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Impact of Gaia-Sausage-Enceladus merger revealed by a single cosmic clock: thick-to-thin disc transition, gas disc shrinking and bar formation

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Natsuki Funakoshi, Daisuke Kawata, Jason L. Sanders, Ioana Ciucă, Robert J. J. Grand, Noriyuki Matsunaga, Brian Thorsbro, HanYuan Zhang

Astrophysics of Galaxies · Submitted 21 Oct 2026 · 297 references

Pith's verdict: A careful kinematic recalibration of the Mira period-age relation onto the APOGEE/BINGO scale, but the main conclusion about GSE triggering the bar rests on an unverified proxy and unquantified metallicity systematics.

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arXiv ID2610.30237 (version 1)
Pith ID7CASAOCN, Pith's permanent record of this paper and its review
CitationBrian Thorsbro, Daisuke Kawata, Hanyuan Zhang, Ioana Ciuc\u{a}, Jason L. Sanders, Natsuki Funakoshi, Noriyuki Matsunaga, Robert J. J. Grand. "Impact of Gaia-Sausage-Enceladus merger revealed by a single cosmic clock: thick-to-thin disc transition, gas disc shrinking and bar formation." arXiv:2610.30237 (2026). https://pith.science/paper/7CASAOCN
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The paper

The paper claims

This paper recalibrates Mira variable ages onto the APOGEE red giant scale, showing that the nuclear stellar disc's star-formation rise and peak at 8.5–10.5 Gyr ago overlaps with the final stage of the Gaia-Sausage-Enceladus merger, the…

The reading

The paper recalibrates the Mira variable period–age relation onto the age scale of APOGEE red giants, whose ages are determined from a Bayesian neural network trained on Kepler asteroseismology. By matching the vertical velocity dispersion of disk Mira variables (as a function of pulsation period) to that of APOGEE red giants (as a function of age), the authors place the rise and peak of nuclear stellar disc (NSD) star formation at 8.5–10.5 Gyr ago. This epoch coincides with the final stage of the GSE-induced starburst, the contraction of the star-forming gas disk, and the thick-to-thin disk transition. If NSD star formation traces bar formation—a key assumption the authors explicitly acknowledge—these coincidences suggest that the GSE merger triggered the formation of the Milky Way's bar.

What carries the argument

The key is a two-step calibration chain: (1) fit the Gaia DR3 proper motions of 2394 O-rich disk Mira variables with a period-dependent quasi-isothermal distribution function to obtain the vertical velocity dispersion as a function of period, σ_z,0(P); (2) match this to the age-dependent vertical velocity dispersion of APOGEE red giants, σ_z,0(τ), from Funakoshi et al. (2025), using a tanh functional form τ(P) = τ₀/2 [1 + tanh((330−P)/P_s)]. The best-fit parameters (τ₀ = 17.05 Gyr, P_s = 213.6 days) convert the NSD star-formation rise at P~280 days and peak at P~330 days into ages of 8.5–10.5 Gyr on the APOGEE/BINGO relative age scale.

Extended reading notes

Core claim

The crucial advance is bringing two independent age scales onto a common footing. The bar formation epoch, previously inferred from NSD Mira variables using an isochrone-based age scale (Sanders et al. 2024), and the GSE merger epoch, inferred from APOGEE red giants using the BINGO asteroseismic age scale (Ciucă et al. 2024), could not be directly compared. By recalibrating the Mira period–age relation via kinematic matching of vertical velocity dispersions, the authors show that the NSD star-formation rise and peak correspond to 8.5–10.5 Gyr on the APOGEE/BINGO scale, overlapping the final stage of the Great Galactic Starburst and the epoch of gas disk shrinking. This temporal alignment, combined with the caveat that the NSD–bar connection is assumed, makes a coherent narrative in which the GSE merger perturbed the disk, drove gas inward, and seeded the bar.

Load-bearing premise

The entire chronological argument rests on the unproven premise that the rise in nuclear stellar disc star formation, as traced by Mira variables, directly dates the epoch of bar formation. The paper explicitly states this assumption and notes that the relation between NSD star formation and bar formation is not precisely established.

