REVIEW 3 major objections 4 minor 23 references
Origin Of Tidal Structures In Modified Gravity
T0 review · 3 major / 4 minor · reviewed 2026-08-14 · deepseek-v4-flash
Pith's one-line read In modified gravity, a close flyby can build most merger-like tidal structures.
desk verdict Reasonable proceedings summary of a single MOND flyby simulation; the existence claim is plausible but the frequency claim is unsupported. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The load-bearing mechanism is a self-consistent MOND N-body simulation of a close Milky Way–Andromeda-like flyby, together with the MOND property of weakened dynamical friction. Dynamical friction normally drags passing galaxies into a merger; in MOND it is weaker for comparable masses, so the two galaxies can reach a 24 kpc pericenter, stir each other tidally, and still separate. The simulated encounter is compared with observed Local Group structures—the loop-like M31 streams, the Milky Way warp, and the disk of satellites—on the grounds that the simulation's initial conditions match the real galaxies' masses, disk orientations, separation, and relative velocity.
What would settle it
Re-run the simulated encounter with higher resolution, different disk orientations, and small perturbations: if the long tails, bridge, streams, warp, and thick disk appear only for one finely tuned geometry and vanish otherwise, the claim that non-merging flybys generically make merger-like structures fails. Alternatively, find an observed shell system whose stellar kinematics can only be produced by a minor merger, or show that the required flyby orbit is excluded by measured proper motions.
Extended reading notes
Core claim
On the paper's own terms, the discovery is that a non-merging close encounter between two disk galaxies in MOND can generate nearly the full catalogue of structures conventionally attributed to mergers. In the simulation, the two model galaxies approach to a pericenter of 24 kpc, exchange material through a tidal tail, and then recede; over the following 6.5 Gyr this single passage produces long tidal arms, a bridge between the galaxies, loop-like streams around the Andromeda analog that host tidal dwarf galaxies, a warp in the Milky Way analog's disk, and a thick disk. Shells are not formed in this run, but the paper cites an earlier demonstration that flybys can make them. The paper concludes that tidal features are therefore not reliable merger fingerprints, and that in a MOND universe many such structures will have been created by encounters that end in separation rather than coalescence.
Load-bearing premise
The entire catalogue of structures rests on one simulated close passage of two model galaxies, with no resolution tests or ensemble of encounters reported, and with the shell item imported from a Newtonian calculation rather than produced in the MOND simulation.
Editorial extensions
If this is right
- A tidal arm, bridge, stream, warp, or thick disk no longer counts by itself as evidence that a galaxy merger has occurred.
- The roughly 20% of massive galaxies seen interacting at redshift 1 may include many pairs that will separate again rather than merge.
- Disks of satellites around galaxies like M31 can arise naturally from satellites formed as tidal dwarfs during a close flyby, without a merger history.
- Merger-rate estimates based on counting tidal features will need to distinguish flyby products from post-merger remnants.
Reading between the lines
- Editorial inference: the same logic extends beyond the Local Group: field ellipticals with shells but no visible companion may be best explained by a flyby whose partner has since moved on.
- Editorial inference: the shell item is the weakest link in the catalogue, because it is imported from a Newtonian simulation rather than produced in MOND; a MOND-specific shell simulation would settle whether it belongs.
- Editorial inference: a testable extension would be to run an ensemble of flybys with varied mass ratios, orientations, and impact parameters; if the structures prove robust across geometry, individual observed systems can be fitted and falsified.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This proceedings paper argues that in the MOND framework, close non-merging galaxy flybys are much more common than in standard dark-matter cosmology, because dynamical friction is weaker, and that many tidal structures traditionally attributed to galaxy mergers can instead be formed by such flybys. The authors present results from a self-consistent MOND N-body simulation of a close Milky Way-Andromeda encounter (Bílek et al. 2018) and claim that this single encounter produced tidal arms, bridges, streams, disk warps, thick disks, and probably also disks of satellites, while shells are claimed on the basis of a Newtonian simulation by Hernquist & Quinn (1988). The abstract further claims that such structures are 'often formed by non-merging encounters', a frequency conclusion that is argued to follow from the high observed interaction rate at high redshift combined with the MOND behavior.
