Euclid: Scaled-up little red dots and other sources with v-shaped spectral energy distributions at z>4
00014 Helsinki, 00044 Frascati, 00078 Monteporzio Catone, 00100 Roma, 00133 Roma, 00146 Rome, 00185 Roma, 0315 Oslo, 077125, 08010 Barcelona, 08028 Barcelona, 08193 Barcelona, 08193 Bellaterra (Barcelona), 08860 Castelldefels, 1), 10), 100, 10025 Pino Torinese (TO), (100) Universit\'e Paris-Saclay, 10125 Torino, (101) Centre National d'Etudes Spatiales -- Centre spatial de Toulouse, (102) Institute of Space Science, 103), (103) INFN-Padova, (104) Caltech/IPAC, (105) Instituto de F\'isica Te\'orica UAM-CSIC, 106, 106), (106) Institut de Recherche en Astrophysique et Plan\'etologie (IRAP), 107), (107) Universit\'e St Joseph, (108) Departamento de F\'isica, (109) Universit\"at Innsbruck, (10) Niels Bohr Institute, (110) Department of Physics, (111) Infrared Processing, 112), (112) Instituto de Astrof\'isica e Ci\^encias do Espa\c{c}o, (113) Universidad Polit\'ecnica de Cartagena, (114) Institute of Space Sciences (ICE, 115, (115) Institut d'Estudis Espacials de Catalunya (IEEC), 116, (116) Universit\'e PSL, 117), (117) Universit\'e Paris-Cit\'e, (118) Aurora Technology for European Space Agency (ESA), (119) Institut d'Astrophysique de Paris, (11) Centro de Astrobiolog\'ia (CAB), 11F of ASMAB, 1200 E. 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Euclid data reveals 16 compact massive candidates for little red dots at z>4
A machine-rendered reading of the paper's core claim, the
machinery that carries it, and where it could break.
The paper selects 233 sources at redshift greater than 4 that display V-shaped spectral energy distributions in Euclid near-infrared images supplemented by Spitzer data over 0.75 square degrees in the COSMOS field. Sixteen of these sources exceed the median compactness of all z>4 galaxies by more than one sigma and are presented as robust candidates for the Little Red Dots and Little Blue Dots previously identified with JWST. These candidates span stellar masses from 10^8.5 to 10^10.5 solar masses, making them substantially more massive than the typical JWST examples, and roughly half appear as old as the universe at the time of observation. Less than 10 percent of the full V-shaped sample, including only one of the compact candidates, matches known active galactic nuclei, indicating that the population is mostly separate from standard AGN. The work shows that wider-area surveys can locate brighter analogues to the JWST population and sets the stage for spectroscopic follow-up to test their nature.
Core claim
Out of 233 sources with V-shaped SEDs at z>4, 16 sources with compactness more than 1 sigma above the median of all z>4 galaxies are identified as robust LRD/LBD candidates. These have stellar masses in the range 10^8.5 to 10^10.5 solar masses, half of them approximately as old as the universe at their redshifts, and their median properties resemble those of Blue DOGs. Less than 10 percent of the V-shaped sources, including only one of the Euclid LBD candidates, correspond to known AGN, forming a population largely disjoint from known AGN.
What carries the argument
Photometric selection of V-shaped spectral energy distributions from Euclid NIR plus Spitzer IRAC data, followed by a compactness cut relative to the median size distribution of z>4 galaxies.
If this is right
Brighter and more massive versions of the JWST LRD/LBD population exist and can be detected with wide-field near-infrared imaging.
Some compact high-redshift galaxies with V-shaped SEDs reach stellar ages comparable to the age of the universe at their observed redshifts.
The V-shaped SED population at z>4 is mostly distinct from the known AGN population at these redshifts.
The 16 candidates provide targets for follow-up observations to test whether they represent a transition stage toward standard AGN.
Where Pith is reading between the lines
These are editorial extensions of the paper, not claims the
author makes directly.
If the reported ages hold, these objects would require extremely rapid star formation and quenching within the first few hundred million years after the Big Bang.
The similarity to Blue DOGs suggests a possible evolutionary link between the V-shaped sources and dust-obscured galaxy populations at lower redshifts.
