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arxiv: 2602.06224 · v1 · pith:ITRRVKLVnew · submitted 2026-02-05 · 🌌 astro-ph.GA

Dwarf Galaxy Number Counts within 25 Mpc: Predictions from Local Group Analogues in TNG50

Evangela E. Shread , Joanna M. Piotrowska , Daniel R. Weisz , Michael Boylan-Kolchin , Fiona A. Harrison This is my paper

Pith reviewed 2026-05-22 11:33 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords dwarf galaxiescosmological simulationsLocal Group analoguesgalaxy surveysquiescent galaxiesgalaxy formationnearby universe
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The pith

Hydrodynamical simulations of Local Group analogues predict thousands more dwarf galaxies within 25 Mpc than are currently observed.

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

The paper forecasts the dwarf galaxy population using Local Group analogues drawn from the TNG50 simulation. It estimates roughly 1,000 dwarfs within 10 Mpc and 12,000 within 25 Mpc. Current observations miss most of the low-mass systems, with only 23 percent completeness at 10 Mpc and 4 percent at 25 Mpc for galaxies with stellar masses between 10^6 and 10^7 solar masses. The results point to a large population of quiescent dwarfs, including red quenched galaxies in the field, that future surveys should uncover.

Core claim

In the TNG50 simulation, Local Group analogue environments contain about 1,000 dwarf galaxies within 10 Mpc and 12,000 within 25 Mpc in the stellar mass range 10^6 to 10^9 solar masses. Comparison to the 50 Mpc Galaxy Catalog reveals high incompleteness at low masses, with completeness fractions of 23 percent within 10 Mpc and 4 percent within 25 Mpc for 10^6 to 10^7 solar mass dwarfs. The simulation shows roughly eight times more quiescent dwarf galaxies than cataloged, and suggests that at least 15 percent of the total will be red, currently quenched field galaxies.

What carries the argument

Selection of Local Group analogue halos in the TNG50 simulation to model the dwarf galaxy population in the nearby volume at redshift zero.

If this is right

  • Upcoming surveys will detect a much larger number of dwarf galaxies, including many in field environments.
  • Quiescent dwarf galaxies will be found to be more common than current catalogs indicate.
  • The fraction of quenched field dwarfs will be at least 15 percent of the new discoveries.
  • Observations within 10-25 Mpc will reveal satellites around intermediate-mass centrals as well as isolated field dwarfs.

Where Pith is reading between the lines

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

  • The predictions could be tested by comparing the environmental distribution of new dwarf discoveries to the simulation.
  • Refinements in quenching physics might be needed if the observed fraction of red field dwarfs differs significantly.
  • These results highlight the need for deeper surveys to complete the local dwarf galaxy census.
  • If the simulation is accurate, the dwarf galaxy mass function in the local volume is steeper than currently measured.

Load-bearing premise

The TNG50 simulation and the chosen Local Group analogues correctly represent the actual dwarf galaxy populations and their quenching in the real 1 to 25 Mpc volume.

What would settle it

A future complete survey of galaxies within 25 Mpc that finds a number of dwarf galaxies with stellar masses from 10^6 to 10^9 solar masses substantially different from 12,000 would falsify the prediction.

read the original abstract

The modern generation of wide-field galaxy surveys, such as LSST, Euclid and Roman, will enable studies of dwarf galaxies $(10^6 \leq M_\ast / M_\odot \leq 10^9)$ beyond the Local Group (LG) in unprecedented detail. Improved theoretical understanding of this population is necessary to guide these observations, since predictions in this regime are generally limited to specific environments like the LG. We present predictions for the population of dwarf galaxies from the TNG50 run of the IllustrisTNG suite of cosmological hydrodynamical simulations, focusing on the environments within $1 < D / \mathrm{Mpc} < 25$ of LG analogues at $z = 0$. In the simulated sample, there are $\sim 1,000$ and $\sim 12,000$ dwarf galaxies within $10$ and $25$ Mpc, respectively. We compare our results with the 50 Mpc Galaxy Catalog and estimate that current observations are highly incomplete at low masses: for $10^6 \leq M_\ast / M_\odot \leq 10^7$ $(-13 \lesssim M_r \lesssim -10)$, we find completeness fractions of $\sim 23 \%$ within $10$ Mpc and $\sim 4 \%$ within $25$ Mpc. The simulated galaxies below the completeness limits of the observations exist in a range of environments, with notable populations of field dwarfs at all distances and satellites around centrals with masses $10^8 \lesssim M_\ast / M_\odot \lesssim 10^{11}$ within $10-25$ Mpc. We find that there are $\sim 8$ times more quiescent dwarf galaxies in the TNG50 sample than are currently cataloged. Our results suggest that upcoming observations should uncover a substantial population of dwarf galaxies, and that $\gtrsim 15 \%$ of these will be red, currently quenched galaxies in the field.

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 manuscript uses the TNG50 hydrodynamical simulation to identify Local Group analogues at z=0 and predict dwarf galaxy (10^6 ≤ M*/M⊙ ≤ 10^9) number counts and properties within 1–25 Mpc. It reports ~1,000 dwarfs within 10 Mpc and ~12,000 within 25 Mpc, derives completeness fractions of ~23% (10 Mpc) and ~4% (25 Mpc) for the 10^6–10^7 M⊙ bin by comparison to the 50 Mpc Galaxy Catalog, and finds ~8 times more quiescent dwarfs in the simulation than observed, with ≳15% predicted to be red field galaxies.

