The effects of the orbital configurations of mergers on reshaping galaxy structures

Guangquan Zeng; Jiang Chang; Ling Zhu; Xinyi Wu; Yu Lei

REVIEW 3 major objections 5 minor 1 cited by

The effects of the orbital configurations of mergers on reshaping galaxy structures

T0 review · 3 major / 5 minor · reviewed 2026-08-06 · deepseek-v4-flash

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Pith's one-line read Merger orbital geometry systematically reshapes remnant galaxies, with spiral-in orbits preserving disks and head-on collisions building hot inner halos.

desk verdict A careful systematic study of merger orbits and remnant structure in TNG100; the headline correlation is plausible but the causal claim needs a cleaner pre-merger control. read the letter →

arxiv 2507.02051 v1 pith:DZ2Z5IUX submitted 2025-07-02 astro-ph.GA

Xinyi Wu , Ling Zhu , Jiang Chang , Guangquan Zeng , Yu Lei This is my paper

classification astro-ph.GA

keywords galaxymergersorbitalconfigurationstructuredecompositiondisksurvivalhotinnerstellarhaloexsitumassIllustrisTNGredHI-richgalaxies

verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

This paper tries to establish that the orbit along which two galaxies merge—not just their mass ratio or gas content—systematically decides what structure the remnant galaxy ends up with. Using 531 merger pairs from the TNG100 cosmological simulation, the authors describe each merger by the relative motion of the pair and the orientations of both galactic disks, then dynamically split every galaxy into disk, warm, bulge, and hot inner stellar halo components. They find that spiral-in orbits align the progenitor disks with the orbital plane and leave remnants rich in disk and warm components, while head-on collisions produce bulge-dominated remnants with large hot inner halos. Almost every merger, regardless of orbit, increases the hot inner stellar halo fraction, which is why that component (unlike the bulge) tracks total accreted stellar mass at $z=0$. A sympathetic reading takes this as evidence that merger geometry is a first-order variable in galaxy evolution, not a secondary correction.

What carries the argument

There are two pieces of machinery. The first is a dynamic decomposition of each galaxy into four components using the circularity $\lambda_z = J_z/J_{\max}(E)$ and radius: disk ($\lambda_z > 0.8$), warm component ($0.8 > \lambda_z > 0.5$ outside the bulge radius), bulge ($\lambda_z < 0.8$, $r < 3.5$ kpc), and hot inner stellar halo ($\lambda_z < 0.5$ outside the bulge radius). The second is a three-angle description of the merger: $\langle \theta_{r,v}\rangle$ measures how radial the infall is (zero for head-on, ninety degrees for spiral-in), while $\langle \theta'_{L_{\rm main},L_{\rm orbit}}\rangle$ and $\langle \theta'_{L_{\rm sat},L_{\rm orbit}}\rangle$ give the orientations of the two galactic spins relative to the orbital plane, folded to 0–90 degrees and summed into $\theta_{\rm sum}$. The orbital angle $\langle \theta_{r,v}\rangle$ does the main carrying: it correlates most strongly with the remnant's disk and warm fractions, and it also correlates with $\theta_{\rm sum}$, showing that orbital geometry and galaxy orientation are tied together in the merger population.

What would settle it

Recompute the component fractions for the same 531 merger pairs at $t_{\rm merger}+2$ Gyr and $t_{\rm merger}+3$ Gyr. If the ordering of remnants by orbital configuration changes materially between one and three gigayears after the merger, or if the $t-1$ Gyr progenitors show tidal distortions that grow with the radiality of the orbit, the claimed orbit–structure relation is an artifact of the chosen snapshot times.

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Extended reading notes

Core claim

The paper's central claim is that the orbital configuration of a merger is a first-order determinant of the remnant's structure. For mergers on spiral-in orbits the disk planes of both galaxies tend to lie nearly parallel to the orbital plane, and the remnant retains higher fractions of disk and warm components and lower fractions of bulge and hot inner stellar halo; for mergers on head-on collision orbits the disk planes tend to lie perpendicular to the orbital plane, producing the opposite outcome. The correlation between the orbital angle $\langle \theta_{r,v}\rangle$ and remnant disk fraction reaches $R=0.66$ for major mergers, comparable in strength to mass-ratio effects. A second load-bearing result is that mergers almost always grow the hot inner stellar halo, increasing its fraction in 93% of cases, so after several mergers a galaxy's inner-halo fraction becomes a faithful record of its total accreted stellar mass. This is offered as an explanation for the observed correlation between inner-halo luminosity and ex situ stellar mass at $z=0$, and as a caution that disk or bulge mass fractions cannot be read as universal merger counters.