Larger-area surveys could measure the volume density of such sources and clarify whether they represent a brief but common phase in early galaxy assembly.
Load-bearing premise
That the combination of V-shaped SED shape and a compactness threshold isolates the same physical population as the JWST LRDs and LBDs without substantial contamination from unrelated high-redshift sources or misclassified AGN.
What would settle it
Deep spectroscopy of the 16 compact candidates that checks for the presence or absence of broad emission lines and other spectral signatures known to characterize JWST-selected LRDs.
Figures
Figures reproduced from
arXiv: 2604.16178
by 00014 Helsinki, 00044 Frascati, 00078 Monteporzio Catone, 00100 Roma, 00133 Roma, 00146 Rome, 00185 Roma, 0315 Oslo, 077125, 08010 Barcelona, 08028 Barcelona, 08193 Barcelona, 08193 Bellaterra (Barcelona), 08860 Castelldefels, 1), 10), 100, 10025 Pino Torinese (TO), (100) Universit\'e Paris-Saclay, 10125 Torino, (101) Centre National d'Etudes Spatiales -- Centre spatial de Toulouse, (102) Institute of Space Science, 103), (103) INFN-Padova, (104) Caltech/IPAC, (105) Instituto de F\'isica Te\'orica UAM-CSIC, 106, 106), (106) Institut de Recherche en Astrophysique et Plan\'etologie (IRAP), 107), (107) Universit\'e St Joseph, (108) Departamento de F\'isica, (109) Universit\"at Innsbruck, (10) Niels Bohr Institute, (110) Department of Physics, (111) Infrared Processing, 112), (112) Instituto de Astrof\'isica e Ci\^encias do Espa\c{c}o, (113) Universidad Polit\'ecnica de Cartagena, (114) Institute of Space Sciences (ICE, 115, (115) Institut d'Estudis Espacials de Catalunya (IEEC), 116, (116) Universit\'e PSL, 117), (117) Universit\'e Paris-Cit\'e, (118) Aurora Technology for European Space Agency (ESA), (119) Institut d'Astrophysique de Paris, (11) Centro de Astrobiolog\'ia (CAB), 11F of ASMAB, 1200 E. California Blvd., 120) ((1) Kapteyn Astronomical Institute, (120) ICL, 1290 Versoix, (12) Department of Mathematics, 13), 1349-018 Lisboa, (13) INAF-Osservatorio Astronomico di Roma, 14 Av. Edouard Belin, (14) Kobayashi-Maskawa Institute for the Origin of Particles, 15, (15) Institute for Particle Physics, 16), 16146, (16) Department of Astronomy, 17, 17), 1749-016 Lisboa, (17) INAF-Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, 18 avenue Edouard Belin, (18) Sterrenkundig Observatorium, (19) School of Physics \& Astronomy, 2), 20122 Milano, 20133 Milano, (20) Instituto de Astrof\'isica de Canarias, 21), 21 avenue Pierre de Coubertin 69627 Villeurbanne Cedex, (21) Universidad de La Laguna, 2200 Copenhagen, 2201 AZ Noordwijk, (22) Max-Planck-Institut f\"ur Astronomie, 2333 CC Leiden, (23) Department of Physics, (24) Jet Propulsion Laboratory, 24 quai Ernest-Ansermet, 25), (25) ESAC/ESA, 26), 2680 Woodlawn Drive, (26) INAF-Osservatorio Astronomico di Brera, 27), (27) IFPU, 28, 2800 Kgs. 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Figure 1.Figure 1: Photometric redshifts obtained from SED fitting with LePhare versus the spectroscopic redshifts from Khostovan et al. (2025) catalogue. The identity line is shown in orange, and the red lines denote the upper and lower bounds, beyond which are the catastrophic outliers, defined as |zspec−zphot|/(1 + zspec) > 0.15. From the 20 726 galaxies in this diagnostic, we have 11.1% outliers, with a dispersion of σ…view at source ↗Figure 2.Figure 2: Example SEDs for the "Cut 1" (top) and "Cut 2" (bottom) sources, with −2.8 < βblue < 0.37, and βred > 0 ("Cut 1") or βred > −1 ("Cut 2") respectively, as defined in Kocevski et al. 2025. The βred and βblue slopes are displayed by the red solid line and blue solid line respectively. The photometric datapoints used to measure these slopes have colours matching the corresponding continuum slope. The grey line…view at source ↗Figure 4.Figure 4: Redshift distribution of z > 4 sources (empty histogram), with the "Cut 1" and "Cut 2" sources in red and yellow respectively. (2025), is shown in