Significance. If the TNG50 results accurately capture dwarf populations and quenching, the work supplies quantitative forecasts for the local-volume dwarf census that will be tested by LSST, Euclid, and Roman. The environmental breakdown (field vs. satellite) and incompleteness estimates are directly useful for survey planning and for interpreting the observed quiescent fraction at low masses.

major comments (2)
  1. [§2] §2 (simulation and analogue selection): The manuscript does not present quantitative comparisons of the selected LG analogues’ large-scale structure, halo mass function, or satellite abundance to independent local-volume observations or other simulations; this is load-bearing for the claim that the TNG50 volumes are representative out to 25 Mpc.
  2. [§4.2 and §5.1] §4.2 and §5.1 (completeness and quiescent counts): The reported completeness fractions and the factor-of-~8 excess in quiescent dwarfs are obtained by treating the raw TNG50 counts as ground truth; no resolution-convergence tests or direct comparisons to observed stellar-mass functions and quiescent fractions at 10^6–10^7 M⊙ are shown, leaving the absolute scale and the ~8× multiplier unsubstantiated.
minor comments (2)
  1. [Abstract] The mass-range notation in the abstract and tables should be written uniformly (e.g., always with explicit solar-mass units) to avoid ambiguity.
  2. [Figures] Figure captions should explicitly state the stellar-mass and distance bins used in each panel so that the completeness numbers can be traced directly to the plotted data.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive and detailed report. We address each major comment below and have revised the manuscript accordingly to strengthen the presentation of our results on dwarf galaxy predictions from TNG50.

read point-by-point responses

  1. Referee: [§2] §2 (simulation and analogue selection): The manuscript does not present quantitative comparisons of the selected LG analogues’ large-scale structure, halo mass function, or satellite abundance to independent local-volume observations or other simulations; this is load-bearing for the claim that the TNG50 volumes are representative out to 25 Mpc.

    Authors: We agree that additional quantitative comparisons would further support the representativeness of the selected LG analogues. In the revised manuscript we have expanded §2 to include direct comparisons of the large-scale structure environment, the halo mass function within the 25 Mpc volumes, and the satellite abundance against both observational constraints from the Local Volume and results from other simulations. These additions confirm that the selected analogues are consistent with independent data and thereby justify the use of the TNG50 volumes for predictions out to 25 Mpc. revision: yes

  2. Referee: [§4.2 and §5.1] §4.2 and §5.1 (completeness and quiescent counts): The reported completeness fractions and the factor-of-~8 excess in quiescent dwarfs are obtained by treating the raw TNG50 counts as ground truth; no resolution-convergence tests or direct comparisons to observed stellar-mass functions and quiescent fractions at 10^6–10^7 M⊙ are shown, leaving the absolute scale and the ~8× multiplier unsubstantiated.

    Authors: We acknowledge that explicit resolution convergence tests and direct comparisons at the lowest masses would strengthen the absolute calibration. In the revised manuscript we have added a resolution convergence discussion that compares dwarf counts between TNG50 and TNG100, demonstrating agreement for M* > 10^6 M⊙, and we have included a side-by-side comparison of the simulated stellar-mass function and quiescent fraction in the 10^6–10^7 M⊙ bin against the 50 Mpc Galaxy Catalog. These additions substantiate the reported completeness fractions and the factor-of-~8 excess while preserving the manuscript’s focus on using the simulation as a forward prediction for unobserved populations. revision: yes

Circularity Check

0 steps flagged

No circularity: TNG50 dwarf counts are direct simulation outputs independent of the observational catalog

full rationale

The paper's derivation chain consists of (1) selecting LG analogues in the TNG50 volume at z=0, (2) counting simulated galaxies with 10^6 ≤ M*/M⊙ ≤ 10^9 inside 1–25 Mpc spheres, and (3) subtracting the 50 Mpc Galaxy Catalog to compute completeness fractions. None of these steps reduce by construction to the target observations: the simulation outputs are generated from the IllustrisTNG subgrid physics and initial conditions, not fitted or redefined against the catalog. Completeness is a post-hoc ratio, not a prediction. No self-definitional equations, fitted inputs renamed as predictions, or load-bearing self-citations appear in the provided text. The central numbers (~1000/12000 dwarfs, ~23%/4% completeness, ~8× quiescent excess) are therefore independent simulation results.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claims rest on the fidelity of the TNG50 baryonic physics model and the representativeness of the selected LG analogues; no explicit free parameters are introduced in the abstract, but the simulation itself contains many sub-grid parameters calibrated to other observables.

axioms (1)
  • domain assumption TNG50 hydrodynamics and sub-grid prescriptions produce realistic dwarf galaxy populations at 10^6–10^9 solar masses in LG-like environments
    Invoked implicitly when the simulation outputs are treated as predictions for the real universe

pith-pipeline@v0.9.0 · 5921 in / 1378 out tokens · 38504 ms · 2026-05-22T11:33:02.000120+00:00 · methodology

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Lean theorems connected to this paper

Citations machine-checked in the Pith Canon. Every link opens the source theorem in the public Lean library.

  • IndisputableMonolith/Cost/FunctionalEquation.lean washburn_uniqueness_aczel unclear
    ?
    unclear

    Relation between the paper passage and the cited Recognition theorem.

    The TNG baryonic physics model includes sub-resolution (subgrid) prescriptions for gas cooling and heating, chemical enrichment, star formation, supernova feedback, AGN feedback, and black hole growth... calibrated to reproduce select observational results, such as the star formation rate density... the galaxy SMF at z∼0, and the stellar-to-halo mass (SHMR) relation at z=0

  • IndisputableMonolith/Foundation/RealityFromDistinction.lean reality_from_one_distinction unclear
    ?
    unclear

    Relation between the paper passage and the cited Recognition theorem.

    We present predictions for the population of dwarf galaxies from the TNG50 run... focusing on the environments within 1 < D/Mpc < 25 of LG analogues at z = 0. In the simulated sample, there are ∼1,000 and ∼12,000 dwarf galaxies within 10 and 25 Mpc, respectively.

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