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REVIEW 2 major objections 6 minor 276 references

Selecting quiescent late-type galaxies from SDSS catalogues overcounts them by about a factor of two because most edge-on cases are misclassified S0s.

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2026-07-31 08:25 UTC pith:32OOWRPZ

load-bearing objection Clear inclination bias in quiescent-LTG selections is real and well shown; the factor-of-two number is a single-assumption estimate that should be treated as illustrative, not definitive. the 2 major comments →

arxiv 2607.24664 v1 pith:32OOWRPZ submitted 2026-07-27 astro-ph.GA

Too many quiescent late-type galaxies or a matter of misclassification?

J. L. Tous , S. I. Raimundo , H. Dom\'inguez S\'anchez This is my paper
classification astro-ph.GA
keywords galaxy morphologyquiescent galaxieslate-type galaxiesS0 galaxiesinclination biasSDSSmorphological classification
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

Large morphological catalogues are now the main way astronomers count how many disc galaxies have stopped forming stars. This paper shows that the usual cut produces a sample that is heavily skewed toward edge-on systems whose spiral arms cannot be seen. Roughly sixty per cent of the supposed quiescent late-types have axis ratios below 0.4, while lenticular (S0) galaxies are under-represented at the same inclinations. The authors argue that most of the excess edge-on objects are simply S0 discs that look late-type when viewed sideways. Correcting for the contamination lowers the true fraction of quiescent late-types from about 3.3 per cent to roughly 1.4 per cent of the parent sample. The result matters because any evolutionary story that treats quiescent spirals as a distinct transitional population is currently built on a contaminated census.

Core claim

The observed excess of edge-on quiescent late-type galaxies is largely an artefact of morphological misclassification: most of those systems are edge-on S0 galaxies that cannot be distinguished from spirals once the arms are hidden by inclination. As a direct consequence, the fraction of true quiescent late-types in SDSS is overestimated by a factor of roughly two.

What carries the argument

The mixture decomposition of the axis-ratio distribution (Eq. 3): the observed b/a histogram of quiescent late-types is written as a linear combination of the unbiased main-sample distribution and a pure contaminant component confined to b/a < 0.4; solving for the mixing weight yields the global misclassification fraction of ~58 per cent.

Load-bearing premise

The calculation treats every misclassified galaxy as living strictly below axis ratio 0.4, so that the contaminant distribution integrates to one inside that bin and nowhere else.

What would settle it

Reclassify the same edge-on quiescent candidates with higher-resolution imaging or integral-field kinematics that can reveal residual spiral structure or a classical bulge; if the majority remain late-type after that test, the mixture correction fails.

Watch this falsifier — get emailed when new claim-graph text bears on it.

If this is right

  • Any published fraction of quiescent late-types drawn from SDSS morphological catalogues without an inclination cut is inflated by a factor of ~2.
  • S0 samples selected the same way are correspondingly incomplete at high inclination.
  • Statistical weights based on the main-sample axis-ratio distribution can restore an unbiased census without discarding every edge-on galaxy.
  • The same inclination check should be applied whenever late-types are selected by other early-type-like properties (old stellar populations, kinematic misalignment).

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Catalogue-trained neural nets will keep reproducing the bias until edge-on S0s are deliberately over-represented or flagged in the training labels.
  • Environment studies that report elevated quiescent-spiral fractions in dense regions may partly be counting stripped S0s viewed edge-on.
  • A clean volume-limited sample with b/a > 0.4 would give a sharper empirical upper limit on how common genuine quenched discs really are.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit.

Referee Report

2 major / 6 minor

Summary. This letter shows that selecting quiescent late-type galaxies (LTGs) from SDSS-based morphological catalogues (NSA + GSWLC + Domínguez Sánchez et al. 2023) produces a strong over-representation of edge-on systems: the b/a distribution of quiescent LTGs peaks at ~0.2 with ~60–69% of objects below b/a = 0.4, versus ~25% for the parent sample, and the independent P_edge diagnostic confirms it. The bias persists under a dust-insensitive D4000 quiescence selection (ruling out dust reddening of inclined star-forming discs as the cause), S0s show a mirror deficit at low b/a, edge-on quiescent LTGs sit with S0s in the SFR–M* plane (Appendix A), and their T-type range (0–4) coincides with the regime of unreliable visual classification. Using a two-component mixture model (Eqs. 2–4) with the main-sample b/a distribution as reference and assuming contaminants are confined to b/a < 0.4, the authors estimate that ~58% of quiescent LTGs may be misclassified edge-on S0s, implying the quiescent-LTG fraction could be overestimated by a factor ~2.4. They recommend excluding edge-on systems or reweighting by inclination.