Load-bearing premise

The analysis assumes that one gigayear before a merger the two galaxies are still unperturbed and that one gigayear after it the remnant has already settled into equilibrium; if either timescale is wrong, the measured structural changes would not reflect the merger's permanent outcome.

Editorial extensions

If this is right

Remnant disk survival in the simulation is predictable from the orbital angle: tangential, spiral-in accretion preserves disks, radial head-on accretion destroys them, with correlation $R=0.66$ for major mergers.
Because the hot inner stellar halo fraction rises after nearly every merger, it accumulates accreted mass across a galaxy's whole history, making it a more dependable indicator of ex situ stellar mass than bulge fraction.
The $z=0$ correlation between inner-halo fraction and ex situ stellar mass emerges from the averaging of many individual mergers, each of which scatters widely around the relation.
Spiral-in mergers can produce red but HI-rich galaxies by depositing HI at large radius with high angular momentum while the stellar body remains quenched.
Disk and bulge fractions should not be used as simple merger counters, since a merger can raise or lower them depending on orbit.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

Beyond the paper: if orbit is this influential, semi-analytic and zoom-in merger models that track only mass ratio and gas fraction will mispredict remnant structure; adding the three angles as inputs would sharpen morphology predictions without new physics.
Beyond the paper: the angle–structure relations could be tested observationally by classifying mergers as prograde versus retrograde from stellar kinematics and comparing the resulting halo and disk properties in the local universe.
Beyond the paper: the 93% hot-halo increase suggests that measuring the hot inner stellar halo of a galaxy could serve as a practical single-number proxy for accumulated merger mass, even when the full merger tree is unavailable.
Beyond the paper: a direct check would be to rerun a subset of the same mergers at higher time resolution or in a different simulation code; if the orbit–structure ordering survives, the result is robust to numerical details.

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Editorial analysis

A structured set of objections, weighed in public.

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

Desk Editor's Note

read the letter

Best read as a systematic phenomenological study, not a proof of causal mechanism. The authors map three orbital angles to four dynamically-decomposed components across 531 merger pairs in a public simulation. That alone is a useful step beyond disk-survival studies like Zeng et al. and Sotillo-Ramos et al. The main result—spiral-in orbits correlate with higher disk/warm fractions and lower bulge/halo fractions—is consistent with earlier work, and the correlations are moderate but real. I like that they checked the disk definition systematically and that the manual exclusions are documented.

The soft spot is the one flagged in the stress test: the headline correlations are computed against f_+1Gyr, not against the change Δf = f_+1Gyr − f_main,−1Gyr. The paper defines Δf and even colors points by it, but the running medians and Pearson coefficients use the post-merger value. Since f_+1Gyr carries memory of the progenitor's structure, and since orbit correlates with main-disk orientation (θsum averages 116° for radial vs 81° for spiral-in), some of the correlation could be inherited. A partial correlation controlling for f_disk,−1Gyr and mass ratio would settle this. It's a fixable weakness, not a fatal one—the Δf trends in the top of Fig. 9 are suggestive, but they're not quantified against angle.

Minor concerns: no error bars on correlation coefficients, the visual exclusion of merger pairs is subjective (though documented), and the RR galaxy discussion rests on two simulated analogues, which is explicitly speculative. The inner halo–ex situ correlation is already known from Zhu et al. (2022a), so that part is confirmatory, not new. Self-citation is not the issue here.

Overall, a useful and well-organized paper that deserves referee time. I'd ask for the Δf-based correlations and partial controls before accepting; the central direction of the effect is likely right, but the causal language in Section 5 should be toned down until then.

Referee Report

3 major / 5 minor

Summary. The manuscript analyzes 531 merger pairs from IllustrisTNG-100, restricting to mergers with mass ratio above 1/5 and main progenitors with disk fraction above 0.1. It decomposes galaxies into disk, warm, bulge, and hot inner stellar halo components using circularity and radius cuts, defines three merger orbital angles (θr,v, θLmain,Lorbit, θLsat,Lorbit), and reports correlations between these angles and the structural fractions of the remnant measured 1 Gyr after the merger. The main claims are that spiral-in/tangential mergers produce remnants with higher disk and warm fractions and lower bulge and hot inner halo fractions, while head-on/radial mergers do the opposite; that the hot inner halo fraction increases in nearly all mergers; and that this accumulated behavior explains the z=0 correlation between inner halo fraction and total ex situ stellar mass. The paper also proposes spiral-in mergers as a possible formation channel for red but HI-rich galaxies, based on two simulated counterparts.

