Euclid: Galaxy morphology and photometry from bulge-disc decomposition of Early Release Observations
Pith reviewed 2026-05-23 02:26 UTC · model grok-4.3
The pith
Bulge-disc decomposition shows single-Sérsic effective radii as intermediate between bulge and disc sizes across all B/T values.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
The fitted galaxies to I_E < 21 span the various Hubble types with ubiquitous bulge and disc components, and a bulge-to-total light ratio B/T taking all values from 0 to 1. The effective radius of the single-Sérsic profile is an intermediate estimate of galaxy size, between the bulge and disc effective radii, depending on B/T. The axis ratio of the single-Sérsic profile is higher than the disc axis ratio, increasingly so with B/T. The model impacts the photometry with -0.08 to 0.01 mag median systematic I_E offsets between single-Sérsic and bulge+disc total magnitudes, and a 0.05 to 0.15 mag dispersion, from low to high B/T. A median 0.3 mag bulge-disc colour difference in rest-frame M_g - M
What carries the argument
Bulge-disc decomposition (Sérsic bulge plus exponential disc) applied via SourceXtractor++ and compared against single-Sérsic profile fits on Euclid optical and near-IR imaging.
If this is right
- Single-Sérsic effective radius acts as a B/T-weighted average of bulge and disc sizes.
- Single-Sérsic axis ratios exceed disc axis ratios by an amount that grows with B/T.
- Total magnitudes from the two models differ by median offsets of -0.08 to 0.01 mag with 0.05-0.15 mag scatter depending on B/T.
- Disc components exhibit 5-10% systematic effective-radius variations between optical and near-IR bands, producing redder-inside color gradients.
- Bulge-dominated galaxies show similar median colors for bulge and disc while disc-dominated galaxies display a 0.3 mag color offset.
Where Pith is reading between the lines
- The observed B/T-dependent relations between single-Sérsic and component parameters could supply empirical corrections for size and photometry catalogs that rely only on single-profile fits.
- The measured disc color gradients point to inside-out growth that repeated observations of the same fields could track across cosmic time.
- Extending the same decomposition pipeline to the full Euclid survey depth would test whether the B/T distribution evolves with redshift or environment.
- Unmodeled bars or rings in a subset of spirals may bias B/T values and color measurements for those objects.
Load-bearing premise
A Sérsic bulge plus exponential disc provides an adequate parametric description of the light distribution for the full range of Hubble types without significant unmodeled residuals.
What would settle it
A large fraction of galaxies showing substantial systematic residuals after bulge-disc fits that require bars, rings, or additional components would indicate the models are incomplete.
Figures
read the original abstract
The background galaxies in Euclid ERO images of the Perseus cluster make up a remarkable sample in its combination of 0.57 deg$^2$ area, 25.3 and 23.2 AB mag depth, as well as 0.1" and 0.3" angular resolutions, in optical and near-IR bands, respectively. We perform a morphological analysis of 2445 and 12,786 galaxies with $I_E < 21$ and $I_E < 23$, respectively. We use single-S\'ersic profiles and the sums of a S\'ersic bulge and an exponential disc to model these galaxies with SourceXtractor++ and analyse their parameters in order to assess their consistencies and discrepancies. The fitted galaxies to $I_E < 21$ span the various Hubble types with ubiquitous bulge and disc components, and a bulge-to-total light ratio B/T taking all values from 0 to 1. The effective radius of the single-S\'ersic profile is an intermediate estimate of galaxy size, between the bulge and disc effective radii, depending on B/T. The axis ratio of the single-S\'ersic profile is higher than the disc axis ratio, increasingly so with B/T. The model impacts the photometry with -0.08 to 0.01 mag median systematic $I_E$ offsets between single-S\'ersic and bulge+disc total magnitudes, and a 0.05 to 0.15 mag dispersion, from low to high B/T. We measure a median $0.3$ mag bulge-disc colour difference in rest-frame $M_g - M_i$ that originates from the disc-dominated galaxies, whereas bulge-dominated galaxies have similar median colours for their components. We also measure redder-inside disc colour gradients based on 5 to 10$\%$ systematic variations of disc effective radii between the optical and near-IR bands. This analysis demonstrates the usefulness and limitations of single-S\'ersic profile modelling and the power of bulge-disc decomposition for characterising the morphology of lenticulars and spirals in Euclid images. We make available the catalogues of best-fit parameters for the morphological and SED fits.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript reports single-Sérsic and Sérsic-bulge plus exponential-disc decompositions performed with SourceXtractor++ on Euclid Early Release Observations of the Perseus cluster field. For samples of 2445 galaxies (I_E < 21) and 12,786 galaxies (I_E < 23), it finds that the galaxies span Hubble types with bulge-to-total ratios B/T covering the full range 0–1, that single-Sérsic effective radii lie between the bulge and disc effective radii in a B/T-dependent manner, that single-Sérsic axis ratios exceed disc axis ratios increasingly with B/T, and that model choice produces median I_E magnitude offsets of −0.08 to +0.01 mag with 0.05–0.15 mag dispersion. Additional results include a median 0.3 mag bulge–disc colour difference (rest-frame M_g − M_i) driven by disc-dominated systems and redder-inside disc colour gradients inferred from 5–10 % band-to-band variations in disc effective radius. Parameter catalogues are released.
