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arxiv: 2512.11035 · v3 · submitted 2025-12-11 · 🌌 astro-ph.GA

The abundance of thin dwarf galaxies: a challenge for cosmological simulations

Jos\'e A. Benavides , Laura V. Sales , Julio F. Navarro , Simon D. M. White , Carlos S. Frenk , Kyle A. Oman , Shaun Cole This is my paper

Pith reviewed 2026-05-16 22:47 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords dwarf galaxiesthin galaxiesaxis ratioscosmological simulationsgalaxy morphologyhydrodynamical simulationsgalaxy formation
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The pith

Simulations contain no dwarf galaxies flatter than c/a=0.2 below 10^9 solar masses, unlike observations that infer 30-40% such thin systems.

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

The paper uses the distribution of projected axis ratios from large observational catalogs to infer how many galaxies are intrinsically thin at different stellar masses. It finds thin galaxies are common across the dwarf range but entirely missing from several leading cosmological hydrodynamical simulations below 10^9 solar masses. The discrepancy is attributed to insufficient resolution and incomplete modeling of the baryonic processes that shape low-mass disks. If the observational inference holds, models of galaxy formation in the dwarf regime remain incomplete.

Core claim

Using data from GAMA, DESI, ALFALFA and nearby galaxy catalogs, the distribution of projected axis ratios q implies that up to 40% of galaxies with 10^9 to 10^10 solar masses are intrinsically flatter than c/a=0.2, with the fraction still reaching 30% near 10^8 solar masses. In contrast, the TNG50, FIREbox and Romulus25 simulations contain no galaxies below 10^9 solar masses with c/a<0.2. The paper states that this absence reflects limitations in numerical resolution and in the treatment of baryonic physics.

What carries the argument

Inference of the intrinsic c/a shape distribution from the observed distribution of projected axis ratios q, under random orientations, compared against the shapes measured directly in simulated galaxy catalogs.

If this is right

  • Baryonic physics implementations in simulations must be revised to allow thin disk structures to form and survive at low masses.
  • Numerical resolution must be increased to capture the processes that produce flat dwarf galaxies.
  • Current models leave the mechanisms regulating thin-disk formation in galaxies less massive than the Milky Way incomplete.
  • Predictions for the morphological mix of the dwarf galaxy population require updating to match observed thin fractions.

Where Pith is reading between the lines

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

  • Matching the observed thin fraction may require explicit inclusion of angular-momentum preservation mechanisms that current feedback schemes suppress.
  • The same shape statistics could be applied to other simulation suites to test whether the absence of thin dwarfs is a universal shortcoming.
  • If resolved, the fix would likely alter predictions for the rotation-curve shapes and gas content of low-mass galaxies.

Load-bearing premise

The mapping from observed projected axis ratios to intrinsic c/a fractions assumes random orientations and a specific family of intrinsic shape distributions without significant selection biases or triaxiality effects.

What would settle it

A higher-resolution simulation run that produces at least one galaxy with stellar mass below 10^9 solar masses and c/a below 0.2, or new observational data showing a substantially lower fraction of thin dwarfs than reported here.

Figures

Figures reproduced from arXiv: 2512.11035 by Carlos S. Frenk, Jos\'e A. Benavides, Julio F. Navarro, Kyle A. Oman, Laura V. Sales, Shaun Cole, Simon D. M. White.