[PITH_FULL_IMAGE:figures/full_fig_p006_4.png]view at source ↗Figure 5.Figure 5: Distributions of our two double power-law sources sample populations (red for "Cut 1" and yellow for "Cut 2"), compared to all z > 4 sources (empty histogram). We show the distributions for stellar mass, colour excess E(B − V), SFR, and age, starting from the top left and clockwise. All SFR values shown in this figure have been calculated from the rest-frame UV luminosities, assuming that all the UV light …view at source ↗Figure 6.Figure 6: Stellar mass versus SFR, with the shaded grey regions representing the distribution of all z > 4 sources, with the contours highlighting the 84th and 95th percentiles. The starburst envelope, as defined in Caputi et al. (2017) and Caputi et al. (2021), is marked by the shaded blue region, above the dashed blue line. The median of the star-formation main sequence (MS) for z ∈ [4, 6] (Rinaldi et al. 2022) …view at source ↗Figure 7.Figure 7: Left: Stellar mass versus compactness, defined as fHE (0. ′′7)/ fHE (1. ′′0), with the shaded grey regions representing the distribution of all z > 4 sources. The yellow dots denote the "Cut 2" sources, and the red dots are the more restrictive "Cut 1" sources. The median of the compactness distributions of the z > 4 population is shown by the solid line, and the shaded blue areas denote the 1σ and 2σ regi…view at source ↗Figure 8.Figure 8: Classical AGN, recognised with different criteria, on the βred– βblue diagram. As previously, the red and yellow shaded regions represent the boundaries of the β "Cut 1" and "Cut 2" selections. 6. Standard AGN as a mostly disjoint population to V-shaped SED sources at z > 4 A number of studies have found that LRDs/LBDs have X-ray luminosities that are significantly weaker than expected in comparison to …view at source ↗Figure 9.Figure 9: LRD/LBD luminosity function in the rest-frame UV (λrest = 1450 Å; top) and optical (λrest = 5100 Å; bottom), at z ∈ [4.5, 6.5]. The number densities are comoving. Φ(x) = 1 ∆x X i V −1 max(A,zmin,zmax) , (5) where ∆x is the width of the M1450 Å and M5100 Å bins, and Vmax is the maximum volume spanned by our Euclid LRDs/LBDs. The maximum volume Vmax is dependent on the survey area (A), and the maximum redshi…view at source ↗Figure 10.Figure 10: Average rest-frame SED of our final Euclid-selected LRD candidates, compared with the average SEDs of JWST-selected LRDs and that of BlueDOGs. This figure is similar to the one presented in Noboriguchi et al. (2023), with this new version including our own data points for the Euclid-selected LRDs. Out of out 16 robust Euclid LRD/LBD candidates, we found that 25% (4 out of 16) preferred the LRD model tem…view at source ↗
read the original abstract
Little Red Dots (LRDs) are some the most intriguing galaxy populations recently identified at z>~4 with JWST. They constitute the most extreme class of a more abundant population of sources with `V-shaped' spectral energy distributions (SEDs) and compact morphologies, which includes also Little Blue Dots (LBDs). Finding brighter analogues to these sources requires surveying sky areas which are significantly larger than those covered with JWST. Euclid deep images are ideally suited for this purpose. We make use of Euclid near-infrared images, complemented by Spitzer Infrared Array Camera (IRAC) data, over 0.75 sq. deg. of the COSMOS field to select a sample of 233 sources with `V-shaped' SEDs at z>4. Out of those, we identify 16 sources with compactness >1sigma above the median of all z>4 galaxies, which we consider robust LRD/LBD candidates in our sample. The stellar masses of these 16 sources are in the range 10^{8.5} - 10^{10.5} Msun, so they are significantly more massive than typical JWST-selected LRDs/LBDs. Interestingly, half of them are about as old as the Universe at their redshifts. In addition, we find that the median photometric properties of the Euclid LRDs/LBDs are similar to those of the so-called Blue Dust-Obscured Galaxies (Blue DOGs). Less than 10% of all our `V-shaped' SED sources, including only one of the Euclid LBDs, correspond to known AGN. The latter mostly constitute a population disjoint to the `V-shaped' SED sources. Spectroscopic follow up of the Euclid LRDs/LBD candidates remains necessary to probe whether they host BLAGN as fainter analogues do and whether constitute a transition phase from these fainter sources to standard AGN.
