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arxiv: 2605.25075 · v1 · pith:V5JOG5BLnew · submitted 2026-05-24 · 🌌 astro-ph.GA

Small and Complex II: Characterizing the Disk and Stellar Envelope of Edge-on z sim 0 Massive Compact Galaxies

A. Schnorr-M\"uller , K. Slodkowski Clerici , F. Pinna , R. Merib-Dias , M. Trevisan , T. V. Ricci , F. Ferrari , A. C. Santiago-Menezes
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W. L. Becker F. Palacios
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Pith reviewed 2026-06-29 23:53 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords massive compact galaxiesphotometric decompositionstellar envelopesedge-on galaxiesrelic galaxiesbulge-disk structureS0 galaxies
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The pith

Massive compact galaxies show smaller but proportionally scaled bulges, disks and envelopes, with surviving disks weakening dry minor merger models for envelope growth.

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

The study fits three-component models to edge-on images of 75 low-redshift massive compact galaxies and compares them to ordinary S0 galaxies of similar mass. All structural parts in the compact sample are smaller, yet the sizes of bulge, disk and envelope remain coupled to one another and the disk still carries a large fraction of the light. These patterns match those found in known relic galaxies, suggesting the two populations share a common structural family rather than having formed through unrelated channels.

Core claim

MCGs host more compact bulges and envelopes as well as shorter and thicker disks than a mass-matched control sample of non-compact S0 galaxies; component sizes are coupled across the sample; median bulge-to-total and disk-to-total flux fractions are similar; MCGs and relic galaxies follow the same component size-mass relations.

What carries the argument

Three-component photometric decomposition of edge-on images that separates bulge, disk and stellar envelope and measures their effective radii, scale lengths, ellipticities and flux fractions.

If this is right

  • Bulge, disk and envelope sizes are smaller in MCGs than in control galaxies but scale together.
  • Substantial disk components survive in MCGs, with median D/T approximately 0.4.
  • Envelope ellipticities range from 0 to 0.7, with MCG envelopes rounder on average.
  • MCGs and relic galaxies occupy the same region in component size-mass space.
  • Dry minor mergers are disfavored as the dominant envelope-building process.

Where Pith is reading between the lines

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

  • If size coupling is physical, any envelope-growth channel must preserve inner-component scaling and disk structure.
  • Kinematic or stellar-population data would be needed to decide whether intermediate-ellipticity envelopes are halos or thick disks.
  • Extending the same decomposition to higher-redshift compact galaxies could test whether the structural family persists across cosmic time.

Load-bearing premise

The three-component photometric model can be fitted to edge-on galaxy images without large degeneracies or systematic biases that would alter the measured sizes, ellipticities or flux fractions.

What would settle it

Re-fitting the same images with altered model assumptions or additional components that removes the observed size coupling or drops the disk light fraction below 20 percent would falsify the central claim.

Figures

Figures reproduced from arXiv: 2605.25075 by A. C. Santiago-Menezes, A. Schnorr-M\"uller, F. Ferrari, F. Palacios, F. Pinna, K. Slodkowski Clerici, M. Trevisan, R. Merib-Dias, T. V. Ricci, W. L. Becker.