Figure 1
Figure 1. Figure 1: — Color (𝑔 − 𝑟) as a function of stellar mass for all galaxies included in our GAMA (black), DESI (cyan) and ALFALFA (magenta) samples. The main panel shows individual galaxies as dots along with 10%, 50% and 90% percentile contours, as well as thick curves tracing the median color as a function of 𝑀★. Horizontal and vertical histograms summarize the distributions in stellar mass and colors for each sample… view at source ↗
Figure 2
Figure 2. Figure 2: — Stellar half-light radius (𝑅half,★) as a function of stellar mass for our GAMA (black) and DESI (cyan) samples. There are no large differences in the size of objects included in our GAMA and DESI sample with 𝑀★ ≳ 109 M⊙, although DESI shows some preference towards larger dwarfs for low-mass objects, 𝑀★ < 109 M⊙. (i.e., no satellites) galaxies in the stellar mass range of interest, 107 < 𝑀★/M⊙ < 1011. To … view at source ↗
Figure 3
Figure 3. Figure 3: — Stellar mass vs. redshift of galaxies in the observational sample. Dots are used for individual galaxies while thick lines show the median trend as a function of 𝑀★. The ALFALFA catalog (magenta) samples a more local volume than that covered by GAMA (black) and DESI (cyan). The redshift range spanned by our samples depend on 𝑀★, although the only explicit cut used to select them was 𝑧 < 0.1. Histograms a… view at source ↗
Figure 4
Figure 4. Figure 4: — Examples of galaxies with different projected axis ratio 𝑞 (columns) in bins of stellar masses (rows) from our DESI sample. Low 𝑞 values are quite clearly associated with edge-on disks, while larger 𝑞 can be associated to thicker/rounder objects and also face-on disks. Note the presence of well defined thin edge-on disks even in our lowest mass systems (left column, top two rows). All images were generat… view at source ↗
Figure 5
Figure 5. Figure 5: — Normalized distributions of projected axis ratio (𝑞) in bins of stellar mass. Different colors correspond to different observational surveys and the number of galaxies in each bin for each case is quoted in the panels. Galaxies with 𝑀★ > 109 M⊙ show relatively flat 𝑞 distributions, but they become peaked at around 𝑞 ∼ 0.4 − 0.5 at lower masses. Notice that all stellar mass bins show at least some fractio… view at source ↗
Figure 6
Figure 6. Figure 6: — Comparison between the observed (projected) 𝑞 distribution and the inferred intrinsic 3D axis ratios for a subsample of dwarfs in the GAMA survey with 𝑀★ = 109 -109.5 M⊙. The observed shapes are shown with the solid black line and are used to derive an intrinsic 𝑐/𝑎 (long-dashed) and 𝑏/𝑎 (dotted) distribution that the low-𝑞 distribution when projected (method described in Sec. 4.1). Solid color lines in … view at source ↗
Figure 7
Figure 7. Figure 7: — Fraction of galaxies in the GAMA sample that are derived to be intrinsically thinner than a given 𝑐/𝑎 cut as a function of stellar mass. We show 𝑐/𝑎 < 0.2 and 0.3 in thick solid and dashed lines, respectively. Shaded regions correspond to the 10th -90th percentiles from repeating the intrinsic shape reconstruction 100 times in each 𝑀★ bin (see Sec. 4.1 for details). The fraction of intrinsically thin gal… view at source ↗
Figure 8
Figure 8. Figure 8: — Examples of 10 highly elongated dwarf galaxies with 𝑞 ≤ 0.2 and 𝑀★ = 107 - 108.5 M⊙. They resemble thin edge-on disks and can be found even among the faintest dwarfs in our sample. Legends in each panel quote the stellar mass, the shape parameter 𝑞, as well as the GAMA identifier. Objects are sorted by increasing 𝑀★, left to right. All images were generated using the Legacy Survey database [PITH_FULL_IM… view at source ↗
Figure 9
Figure 9. Figure 9: — Fraction of galaxies that are derived to be intrinsically thinner than 𝑐/𝑎 ≤ 0.2 (left) and 𝑐/𝑎 ≤ 0.3 (right) as a function of stellar mass. Thick solid lines correspond to our observational samples while dashed lines are used to display the results of cosmological simulations. Different colors highlight the specific survey or simulation name, as quoted in the legend. In all observational surveys, the fr… view at source ↗
Figure 10
Figure 10. Figure 10: — Normalized distribution of projected axis ratio (𝑞) in stellar mass bins for our simulated galaxy samples: TNG50 (orange), FIREbox (red), Romulus (green). For reference we include the observed 𝑞 distribution from the GAMA sample, as shown in [PITH_FULL_IMAGE:figures/full_fig_p011_10.png] view at source ↗
Figure 11
Figure 11. Figure 11: — Edge-on projection of examples of the flattest galaxies in the TNG50 simulation, in different bins of stellar mass. The rotation is determined by the angular momentum of the galaxy disk. Images were generated using the Py-SPHViewer code (Benítez-Llambay 2017). bin (107 < 𝑀★/M⊙ < 108 ), 10 to 20% of dwarfs are thinner than 𝑐/𝑎 = 0.2, a clear demonstration that thin stellar disks are able to form and surv… view at source ↗
Figure 12
Figure 12. Figure 12: — Comparison of stellar mass (left) and shape parameter 𝑞 (right) for a sample of objects in common in the DESI and GAMA catalogs. Red dashed lines in each panel indicate the 1:1 relation, while a black dashed curve shows the best-fit linear relation to the data. We find good agreement between the properties of galaxies in common in both catalogs. The scatter is 𝜎𝑀★ ∼ 0.12 and 𝜎𝑞 ∼ 0.032 for the stellar m… view at source ↗
Figure 13
Figure 13. Figure 13: — Comparison of q values measured for a simulated galaxy with intrinsic axis ratios 𝑏/𝑎 = 1 and 𝑐/𝑎 = 0.2, generated with 105 stellar particles. The face-on projection (XY) is shown in the left panel, the edge-on projection (XZ) in the middle panel, and an inclined projection in the right panel. At the top of each panel, the axis ratio calculated using the 2D mass-weighted inertia tensor is shown. The gre… view at source ↗
read the original abstract

