plausible slope variations would be useful for luminosity-function users.
- Minor presentation: a few figure captions (e.g., Figs. 2–3) could more explicitly state the smoothing kernels and that spectra are not telluric-corrected; the naming convention footnote is clear but could be moved earlier for first-time readers.
Circularity Check
No significant circularity: spectroscopic census of Euclid candidates is independent of the photometric selection models.
full rationale
This is an observational discovery paper. Candidates are selected from Euclid photometry via multiple ML/probabilistic methods (XDHZQSO, XGBoost, SED fitting, BMC) that use synthetic quasar photometry trained on lower-z SDSS BOSS/eBOSS samples and empirical contaminant densities; those models only rank candidates for follow-up. Confirmation of the 31 quasars, their redshifts (visual Lyα break / emission), and the z≈7.77 record rest entirely on independent Keck/Magellan/LBT spectra (Figs. 2–3, 6–7, App. C; Tables A.1–A.3). No free parameter is fitted to the high-z sample and then re-presented as a prediction; the count of 31 and the redshift record are not forced by the selection priors. Mild self-citations (e.g. Yang et al. 2024 selection framework, synthetic models in prep.) are standard and non-load-bearing for the discovery claim. Score 0 is appropriate.
Axiom & Free-Parameter Ledger
free parameters (3)
- spectral slope α for M1450 conversion =
-1.7
- CLASS_STAR threshold and color cuts =
CLASS_STAR>0.6; IE-JE>3.05
- PQSO and Pq probability thresholds =
0.85 / 0.5
axioms (3)
- domain assumption Flat ΛCDM cosmology with Planck 2020 parameters (Ωm=0.3111, h=0.6766, etc.)
- domain assumption Visual location of Lyα break or emission gives redshift to ∼0.05–0.1 accuracy
- domain assumption Sources consistent with the JE PSF (Sérsic Reff overlapping REE50) are point-like and therefore quasars rather than extended galaxies
read the original 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.
Figures
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