Figure 1
Figure 1. Figure 1: Sample selection. MCGs are shown as red cir￾cles, CSGs as green triangles, relic galaxies as blue dia￾monds (with the prototypical relic galaxy NGC 1277 high￾lighted as a black diamond) and SDSS quiescent edge-on S0 galaxies as orange circles. The top panel displays the stellar mass–velocity dispersion diagram, the middle panel shows the velocity dispersion–effective radius diagram, and the bottom panel pr… view at source ↗
Figure 2
Figure 2. Figure 2: r-band image, best-fitting model, and residuals for an MCG (top) and a CSG (bottom). The image and model panels share the same logarithmic flux scale. The residuals are scaled separately to emphasize structural features. sample are viewed exactly edge-on, all have inclinations > 70◦ . This allows a structural analysis broadly com￾parable to that employed for the MCGs, requiring only minor adjustments to th… view at source ↗
Figure 3
Figure 3. Figure 3: Structural properties of MCGs and CSGs. The first row shows the effective radius, S´ersic index and ellipticity of the bulge component. The second row presents the disk scale length, scale height, and the ratio between them. In the third row, we show the effective radius and ellipticity of the stellar envelope, modeled with a fixed S´ersic index of n = 1. The fourth row displays the flux-to-total ratio of … view at source ↗
Figure 4
Figure 4. Figure 4: A comparison between relic galaxies and MCGs. The top panels displays HST images of three relic galaxies, while the bottom panel shows MCGs with similar stellar mass, velocity dispersion, effective radius, and ellipticity of the outermost component. The morphological similarity between relics and MCGs is clear. 4.3. Size-mass Relations of the Bulge, Disk, and Envelope In [PITH_FULL_IMAGE:figures/full_fig_… view at source ↗
Figure 5
Figure 5. Figure 5: Size–mass relations for the bulge (top left), disk (top right), and envelope (bottom left), together with the scale height–disk mass relation (bottom right). MCGs are shown as red circles, CSGs as green triangles, and relic galaxies as blue diamonds, with the prototypical relic galaxy NGC 1277 highlighted as a black diamond. Best-fit relations from the literature for local and high-redshift galaxies are ov… view at source ↗
Figure 6
Figure 6. Figure 6: Coupling between structural components. Disk scale length versus bulge effective radius is shown in the top panel, and disk scale length versus envelope effective radius in the bottom panel. MCGs are shown as red circles, CSGs as green triangles, and relics as blue diamonds with the pro￾totypical relic galaxy NGC 1277 highlighted as a black dia￾mond. A linear fit to the combined MCG and relic distri￾bution… view at source ↗
Figure 7
Figure 7. Figure 7 [PITH_FULL_IMAGE:figures/full_fig_p013_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: Distribution of MCGs, CSGs, relics, and 1.5 < z < 3 quiescent galaxies in the M⋆–Re plane. MCGs, CSGs, and relics are color-coded by stellar population age. The dashed, dot-dashed, and solid lines represent the com￾pactness thresholds proposed by van der Wel et al. (2014), Barro et al. (2017), and Spiniello et al. (2021), respectively. Despite their differing sizes, relics and MCGs exhibit simi￾larly old s… view at source ↗
Figure 9
Figure 9. Figure 9: HST WFPC2/F606W image, best-fitting model, and residuals for MCG ID: 109. The bulge of this galaxy was fitted with a point source, which gives an upper limit of ∼ 0.2 kpc for Re,bulge. APPENDIX A. MCGS WITH HST IMAGING Of the 1858 SDSS MCGs in the Clerici et al. (2024) sample, four have HST archival imaging available. In table 3 we list the Proposal ID, PI, instrument and filter of these observations. We p… view at source ↗
read the original abstract

We present multi-component photometric decompositions of $r$-band Hyper Suprime-Cam images for a sample of 75 edge-on massive compact galaxies (MCGs) at $z < 0.1$, selected as $+2\sigma$ outliers in the stellar mass-velocity dispersion relation and $-2\sigma$ outliers in the velocity dispersion-size relation. MCGs are composed of compact bulges and disks embedded within stellar envelopes of unclear physical nature. Comparing MCGs to a mass- and redshift-matched control sample of non-compact edge-on S0 galaxies with a similar three-component structure, we find that the smaller sizes of MCGs are not driven by a single component. MCGs host more compact bulges and envelopes ($R_\mathrm{e,bulge} \sim 0.3$ versus $0.5$kpc; $R_\mathrm{e,env} \sim 4.4$ versus $5.7$kpc), as well as shorter and thicker disks ($h_R \sim 1.1$ versus $1.7$kpc; $h_R/z_0 \sim 3.9$ versus $5.3$). The sizes of the structural components are coupled, suggesting their formation processes are linked. Median bulge- and disk-to-total flux fractions are similar in both samples, with $B/T \sim 0.3$ and $D/T \sim 0.4$. Envelope ellipticities span $\epsilon_\mathrm{env} \sim 0$-$0.7$, with MCGs exhibiting rounder envelopes. Low- and high-ellipticity envelopes are broadly consistent with stellar halos and thick disks, respectively. However, the nature of intermediate ellipticity envelopes remains ambiguous from photometry alone. The coupling between component sizes, together with the survival of a substantial disk component, argues against dry minor mergers as the dominant envelope-building mechanism. A comparison with 8 relic galaxies reveals that MCGs and relics share similar bulge-disk-envelope structures and follow the same component size-mass relations, consistent with belonging to a common structural family.

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 pith above is the substance, this is the friction.

Referee Report

3 major / 2 minor

Summary. The paper reports three-component (bulge+disk+envelope) photometric decompositions of HSC r-band images for 75 edge-on massive compact galaxies (MCGs) at z<0.1. It finds that MCGs have systematically smaller bulges (R_e,bulge ~0.3 vs 0.5 kpc), shorter/thicker disks (h_R ~1.1 vs 1.7 kpc), and smaller envelopes (R_e,env ~4.4 vs 5.7 kpc) than a mass-matched control sample of edge-on S0s, with coupled component sizes, similar B/T~0.3 and D/T~0.4, rounder envelopes in MCGs, and structural similarity to 8 relic galaxies. The authors argue that size coupling plus disk survival disfavors dry minor mergers as the dominant envelope-building process.