Editorial analysis
A structured set of objections, weighed in public.
Desk editor's note, referee report, simulated authors' rebuttal, and a
circularity audit. Tearing a paper down is the easy half of reading it; the
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Referee Report
3 major / 2 minor
Summary. This paper uses Euclid near-IR and Spitzer IRAC photometry over 0.75 deg² in COSMOS to select 233 sources with V-shaped SEDs at z>4. Of these, 16 are identified as compact (compactness >1σ above the median of all z>4 galaxies) and presented as robust scaled-up LRD/LBD candidates with stellar masses 10^{8.5}–10^{10.5} M_⊙. Half are reported to have photometric stellar ages comparable to the age of the Universe at their redshifts; the sample shows median properties similar to Blue DOGs and <10% overlap with known AGN. Spectroscopic follow-up is recommended to test for broad-line AGN.
Significance. If the photometric selection and derived properties hold, the work would usefully extend the LRD/LBD population to higher masses and wider survey areas, suggesting these sources are more massive and potentially older than typical JWST examples and may overlap with Blue DOGs. This has implications for early galaxy assembly and the transition to AGN. The use of public wide-field data is a practical strength, but the central claims rest on untested aspects of the SED modeling.
major comments (3)
[Abstract] Abstract: The headline result that half of the 16 compact V-shaped sources have stellar ages 'about as old as the Universe at their redshifts' is obtained from SED fitting on Euclid NIR + IRAC bands only. At z>4, V-shaped SEDs are shaped by strong breaks or dust, so parametric fits are sensitive to SFH and attenuation assumptions; the manuscript provides no details on the fitting code, assumed SFHs, dust laws, or tests against non-parametric models or full posterior age distributions.
[Candidate selection] Candidate selection: The V-shaped SED color cuts and the compactness threshold (>1σ above the z>4 median) are free parameters that directly define the final sample of 16 sources. No sensitivity tests, alternative thresholds, or justification for these choices are described, undermining the claim that these 16 are robust scaled-up analogues of JWST LRDs/LBDs.
[Stellar mass and age estimates] Stellar masses and redshifts: The reported mass range 10^{8.5}–10^{10.5} M_⊙ and the z>4 selection lack reported uncertainties, photometric redshift quality metrics, or discussion of possible biases from the limited number of bands and strong spectral features. These quantities are load-bearing for the 'scaled-up' and 'more massive' conclusions.
minor comments (2)
The abstract refers to 'Blue Dust-Obscured Galaxies (Blue DOGs)' without citing the defining papers; a brief reference would help readers place the median-property comparison.
Clarify the exact number of photometric bands entering the SED fits and any fixed assumptions (metallicity, IMF) used when deriving the masses and ages of the 16 candidates.
Simulated Author's Rebuttal
3 responses · 0
unresolved
We thank the referee for their constructive and detailed comments, which have helped us improve the clarity and robustness of the manuscript. We address each major comment point by point below and have made revisions to the manuscript where appropriate.
read point-by-point responses
Referee: [Abstract] Abstract: The headline result that half of the 16 compact V-shaped sources have stellar ages 'about as old as the Universe at their redshifts' is obtained from SED fitting on Euclid NIR + IRAC bands only. At z>4, V-shaped SEDs are shaped by strong breaks or dust, so parametric fits are sensitive to SFH and attenuation assumptions; the manuscript provides no details on the fitting code, assumed SFHs, dust laws, or tests against non-parametric models or full posterior age distributions.