Significance. If the causal interpretation is upheld, the paper would establish orbital configuration, not only mass ratio or gas fraction, as a first-order determinant of merger-driven structural transformation, and would connect single-merger orbital properties to the scatter in structural indicators of ex situ mass at z=0. The work is based on a public simulation, uses a clearly defined sample and explicitly defines merger-induced changes Δf* even though the headline correlations are not computed on those changes. The proposed RR-galaxy channel is interesting but rests on two objects and is appropriately framed as a possibility. The main weakness is that the causal reading of the central correlations is underdetermined without controlling for progenitor structure, so the significance is conditional on additional analysis.

major comments (3)

[Section 3.2, Figs. 6 and 8] The central causal claim—that orbital configuration reshapes remnant structure—is based on correlations between the orbital angle ⟨θr,v⟩ and the post-merger fractions f*+1Gyr (R = 0.66, 0.66, −0.36, −0.30 for major mergers), not on correlations with the merger-induced change Δf* = f*+1Gyr − f*main,-1Gyr that is defined in Section 3.2. This distinction matters because Section 3.1 and Summary item 1 show that the orbital configuration is itself correlated with the pre-merger disk orientation: for major mergers, θsum averages 116° on radial orbits versus 81° on spiral-in orbits. If high-disk-fraction progenitors preferentially end up on spiral-in orbits, the reported f*+1Gyr–angle relations could simply reflect initial conditions rather than the merger itself. Please add (i) plots and Pearson/Spearman correlations of Δfdisk, Δfwarm, Δfbulge, and Δfhalo versus ⟨θr,v⟩; (ii) partial correlations of f*+1Gyr with ⟨θr,v⟩ controlling for f*,-1Gyr, merger mass ratio, and gas fraction; and (iii) fdisk,-1Gyr versus ⟨θr,v⟩ and θsum. Without these, the abstract's 'lead to higher fractions' is not supported by the presented statistics.
[Section 2.3 and Appendix A] The visual filter that excludes pairs with pre-merger disturbance or post-merger substructure is described only by four representative examples and no count. Because the t+1Gyr snapshot is the operational definition of the settled remnant for all 531 pairs, and because the likelihood of being out of equilibrium could plausibly depend on orbital configuration, this subjective filter can bias all of the reported correlations. Please quantify the filter: report how many pairs were excluded, give the distribution of the three angles for excluded versus included pairs, and test sensitivity to an automated equilibrium criterion such as the substructure mass fraction or the convergence of the component fractions between t+1Gyr and t+2Gyr.
[Section 3.2 and 3.3, Figs. 6 and 8] The Pearson correlation coefficients are reported without uncertainties, p-values, or a statement about whether they are computed on individual points or on the binned running medians. Given the modest values for bulge and halo (R = −0.36 and −0.30) and the highly non-Gaussian angle distributions shown in Fig. 6, the paper should provide bootstrap confidence intervals and Spearman rank coefficients so that the relative strength of the correlations, and in particular the claim that 'the correlations are much weaker in minor mergers', is quantitatively grounded.

minor comments (5)

[Section 2.2] The disk component is defined as λz > 0.8 without a radial restriction, while warm, bulge, and halo are confined to rcut < r < rmax or r < rcut. Please clarify whether the four fractions are normalized to total stellar mass and whether they are intended to partition the galaxy; as written, stellar mass outside rmax with λz > 0.8 is counted only in fdisk, so the four fractions need not sum to unity.
[Abstract and Section 3.3] The abstract quotes 93% for the hot inner stellar halo increase, but Section 3.3 only says 'nearly always increases'; please state where the 93% is obtained or remove the number from the abstract.
[Fig. 9 bottom panels] The claimed strong positive correlation between fhalo,z=0 and the ex situ stellar mass fraction is not quantified; please add a correlation coefficient, a fit, or a scatter estimate so that the strength of the relation can be compared with the scatter shown in the panel.
[Figs. 3 and 5] The captions contain non-English placeholder text ('给Sample exhibition添加Redshift z' and '* theta_r,v调整为被90度折叠后的') that should be removed before submission.
[Section 3.1] Equation 5 and the citation to Cai et al. (2013) appear questionable as the source for the isotropic PDF p(θ) = sin θ; please verify the reference or cite a standard source for the isotropic distribution of 3D angles.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the orbit-structure correlations are direct measurements of independently defined quantities, and the only self-citations are transparent, non-load-bearing method and consistency references.