Significance. If it holds, this is a practically important caution for a large user community: quiescent-LTG fractions are inputs to quenching-pathway and morphology-transformation studies, and a factor-of-two contamination correction materially changes their interpretation. The letter's strengths are its reproducibility (entirely public catalogues with URLs provided), its falsifiable and simple mitigation recipes (exclude edge-on systems, or apply the b/a reweighting of Eq. 5), and the convergence of multiple independent diagnostics rather than a single statistic. The demonstration that the bias survives a dust-insensitive quiescence definition properly closes the main alternative explanation. The quantitative correction factor is less secure than the qualitative bias, but it is presented with appropriate hedging and, with the requested sensitivity check, would be a defensible estimate.

major comments (2)
  1. [§3.2] Eqs. (2)–(3): the headline numbers (φ_mis ≈ 58%, φ_true ≈ 1.4%, factor ≈ 2.4) rest entirely on the assumption that misclassified galaxies populate exclusively b/a < 0.4, so that the integrated F_mis equals unity on that interval, and that genuine quiescent LTGs share the main-sample b/a distribution. Neither assumption is varied or empirically justified. If real contaminants leak above b/a = 0.4 (plausible for moderately inclined S0s whose bulge–disc contrast is still hidden), φ_mis is underestimated; if genuinely quenched discs are intrinsically thicker than the average disc population, it is overestimated. Since the authors have already verified that star-forming LTGs follow the main-sample distribution, a sensitivity check is cheap: repeat the calculation with the boundary at b/a = 0.3 and 0.5, and with the LTG or star-forming-LTG sample (rather than the morphology-mixed main sample)
  2. [§3.2 / §3.1] The main galaxy sample is used as the unbiased reference distribution, but it contains the very contaminants at issue (the misclassified edge-on S0s remain in it). The authors note the contaminants are <2% of the main sample, so the effect is small, but the reference is also morphology-mixed: it includes ellipticals (skewed to high b/a) and any other inclination-correlated selections. The text asserts the main-sample distribution is 'approximately uniform' and the footnote gives 0.25 below b/a = 0.4, consistent with thin discs with q0 ~ 0.2, so this choice is likely benign — but it should be demonstrated, not asserted. Please show explicitly that using the full LTG sample (or star-forming LTGs) as the reference leaves the 0.25 fraction, and hence φ_mis, essentially unchanged.
minor comments (6)
  1. [§3.1 vs footnote 4] Internal numerical inconsistency: §3.1 states that 'around sixty per cent' of quiescent LTGs have b/a < 0.4, while footnote 4 gives 0.69. Since 0.69 is the value entering Eq. (3), the ~60% figure in the text (and the matching statement for P_edge) should be reconciled.
  2. [§2] The stated percentages for the comparison samples do not match the counts: 85,554 S0s and 57,554 Es correspond to 19.3% and 13.0% of the 442,872-object main sample, not the quoted 22% and 11%. Please check the denominators.
  3. [§3.2, Eqs. (2)–(4)] Eqs. (2)–(3) use F for both the differential b/a distribution and its integral over b/a < 0.4; the transition from the pointwise mixture to integrated fractions should be written explicitly (e.g., with separate notation for pdf and CDF) to avoid confusion.
  4. [Appendix A] The edge-on cut P_edge > 0.7 is introduced without justification; a sentence on the sensitivity of the Appendix A conclusion to this threshold would help, especially since §3.3 recommends exactly this kind of cut to other users.
  5. [Figures 1–2] No statistical uncertainties are shown on the histograms in Figs. 1–2. With ~14,600 quiescent LTGs Poisson errors are small, but they should be indicated (or stated) so the KS results and the excess are visually quantified.
  6. [§3.2, footnote 5] The claim that the bias also affects kinematically misaligned LTG selections (footnote 5) is asserted without any supporting figure or number; either provide a brief quantitative statement or soften to 'will be explored in future work'.

Circularity Check

0 steps flagged

No significant circularity: empirical histogram comparison plus a transparent mixture estimator, not a derivation forced by its inputs.

full rationale

The paper’s load-bearing chain is observational, not definitional. It selects quiescent LTGs from public catalogues (NSA, GSWLC, Domínguez Sánchez morphologies), compares their observed b/a and P_edge distributions to the parent sample and to S0/E controls (Figs. 1–2), notes a mirror deficit of edge-on S0s, and checks that the excess survives a dust-insensitive D4000 cut. The quantitative claim (φ_mis ≈ 58%, factor-of-~2 overestimate) comes from an explicit mixture model (Eqs. 2–4) that treats the main-sample b/a distribution as the unbiased reference and assumes contaminants lie only at b/a < 0.4 so that F_mis integrates to 1 on that interval. That assumption is a modeling choice that can be varied; it does not make φ_mis equal to an input by construction, nor is any fitted parameter renamed as a prediction. Self-citations (Domínguez Sánchez catalogues; Tous et al. quiescence offset) supply data products and a selection cut, not a uniqueness theorem or a result that forces the target fraction. Cross-checks with independent T-types (Vázquez-Mata) and Bom et al. further keep the argument externally anchored. No self-definitional loop, no fitted-input-as-prediction, and no load-bearing self-citation chain.