Authors: We agree that the original manuscript lacked sufficient methodological detail on the SED fitting to support the age claims. In the revised version we have added a new subsection in the Methods section that specifies the fitting code, the assumed parametric star-formation histories, the dust attenuation law, and direct comparisons with non-parametric SFH models. We also include the full posterior age distributions for the 16 compact sources, demonstrating that the result that half are comparable in age to the Universe at their redshifts is robust within the explored modeling assumptions.
revision: yes
Referee: [Candidate selection] Candidate selection: The V-shaped SED color cuts and the compactness threshold (>1σ above the z>4 median) are free parameters that directly define the final sample of 16 sources. No sensitivity tests, alternative thresholds, or justification for these choices are described, undermining the claim that these 16 are robust scaled-up analogues of JWST LRDs/LBDs.
Authors: The V-shaped color selection is directly motivated by the color criteria used in the JWST LRD/LBD literature to ensure we identify analogous sources at brighter magnitudes. The compactness threshold (>1σ above the median) follows standard practice for identifying morphological outliers. We acknowledge that the original submission did not include sensitivity tests. We have now added an appendix presenting results for modest variations in both the color cuts (±0.2 mag) and the compactness threshold (0.5σ and 1.5σ), showing that the median properties and the conclusion that the sources are scaled-up analogues remain stable. A brief justification for the fiducial choices has also been inserted in Section 3.
revision: yes
Referee: [Stellar mass and age estimates] Stellar masses and redshifts: The reported mass range 10^{8.5}–10^{10.5} M_⊙ and the z>4 selection lack reported uncertainties, photometric redshift quality metrics, or discussion of possible biases from the limited number of bands and strong spectral features. These quantities are load-bearing for the 'scaled-up' and 'more massive' conclusions.
Authors: We have revised the text to report the 16th–84th percentile uncertainties on stellar masses and photometric redshifts for the 16 sources. We now include standard photo-z quality metrics (e.g., χ² and odds) and have added a paragraph discussing possible biases arising from the limited number of bands and strong spectral breaks. Mock-catalog tests quantifying the impact on mass and redshift recovery have been included to support the claim that these Euclid sources are more massive than typical JWST LRDs/LBDs.
revision: yes
Circularity Check
0 steps flagged
No significant circularity in observational selection and SED-derived properties
full rationale
The paper conducts direct photometric source selection of V-shaped SEDs at z>4 from Euclid NIR and Spitzer IRAC imaging over 0.75 sq. deg., applies a compactness cut relative to the median of z>4 galaxies, and performs standard SED fitting to derive stellar masses (10^8.5-10^10.5 Msun) and ages. No equations, predictions, or first-principles derivations are presented that reduce by construction to the input photometry or to self-cited results. The central claims (16 robust candidates, half as old as the Universe at their redshifts, <10% known AGN) are outputs of data analysis and fitting, not tautological re-expressions of inputs. Any self-citations are non-load-bearing and do not create a circular chain.
The central claim rests on standard photometric assumptions rather than new derivations; free parameters are selection thresholds chosen by the authors.
free parameters (2)
compactness threshold Set at >1 sigma above the median of all z>4 galaxies to define robust candidates
V-shaped SED color cuts Specific wavelength-dependent flux ratios chosen to match JWST LRD definition
axioms (2)
domain assumptionPhotometric redshifts from Euclid+Spitzer data accurately place sources at z>4 with low contamination Invoked for the initial sample of 233 sources
domain assumptionSED fitting yields reliable stellar masses and ages for these sources Used to report the 10^{8.5}-10^{10.5} Msun range and half being as old as the universe
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· methodology
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Toulouse, France 107 Université St Joseph; Faculty of Sciences, Beirut, Lebanon 108 Departamento de Física, FCFM, Universidad de Chile, Blanco Encalada 2008, Santiago, Chile 109 Universität Innsbruck, Institut für Astro- und Teilchenphysik, Technikerstr. 25/8, 6020 Innsbruck, Austria 110 Department of Physics and Helsinki Institute of Physics, Gustaf Häll...
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