Significance. If the central claims hold, the paper offers a genuinely different interpretation of tidal structures in galaxies, with implications for how interacting systems are classified at high redshift and for the role of mergers in galaxy evolution. The simulation itself is a rare self-consistent MOND N-body experiment that reproduces several observed Local Group features quantitatively (e.g., the MW-M31 streams and disk warp), and the paper is explicit about which structures were actually generated in the MOND run and which were not. The significance is, however, limited by the fact that the evidence for the 'most types' claim is a single, purpose-tuned encounter, and by the absence of MOND simulations for shell formation.
major comments (3)
- [Abstract and §3] The frequency claim that tidal structures are 'often formed by non-merging encounters' is not supported by the evidence presented. The paper relies on one self-consistent MOND simulation with initial conditions chosen to reproduce the observed Milky Way-Andromeda system, which is a single point in the parameter space of impact parameter, mass ratio, relative velocity, and orbital orientation. No ensemble, parameter survey, or statistical argument is provided to show how often such encounters occur in a MOND universe. The frequency conclusion goes beyond the existence claim and should either be removed or supported with a broader set of simulations or an explicit population argument.
- [§3, Figure 3 (shells)] The paper states explicitly that 'Shells were not formed in our simulation but Hernquist & Quinn (1988) demonstrated that non-merging flybys can produce them.' This means that the inclusion of shells in the list of structures 'formed by non-merging encounters' in MOND is not demonstrated by the authors' MOND simulation; it is an extrapolation from a Newtonian calculation. Because dynamical friction and the external field effect are different in MOND, it cannot be taken for granted that the shell-formation mechanism carries over. The authors should either present a MOND simulation of shell formation or clearly label shells as a Newtonian-based expectation rather than a result of their simulation.
- [§3 (initial conditions and numerics)] The entire existence claim rests on the Bílek et al. (2018) simulation, but this paper provides no details of the N-body code, the MOND interpolation function, particle number, softening length, time integration, or resolution and convergence tests. Without such information, a reader of this paper cannot assess the reliability of the simulated encounter or the robustness of the resulting tidal structures. A brief summary of the numerical setup, or at least a clear statement that those details are given in the cited companion paper, is needed to make the present contribution self-contained enough for its claims.
minor comments (4)
- [Abstract] The phrase 'MOND theory' is redundant (MOND stands for Modified Newtonian Dynamics), and 'suggests an adjustments' should be 'suggests an adjustment' or 'suggests adjustments'.
- [Figure 3 caption] The name 'Hernquist' is misspelled as 'Herqnquist' in the figure caption; please correct.
- [§3, disk of satellites] The identification of the simulated structure with the observed disk of satellites is made qualitatively ('similar size, mass, rotation, orientation'); a quantitative comparison or a measure of statistical significance would strengthen this claim.
- [§3, 'nothing else was tuned'] The statement that 'nothing else was tuned' is potentially misleading, because choices such as the MOND interpolation function, the baryonic mass models, and the initial particle distributions are not specified. Please clarify what was fixed and what was allowed to vary in constructing the initial conditions.
Circularity Check
No circularity: the claimed tidal structures emerge from a parameter-free MOND flyby simulation; the paper's self-citations are real numerical evidence and its weaknesses are extrapolation, not circular reduction.
full rationale
The paper's central claim is that non-merging encounters in MOND can generate structures traditionally attributed to mergers. The supporting evidence is the authors' prior self-consistent MOND N-body simulation (Bilek et al. 2018), but the manuscript explicitly states that the initial conditions were set only to reproduce observed Milky Way and M31 masses, sizes, disk inclinations, distance, and relative velocity, with 'nothing else tuned.' The tidal arms, bridges, streams, disk warp, thick disk, and satellite-disk feature are emergent outputs of the encounter dynamics, and they are compared to observed Local Group structures rather than used to adjust any parameter. No equation in the paper defines a target structure in terms of an input, and no fitted quantity is renamed as a prediction. The self-citations (Bilek et al. 2018, Kroupa 2015) are either an independent numerical experiment or auxiliary review support; the core inference does not reduce to them because the simulation is parameter-free with respect to the claimed structures and is externally falsifiable against real MW/M31 features. The acknowledged limitations—shells were not formed in the MOND run and are imported from the Newtonian calculation of Hernquist & Quinn (1988), and the frequency claim ('often') is extrapolated from a single encounter plus observed interaction fractions—are evidential and robustness weaknesses, not circular reductions. The paper also transparently notes where simulations are still missing (minor mergers). No self-definitional step, fitted-input-as-prediction, or self-citation chain can be exhibited, so the appropriate finding is no significant circularity.
Assumptions & free parameters
assumptions (3)
- domain assumption MOND modifies gravity in the low-acceleration regime and is a valid description of galactic dynamics.
- domain assumption The Bilek et al. (2018) N-body simulation with the stated initial conditions reliably represents a Milky Way and M31 close flyby.
- domain assumption Hernquist and Quinn (1988) Newtonian flyby simulations imply that shells would also form in MOND flybys.