Axiom & Free-Parameter Ledger

4 free parameters · 4 axioms · 0 invented entities

The central claim rests on standard thin-disc projection geometry, public catalogue labels and SFRs, a conventional quiescence offset, and one modeling choice that all contaminants lie at b/a < 0.4. No new physical entities are introduced. Free parameters are selection thresholds and that geometric cut, not fitted dynamical constants.

free parameters (4)
  • quiescence offset threshold = −1.1 dex
    Offset of −1.1 dex from the Renzini & Peng (2015) main sequence (Eq. 1) defines the quiescent sample; the paper states conclusions are robust to alternatives but the reported fractions use this cut.
  • P_LTG classification threshold = 0.5
    P_LTG ≥ 0.5 selects late-types; stricter cuts are said not to change conclusions, but the headline sample uses 0.5.
  • b/a contaminant boundary = 0.4
    Section 3.2 assumes misclassified galaxies populate exclusively b/a < 0.4 so F_mis integrates to 1 over that interval; this choice directly sets φ_mis ≈ 58%.
  • P_edge edge-on cut (Appendix A) = 0.7
    P_edge > 0.7 defines the edge-on QLTG subsample compared to S0s in the SFR–M* plane.
axioms (4)
  • domain assumption For thin, nearly axisymmetric discs, projected axis ratio b/a primarily traces inclination, so a randomly oriented disc population has an approximately uniform b/a distribution.
    Stated in §3.1 and used as the null expectation against which the QLTG excess is judged; footnote notes it fails for slow-rotator ETGs.
  • domain assumption The main NSA galaxy sample’s b/a distribution is an unbiased reference for the intrinsic inclination distribution of LTGs.
    Used in Eqs. 2–4 (§3.2) to separate F_MGS from F_mis; the main sample includes ETGs, which the text argues only mildly affect the low-b/a end.
  • domain assumption Morphological probabilities (P_LTG, P_S0, T-type, P_edge) from Domínguez Sánchez et al. (2023) and SFRs/M* from GSWLC-Salim et al. (2018) are sufficiently accurate for population statistics.
    Entire sample construction in §2 rests on these catalogues; dust-corrected SFRs and D4000 cross-check are used to argue the bias is morphological, not attenuation-driven.
  • standard math Flat ΛCDM with H0 = 70 km s−1 Mpc−1 and Ωm = 0.3.
    Cosmology stated at end of §1; only needed for the volume-limited subsample absolute-magnitude cut.

pith-pipeline@v1.2.0-grok45-kimik3 · 16200 in / 3240 out tokens · 58712 ms · 2026-07-31T08:25:53.565059+00:00 · methodology

0 comments
read the original abstract

In the local Universe, the terms late-type and early-type are used to describe, respectively, disc galaxies with spiral arms and more bulge-dominated systems lacking spiral structure. These morphologies are often associated with different star formation levels, motivating the classical scenario where late-type and early-type galaxies are seen, respectively, as star-forming and "red and dead" systems. To build a comprehensive picture of galaxy evolution, we must understand the relationship between morphology and star formation, and be able to explain more exotic configurations, such as quiescence in late-type galaxies. Large morphological catalogues are valuable tools to address these questions, but they must not be used blindly. We show that selecting quiescent galaxies classified as late-type in these catalogues leads to a clear excess of edge-on systems, with unreliable morphologies, compared with the general galaxy population in the Sloan Digital Sky Survey. Consequently, the fraction of quiescent late-type galaxies in this survey could be overestimated by a factor of two. Conversely, S0 galaxies, considered early-type discs without spiral arms, are under-represented at high inclinations. We conclude that the excess of edge-on quiescent late-type galaxies can be explained if most of these systems are misclassified edge-on S0 galaxies. We make recommendations to remove this bias.

Figures

Figures reproduced from arXiv: 2607.24664 by H. Dom\'inguez S\'anchez, J. L. Tous, S. I. Raimundo.

Figure 2
Figure 2. Figure 2: Fraction of galaxies in the different samples as a function of the probability of being edge on (Domínguez Sánchez et al. 2023). The main galaxy sample is represented by the grey dashed lined, late-type galaxies by the blue solid line, quiescent late-type galaxies by the salmon solid boxes, lenticulars by the green dotted line, and ellipticals by the brown dashed-dotted line. The fraction of edge-on, quies… view at source ↗

discussion (0)

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Reference graph

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