We study the prevalence of thin galaxies as a function of stellar mass in the range $10^7 < M_{\star} / \rm{M_\odot} < 10^{11}$ using data from the GAMA, DESI, ALFALFA, and Nearby Galaxy catalogs. We use the distribution of projected axis ratios, $q$, to infer the abundance of intrinsically flat galaxies needed to reproduce the observed abundance of highly elongated systems in projection. We find that as many as $40\%$ of galaxies in the mass range $10^9<M_{\star}/\rm{M_\odot}<10^{10}$ are intrinsically flatter than $1$:$5$ (i.e., $c/a<0.2$), a fraction that rises to $\sim 80\%$ for $c/a<0.3$. Although the incidence of thin galaxies decreases towards lower and higher $M_{\star}$, they are still quite common in dwarfs: $\sim 30\%$ and $\sim 65\%$ of $\sim 10^8 ~ \rm{M_\odot}$ galaxies are inferred to be intrinsically flatter than $c/a=0.2$ and $0.3$, respectively. A comparison of these results with several state-of-the-art cosmological hydrodynamical simulations (TNG50, FIREbox, Romulus25) reveals a distinctive lack of thin simulated dwarfs. In particular, there are no $M_{\star} < 10^9 ~ \rm{M_{\odot}}$ simulated galaxies flatter than $c/a=0.2$, in clear contrast with observational samples. This discrepancy likely reflects limitations in resolution and in the treatment of baryonic physics, suggesting that our understanding of the mechanisms regulating the formation of disk galaxies less massive than the Milky Way is still quite incomplete. Our results present a clear challenge to current numerical models of dwarf galaxy formation, which future models should attempt to meet.

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

2 major / 2 minor

Summary. The paper infers the intrinsic fraction of thin dwarf galaxies (c/a < 0.2 and c/a < 0.3) from the observed distribution of projected axis ratios q in GAMA, DESI, ALFALFA, and Nearby Galaxy catalogs across 10^7 < M⋆/M⊙ < 10^11. It reports ~30% of ~10^8 M⊙ galaxies are intrinsically flatter than c/a=0.2 (rising to ~65% for c/a<0.3), with even higher fractions at 10^9–10^10 M⊙, but finds that TNG50, FIREbox, and Romulus25 simulations contain no M⋆ < 10^9 M⊙ galaxies with c/a < 0.2. The discrepancy is attributed to insufficient resolution and incomplete baryonic physics in the simulations.

Significance. If the deprojection is robust, the result supplies a quantitative observational benchmark showing that current hydrodynamical simulations systematically underproduce thin disks in the dwarf regime, highlighting gaps in the modeling of angular-momentum acquisition and feedback for M⋆ ≲ 10^9 M⊙ systems.