Cite this review
Pith. "Pith review of Origin Of Tidal Structures In Modified Gravity." pith.science (2026). https://pith.science/paper/KKP4QTNA
@misc{pith2026190807537,
author = {Pith},
title = {Pith review of: Origin Of Tidal Structures In Modified Gravity},
year = {2026},
howpublished = {\url{https://pith.science/paper/KKP4QTNA}},
note = {Machine review of arXiv:1908.07537}
}
abstract
The missing mass problem has not been solved decisively yet. Observations show that if gravity is to be modified, then the MOND theory is its excellent approximation on galactic scales. MOND suggests an adjustments of the laws of physics in the limit of low accelerations. Comparative simulations of interacting galaxies in MOND and Newtonian gravity with dark matter revealed two principal differences: 1) galaxies can have close flybys without ending in mergers in MOND because of weaker dynamical friction, and 2) tidal dwarf galaxies form very easily in MOND. When this is combined with the fact that many interacting galaxies are observed at high redshift, we obtain a new perspective on tidal features: they are often formed by non-merging encounters and tidal disruptions of tidal dwarf galaxies. Here we present the results from our self-consistent MOND $N$-body simulation of a close flyby of two galaxies similar to the Milky Way. It turns out that most types of the structures that are traditionally assigned to galaxy mergers can be formed by non-merging encounters, including tidal arms, bridges, streams, shells, disk warps, thick disks, and most probably also disks of satellites. The success of MOND in explaining the dynamics of galaxies hints us that this way of formation of tidal structures should be considered seriously.
Reference graph
Works this paper leans on
-
[1]
2019, A&A, 625, A32 B´ ılek, M., Thies, I., et al
B´ ılek, M., Samurovi´ c, S., & Renaud, F. 2019, A&A, 625, A32 B´ ılek, M., Thies, I., et al. 2018, A&A, 614, A59
work page 2019
- [2]
- [3]
- [4]
-
[5]
Combes, F. & Tiret, O. 2010, in American Institute of Physics Conference Series, Vol. 1241, American Institute of Physics Conference Series, ed. J.-M. Alimi & A. Fu¨ ozfa, 154–161
work page 2010
-
[6]
Famaey, B. & McGaugh, S. S. 2012, Living Re- views in Relativity, 15, 10
work page 2012
-
[7]
Ferguson, A. M. N. & Mackey, A. D. 2016, Tidal Streams in the Local Group and Be- yond, 420, 191
work page 2016
-
[8]
Gentile, G., Famaey, B., & de Blok, W. J. G. 2011, A&A, 527, A76
2011
Show all 23 references
-
[9]
& Quinn, P
Hernquist, L. & Quinn, P. J. 1988, ApJ, 331, 682
1988
-
[10]
Karachentsev, I. D. 2012, Astrophysical Bul- letin, 67, 123
2012
-
[11]
2015, Canadian Journal of Physics, 93, 169
Kroupa, P. 2015, Canadian Journal of Physics, 93, 169
2015
-
[12]
S., et al
Lelli, F., McGaugh, S. S., et al. 2017, ApJ, 836, 152
2017
-
[13]
S., Blitz, L., & Heiles, C
Levine, E. S., Blitz, L., & Heiles, C. 2006, ApJ, 643, 881
2006
-
[14]
& Milgrom, M
McGaugh, S. & Milgrom, M. 2013, ApJ, 775, 139 Tidal Structures In Modified Gravity 5
2013
-
[15]
S., Lelli, F., & Schombert, J
McGaugh, S. S., Lelli, F., & Schombert, J. M. 2016, Physical Review Letters, 117, 201101
2016
-
[16]
1983, ApJ, 270, 365
Milgrom, M. 1983, ApJ, 270, 365
1983
-
[17]
2013, Phys
Milgrom, M. 2013, Phys. Rev. Let., 111, 041105
2013
-
[18]
2014, Scholarpedia, 9, 31410
Milgrom, M. 2014, Scholarpedia, 9, 31410
2014
-
[19]
2019, Phys
Milgrom, M. 2019, Phys. Rev. D, 99, 044041
2019
-
[20]
2008, MNRAS, 386, 2194
Nipoti, C., Ciotti, L., et al. 2008, MNRAS, 386, 2194
2008
-
[21]
2007, MNRAS, 381, L104
Nipoti, C., Londrillo, P., & Ciotti, L. 2007, MNRAS, 381, L104
2007
-
[22]
& Combes, F
Tiret, O. & Combes, F. 2007, in SF2A-2007: Proceedings of the Annual meeting of the French Society of Astronomy and Astro- physics, ed. J. Bouvier, A. Chalabaev, & C. Charbonnel, 356
2007
-
[23]
2013, A&A, 557, L3
Zhao, H., Famaey, B., et al. 2013, A&A, 557, L3
2013
Reviewed August 14, 2026 · model on record in the stance chip above.
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