Significance. If the decompositions hold, the work supplies direct observational evidence that structural components in MCGs are linked in size and that a substantial disk persists, thereby constraining envelope formation channels and placing MCGs and relics in a common structural family. The sample comparison and quantitative size-mass relations are strengths of the analysis.

major comments (3)
  1. [Methods (photometric decomposition)] Methods section on photometric decomposition: no mock-galaxy recovery tests, convergence diagnostics, or alternative modeling (e.g., two-component runs) are described to bound degeneracies between the exponential disk scale length h_R, envelope R_e,env, and their ellipticities in edge-on projections. All reported size differences, flux fractions, and the merger-mechanism inference rest on these parameters.
  2. [Results (size and ellipticity comparisons)] Results (component size and ellipticity comparisons): the median values (R_e,bulge 0.3 vs 0.5 kpc; h_R 1.1 vs 1.7 kpc; envelope ellipticities 0–0.7) and the claim of coupled sizes lack reported uncertainties or degeneracy bounds, so it is unclear whether the differences exceed possible systematic biases from the edge-on geometry.
  3. [Discussion] Discussion of merger mechanism: the argument that size coupling plus disk survival disfavors dry minor mergers inherits the same unquantified systematic risk as the underlying fits; an explicit test (e.g., how much bias would be needed to erase the reported differences) is absent.
minor comments (2)
  1. Notation for scale lengths (h_R, z_0) and effective radii should be defined once in the text and used consistently in all tables and figures.
  2. The relic-galaxy comparison sample size (N=8) is small; a brief statement on how selection effects were assessed would improve clarity.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for their constructive comments, which identify key areas where additional validation will strengthen the analysis of our photometric decompositions. We address each major comment below and will revise the manuscript accordingly.

read point-by-point responses

  1. Referee: Methods section on photometric decomposition: no mock-galaxy recovery tests, convergence diagnostics, or alternative modeling (e.g., two-component runs) are described to bound degeneracies between the exponential disk scale length h_R, envelope R_e,env, and their ellipticities in edge-on projections. All reported size differences, flux fractions, and the merger-mechanism inference rest on these parameters.

    Authors: We agree that the Methods section would benefit from explicit validation. In the revised manuscript, we will add mock-galaxy recovery tests tailored to edge-on projections, report convergence diagnostics from the fitting procedure, and include two-component model comparisons to quantify potential degeneracies between the disk and envelope parameters. revision: yes

  2. Referee: Results (component size and ellipticity comparisons): the median values (R_e,bulge 0.3 vs 0.5 kpc; h_R 1.1 vs 1.7 kpc; envelope ellipticities 0–0.7) and the claim of coupled sizes lack reported uncertainties or degeneracy bounds, so it is unclear whether the differences exceed possible systematic biases from the edge-on geometry.

    Authors: We will revise the Results section to include uncertainties on all median values and size comparisons, derived from bootstrap resampling and fit error propagation. We will also add a discussion of possible systematic biases from edge-on geometry and assess whether they could account for the observed differences and size coupling. revision: yes

  3. Referee: Discussion of merger mechanism: the argument that size coupling plus disk survival disfavors dry minor mergers inherits the same unquantified systematic risk as the underlying fits; an explicit test (e.g., how much bias would be needed to erase the reported differences) is absent.

    Authors: We will expand the Discussion to include an explicit sensitivity analysis quantifying the level of systematic bias in component sizes that would be required to erase the reported differences. This will directly test the robustness of the inference against dry minor mergers. revision: yes

Circularity Check

0 steps flagged

No circularity: empirical photometric measurements and sample comparisons

full rationale

The paper reports direct results from fitting three-component models to HSC images of edge-on galaxies and comparing the resulting sizes, ellipticities, and flux fractions to a control sample and to relics. No derivation, equation, or prediction is presented that reduces by construction to its own inputs; the central claims (size coupling, disk survival, disfavoring dry minor mergers) follow from the measured quantities themselves rather than from any self-referential step or self-citation chain. The work is therefore self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

Review performed on abstract only; full methods, fitting details, and data products are unavailable. The central results rest on the validity of the three-component photometric model and the appropriateness of the control-sample matching.

axioms (1)
  • domain assumption Photometric decomposition into bulge, disk, and envelope components can be performed reliably on edge-on galaxy images without major degeneracies affecting size and ellipticity measurements.
    Invoked implicitly throughout the abstract as the basis for all reported component sizes, ellipticities, and flux fractions.

pith-pipeline@v0.9.1-grok · 5992 in / 1408 out tokens · 40264 ms · 2026-06-29T23:53:08.554395+00:00 · methodology

discussion (0)

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Reference graph

Works this paper leans on

3 extracted references · 2 canonical work pages

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    APPENDIX A.MCGS WITH HST IMAGING Of the 1858 SDSS MCGs in the Clerici et al

    The bulge of this galaxy was fitted with a point source, which gives an upper limit of∼0.2 kpc forR e,bulge. APPENDIX A.MCGS WITH HST IMAGING Of the 1858 SDSS MCGs in the Clerici et al. (2024) sample, four have HST archival imaging available. In table 3 we list the Proposal ID, PI, instrument and filter of these observations. We performed multi-component ...

    2024