full rationale

The paper's central claim is an empirical correlation between merger orbital angles (⟨θr,v⟩, ⟨θ′_Lmain,Lorbit⟩, ⟨θ′_Lsat,Lorbit⟩) and dynamically decomposed structural fractions in the remnant. These quantities are measured independently from TNG100 kinematics and merger trees; no parameter is fitted, and no equation defining an angle or a fraction presupposes the target correlation. The only self-citations are to Zhu et al. (2022a), which supplies the four-component decomposition thresholds (λz > 0.8 for disk, etc.) and previously reported the hot-inner-halo/ex-situ-mass relation. Neither is load-bearing here: the decomposition is applied as a fixed, stated algorithm, and the halo–ex-situ correlation is re-derived directly in Fig. 9 rather than assumed from the citation. The skeptical concern that post-merger fractions partly inherit pre-merger progenitor structure is a possible confounding or control issue, not circularity; the paper also defines and plots Δf* = f*,+1Gyr − f*,main@−1Gyr and reports orbit-dependent changes. Manual exclusion of substructured remnants is a selection choice, not a definitional reduction. No step reduces to its own input, so the circularity score is 0.

Assumptions & free parameters 4 free parameters · 4 assumptions · 0 invented entities

The analysis introduces no new physical entities. It uses a new parameter theta_sum (sum of two spin-orbit angles) but that is a derived variable, not an invented entity. The fixed thresholds and timescales are the main free choices, carried over from prior work rather than fitted here.

free parameters (4)

rcut = 3.5 kpc
Radial threshold separating bulge (inside) from warm component and hot inner stellar halo (outside). Fixed from Zhu et al. 2022a; central fractions depend on this choice and sensitivity is not tested.
rmax rule = 2Re for Re > 3.5 kpc, else 7 kpc
Outer boundary for warm component and hot inner halo in the decomposition. Affects mass fractions; no sensitivity test reported.
progenitor/remnant sampling times = 1 Gyr before and after merger
Defines the before and after structures. If galaxies are not unperturbed or settled at these times, the measured changes in component fractions are biased. Manual exclusion partially addresses this.
main progenitor disk fraction cut = fdisk > 0.1
Sample selection criterion that removes dispersion-dominated progenitors. This restricts the sample to disk-bearing galaxies and affects the generality of the conclusions.

assumptions (4)

domain assumption TNG100 simulation faithfully represents galaxy formation physics
The simulation's galaxy formation model is assumed to reproduce real merger physics well enough for the conclusions. Invoked in Section 2.1.
domain assumption Sublink-gal merger trees correctly identify true merger pairs
The merger trees are assumed to correctly identify true merger pairs and their main/satellite progenitors. Invoked in Section 2.1.
domain assumption Instantaneous lambda_z values are valid for orbital decomposition
The paper uses instantaneous 6D phase-space values of lambda_z instead of orbit-averaged values (Section 2.2). This approximation could misclassify some orbits.
domain assumption Subhalo spin direction represents the stellar disk orientation
The spin direction from the TNG catalogue, computed from all particles and cells, is assumed to trace the stellar disk plane. Invoked in Section 2.4.

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Cite this review

Pith. "Pith review of The effects of the orbital configurations of mergers on reshaping galaxy structures." pith.science (2026). https://pith.science/paper/DZ2Z5IUX