Significance. If the two-component model is shown to be adequate, the work supplies one of the largest existing samples of bulge–disc decompositions in deep, high-resolution Euclid imaging and quantifies practical differences between single-Sérsic and two-component photometry and structural parameters. The public catalogues constitute a concrete resource for future Euclid morphological studies.
major comments (1)
- [Abstract and Methods] The central claim that a fixed Sérsic-bulge + exponential-disc model supplies an adequate parametric description across the full range of Hubble types (yielding reliable B/T values from 0 to 1 and interpretable component radii) is load-bearing for the interpretation of all reported trends. The manuscript does not report aggregate goodness-of-fit statistics (e.g., distributions of reduced χ²) or systematic residual maps that would demonstrate the absence of unmodeled bars, rings or spiral structure (see abstract and the description of the fitting procedure). Without these checks the claimed B/T coverage and the intermediate single-Sérsic radius could partly reflect model mismatch rather than intrinsic structure.
Simulated Author's Rebuttal
We thank the referee for their constructive comments, which help clarify the presentation of our results. We address the single major comment below.
read point-by-point responses
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Referee: [Abstract and Methods] The central claim that a fixed Sérsic-bulge + exponential-disc model supplies an adequate parametric description across the full range of Hubble types (yielding reliable B/T values from 0 to 1 and interpretable component radii) is load-bearing for the interpretation of all reported trends. The manuscript does not report aggregate goodness-of-fit statistics (e.g., distributions of reduced χ²) or systematic residual maps that would demonstrate the absence of unmodeled bars, rings or spiral structure (see abstract and the description of the fitting procedure). Without these checks the claimed B/T coverage and the intermediate single-Sérsic radius could partly reflect model mismatch rather than intrinsic structure.
Authors: We agree that aggregate goodness-of-fit statistics and residual maps would strengthen the manuscript's support for the two-component model. The current text does not include distributions of reduced χ² or systematic residual maps. In the revised version we will add the distribution of reduced χ² for both the single-Sérsic and bulge+disc fits on the I_E < 21 sample, together with a representative set of residual maps (including cases across the B/T range) to illustrate that unmodeled features such as bars or strong spiral arms are not dominant. This addition will directly address the concern that the reported B/T coverage and radius trends could be influenced by model mismatch. We note that the abstract already states that the work demonstrates both the usefulness and the limitations of the modeling approach. revision: yes
Circularity Check
No circularity: all quantities are direct least-squares fit outputs to pixel data
full rationale
The paper reports parameters (B/T ratios, effective radii, axis ratios, colours) obtained by fitting single-Sérsic and Sérsic+exponential models to Euclid ERO galaxy images using SourceXtractor++. No derivation chain, prediction, or uniqueness claim reduces by the paper's equations to a fitted input or self-citation. The central results are empirical outputs of the fits; model adequacy is an assumption but does not create definitional circularity. This matches the reader's assessment of score 1.0 with no load-bearing self-referential steps.
Axiom & Free-Parameter Ledger
axioms (1)
- domain assumption Sérsic bulge plus exponential disc profiles adequately describe the light distribution across the Hubble types present in the sample
Lean theorems connected to this paper
-
IndisputableMonolith/Cost/FunctionalEquation.leanwashburn_uniqueness_aczel unclear?
unclearRelation between the paper passage and the cited Recognition theorem.
We use single-Sérsic profiles and the sums of a Sérsic bulge and an exponential disc to model these galaxies with SourceXtractor++
-
IndisputableMonolith/Foundation/RealityFromDistinction.leanreality_from_one_distinction unclear?
unclearRelation between the paper passage and the cited Recognition theorem.
The fitted galaxies span the various Hubble types with ubiquitous bulge and disc components, and a bulge-to-total light ratio B/T taking all values from 0 to 1
What do these tags mean?
- matches
- The paper's claim is directly supported by a theorem in the formal canon.
- supports
- The theorem supports part of the paper's argument, but the paper may add assumptions or extra steps.
- extends
- The paper goes beyond the formal theorem; the theorem is a base layer rather than the whole result.
- uses
- The paper appears to rely on the theorem as machinery.
- contradicts
- The paper's claim conflicts with a theorem or certificate in the canon.
- unclear
- Pith found a possible connection, but the passage is too broad, indirect, or ambiguous to say the theorem truly supports the claim.
Forward citations
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Reference graph
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discussion (0)
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