major comments (2)
  1. [Methods (deprojection procedure)] The conversion from the observed q distribution to the quoted intrinsic fractions (~30% with c/a<0.2 at 10^8 M⊙) relies on the assumptions of isotropic random orientations and a specific parametric family of intrinsic shapes (likely oblate). Triaxiality would broaden the range of projected q for a given minimum axis ratio and could therefore reduce the inferred thin fraction; the manuscript should present a quantitative test of how the reported percentages change under triaxial models.
  2. [Results (simulation comparison)] The headline discrepancy (zero simulated galaxies with c/a<0.2 below 10^9 M⊙ versus ~30% inferred observationally) is load-bearing. The paper must demonstrate that the simulated samples are selected with identical mass bins, completeness cuts, and projection statistics as the observational catalogs, and report the raw number of simulated objects per bin so that the statistical significance of the null result can be assessed.
minor comments (2)
  1. [Abstract and Results] Clarify in the abstract and §4 whether the quoted fractions (40%, 30%, 80%, 65%) are medians, means, or upper limits and whether they include uncertainties from the deprojection modeling.
  2. [Data section] Add a brief statement on possible orientation biases or selection effects in the GAMA/DESI/ALFALFA samples that could affect the low-q tail.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their careful reading and constructive comments on our manuscript. We address each major comment below and describe the revisions we will make to improve the clarity and robustness of the analysis.

read point-by-point responses

  1. Referee: The conversion from the observed q distribution to the quoted intrinsic fractions (~30% with c/a<0.2 at 10^8 M⊙) relies on the assumptions of isotropic random orientations and a specific parametric family of intrinsic shapes (likely oblate). Triaxiality would broaden the range of projected q for a given minimum axis ratio and could therefore reduce the inferred thin fraction; the manuscript should present a quantitative test of how the reported percentages change under triaxial models.

    Authors: We agree that the deprojection relies on the standard assumptions of oblate intrinsic shapes and isotropic orientations. While these assumptions are widely used in the literature for axis-ratio studies, we acknowledge that triaxiality could affect the inferred fractions. In the revised manuscript we will add a quantitative test that explores the impact of triaxial models on the recovered thin-galaxy fractions, thereby demonstrating the robustness of the reported percentages. revision: yes

  2. Referee: The headline discrepancy (zero simulated galaxies with c/a<0.2 below 10^9 M⊙ versus ~30% inferred observationally) is load-bearing. The paper must demonstrate that the simulated samples are selected with identical mass bins, completeness cuts, and projection statistics as the observational catalogs, and report the raw number of simulated objects per bin so that the statistical significance of the null result can be assessed.

    Authors: We will revise the simulation-comparison section to explicitly confirm that the TNG50, FIREbox, and Romulus25 samples are selected with the same stellar-mass bins, completeness cuts, and projection statistics applied to the observational catalogs. We will also tabulate the raw number of simulated galaxies in each mass bin so that the statistical significance of the null result for galaxies with c/a < 0.2 below 10^9 M⊙ can be directly evaluated. revision: yes

Circularity Check

0 steps flagged

No significant circularity in the derivation chain

full rationale

The paper infers the intrinsic thin-galaxy fraction directly from observed projected axis-ratio distributions in external catalogs (GAMA, DESI, ALFALFA) by forward-modeling under standard assumptions of random orientations and a parametric oblate shape family. This inference is performed independently of any simulation output and does not reduce to a fitted parameter, self-citation, or self-definitional step. The subsequent comparison to TNG50/FIREbox/Romulus25 outputs is a straightforward external benchmark; no equation or claim in the provided text equates the observational result to simulation inputs by construction. The central discrepancy is therefore not forced by the paper's own definitions or prior self-citations.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The central claim rests on the validity of inferring intrinsic axis ratios from projected ones under standard assumptions about orientations and shape distributions; no new entities are postulated and no free parameters are fitted to the target result.

axioms (2)
  • domain assumption Galaxies are randomly oriented with respect to the line of sight
    Required to convert the observed distribution of projected axis ratios q into an intrinsic c/a distribution.
  • domain assumption The intrinsic shapes of galaxies can be described by a continuous distribution of axis ratios without strong triaxiality or selection biases
    Invoked when backing out the fraction of galaxies flatter than c/a = 0.2 or 0.3 from the projected data.

pith-pipeline@v0.9.0 · 5685 in / 1401 out tokens · 36722 ms · 2026-05-16T22:47:35.781052+00:00 · methodology

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Forward citations

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Reference graph

Works this paper leans on

1 extracted references · 1 canonical work pages · cited by 1 Pith paper · 1 internal anchor

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    work page internal anchor Pith review Pith/arXiv arXiv doi:10.1063/1.1848322 2009