@misc{pith2026250702051,
  author       = {Pith},
  title        = {Pith review of: The effects of the orbital configurations of mergers on reshaping galaxy structures},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/DZ2Z5IUX}},
  note         = {Machine review of arXiv:2507.02051}
}

abstract

We performed a systematic analysis of how the orbital configuration of a merger can influence the structural formation of remnant galaxies using 531 merger pairs selected from IllustrisTNG-100. We comprehensively described the merger orbital configuration, considering the relative orbital motion of the merger pair and their disk orientations. We quantified the galaxy structures by dynamically defining four components: bulge, disk, warm component, and hot inner stellar halo. For mergers on spiral-in orbits, the disk planes of the two merging galaxies tend to be aligned with the orbital plane, leading to higher fractions for the disk and warm components, as well as lower fractions for the bulge and hot inner stellar halo components in the remnant galaxy. For mergers on direct collision orbits, the disk planes of the two galaxies tend to be perpendicular to the orbital plane, leading to lower fractions for disk and warm components, as well as higher fractions of the bulge and hot inner stellar halo in the remnant. Mergers can lead to either an increase or decrease in the disk and bulge mass fraction in the remnant compared to the progenitor galaxy, depending on the merger orbital configurations; however, in 93% of cases, mergers cause an increase in the hot inner stellar halo. As a result, the luminosity fraction of the hot inner stellar halo (but not the bulge) in galaxies at $z=0$ is highly correlated with its total ex situ stellar mass. In addition, we find that merger on spiral-in orbits is one of the possible reasons for the formation of recently discovered red but HI-rich (RR) galaxies.

Figures

Figures reproduced from arXiv: 2507.02051 by the authors.

**Figure 1.** Stellar mass evolution of a merger pair. The solid black line and dashed black line represent the mass growth of the main galaxy and satellite galaxy, respectively. The three horizontal lines mark the definition of main galaxy mass (M∗,main), satellite galaxy mass (M∗,sat), and remnant galaxy mass (M∗,+1Gyr). The three gray vertical lines represent the merger time (tmerger), 1 Gyr before (tmerger − 1 Gyr), and 1 Gy… view at source ↗

**Figure 2.** Decomposition of galaxy structure adopted in this paper. We show the main progenitor, the satellite, and the remnant galaxy of a merger pair as the same one shown in [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗

**Figure 3.** Merger mass ratio (M∗,sat/M∗,main) vs. remnant galaxy mass (M∗,+1Gyr) of merger pairs used in this paper. Each dot represents one merger pair and we have 531 merger pairs with a wide range of merger ratios. The color of the dots represent the redshift of the merger time. 2.3. Sample selection We first searched for merger pairs in the merger trees of all central galaxies with M∗ > 1010 M⊙ at z = 0 in TNG100. Then we… view at source ↗

Figures from the paper (6 more)

**Figure 4.** Figure 4: A diagram illustrating the relative position of two galaxies during the merger. The blue plane represents the main galaxy, while the green plane represents the satellite galaxy and the gray plane represents the orbital plane, with corresponding colored vectors indicati…

**Figure 6.** Figure 6: Disk fraction in the remnant galaxy at 1 Gyr after the merger as a function of the three angles. The columns from left to right are ⟨θ ′ Lmain,Lorbit ⟩, ⟨θ ′ Lsat,Lorbit ⟩, and ⟨θr,v⟩. The rows from the second top to the bottom corresponds to mergers with mass ratio of…

**Figure 7.** Figure 7: Disk fraction in the remnant galaxy (fdisk,+1Gyr) as a function of ⟨θ ′ Lmain,Lorbit ⟩, ⟨θ ′ Lsat,Lorbit ⟩, and ⟨θr,v⟩ for the mergers with mass ratio over 1/2. The dots in panels (a) and (b) are colored with fdisk,+1Gyr. The dashed lines in panel (a) denote θsum = 30◦…

**Figure 8.** Figure 8: Mass fraction of different orbital components in the remnant galaxy at 1 Gyr after the merger as a function of ⟨θr,v⟩. Columns from left to right: Disk, warm component, bulge, and hot inner stellar halo. Rows from top to bottom: Mergers with mass ratios of 1/2-2, 1/3-1…

**Figure 9.** Figure 9: Top: Change in the mass fractions of disk, warm, bulge, and hot inner stellar halo vs. M∗,sat/M∗,+1Gyr for all the merger pairs. The colors of the top panels represent ⟨θr,v⟩. The circle, plus, and star symbols denote mass ratios of 1/2 − 2, 1/3 − 1/2, and 1/5 − 1/3, r…

**Figure 10.** Figure 10: RHI/R∗ vs. MHI/M∗ in the remnant galaxies at z=0 for our sample, colored by ⟨θr,v⟩(left) and specific star formation rate (right). In both panels, two galaxies marked by stars exhibit RR galaxies with high values of MHI/M∗ and RHI/R∗, yet their star formation appears …

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Pith tools

Reviewed August 6, 2026 · model on record in the stance chip above.

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