Editorial extensions

If this is right

  • If NSD star formation indeed traces bar formation, the Milky Way bar formed 8.5–10.5 Gyr ago, making it among the oldest known Galactic structures and placing its origin in the same era as the last major merger.
  • The temporal overlap with the GSE-induced starburst and gas disk contraction suggests a causal chain: gas-rich merger → tidal perturbation → gas inflow → starburst → bar formation, consistent with the Auriga simulation Au 18 (Merrow et al. 2024).
  • The coincidence with the thick-to-thin disk transition implies that bar formation occurred during a fundamental transformation of the Milky Way disk, potentially linking bar dynamics to the shutdown of thick-disk formation.
  • Mira variables prove to be powerful probes of Galactic archaeology, but their period–age relation requires careful cross-calibration; future Gaia data releases and line-of-sight velocity measurements will significantly improve the precision.

Reading between the lines

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

  • The assumption that NSD star formation equals bar formation could be tested by comparing the NSD star-formation history with independent bar-age indicators, such as the ages of stars on the bar’s corotation resonance or in the inner ring. If bar formation predates or postdates the NSD buildup by more than a few Gyr, the scenario would be refuted.
  • A metallicity-dependent Mira period–age relation could shift the inferred ages for the more metal-rich NSD Miras relative to the disk Miras used for calibration. Quantifying this shift is essential before the alignment with the GSE epoch can be regarded as robust.
  • The relative age scale of APOGEE/BINGO may stretch ages older than ~10 Gyr (Anders et al. 2023). If so, the true absolute age of the bar could be younger than 8.5 Gyr, potentially bringing it closer to other estimates (e.g. 3–4 Gyr from Nepal et al. 2024). A cross-calibration with a different absolute age anchor would resolve this tension.
Original abstract

Whether the formation of the Galactic bar was connected to the Gaia-Sausage-Enceladus (GSE) merger, the last significant merger experienced by the Milky Way, remains an open question. The bar formation epoch has been inferred from the star formation history of the nuclear stellar disc (NSD) traced by Mira variables, and the GSE merger epoch from the age-metallicity relation of red giants. However, these epochs rest on different stellar tracers with different age scales and cannot be compared directly. We recalibrate the Mira period-age relation onto the age scale of APOGEE red giants, with ages from a Bayesian neural network trained on Kepler asteroseismic ages. We fit the Gaia DR3 proper motions of 2394 O-rich disc Mira variables with an action-based quasi-isothermal distribution function whose parameters vary with period, and derive the period-age relation by matching their vertical velocity dispersion as a function of period to that of the APOGEE red giants as a function of age in Funakoshi et al. (2025). This places the NSD star-formation rise and peak at 8.5-10.5 Gyr ago on the red giant age scale, overlapping with the final stage of the GSE-induced starburst, thick-to-thin disc transition and the contraction of the star-forming gas disc. If NSD star formation traces bar formation, these coincidences suggest a possible scenario in which the GSE seeded the bar formation, while fresh gas supplied by the GSE and outer disc gas driven inward by its tidal torques, increased the baryonic dominance of the inner Galaxy.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13.0 via TypeSafe API, question bank abstract-p1-20260923, 2026-10-23: 1 request, 5,969 input tokens.

What the abstract itself states, as probabilities: claims first 0.05 · states limitation 0.51 · human participants 0.02 · controlled comparison 0.07 · randomized assignment 0.01 · states quantitative result 0.97 · compares with prior results 0.84 · mentions code or data availability 0.03.

The abstract, sentence by sentence

arXiv listing abstract v1. Results and the main claim are underlined; .

Whether the formation of the Galactic bar was connected to the Gaia-Sausage-Enceladus (GSE) merger, the last significant merger experienced by the Milky Way, remains an open question. The bar formation epoch has been inferred from the star formation history of the nuclear stellar disc (NSD) traced by Mira variables, and the GSE merger epoch from the age-metallicity relation of red giants. However, these epochs rest on different stellar tracers with different age scales and cannot be compared directly. We recalibrate the Mira period-age relation onto the age scale of APOGEE red giants, with ages from a Bayesian neural network trained on Kepler asteroseismic ages. We fit the Gaia DR3 proper motions of 2394 O-rich disc Mira variables with an action-based quasi-isothermal distribution function whose parameters vary with period, and derive the period-age relation by matching their vertical velocity dispersion as a function of period to that of the APOGEE red giants as a function of age in Funakoshi et al. (2025). This places the NSD star-formation rise and peak at 8.5-10.5 Gyr ago on the red giant age scale, overlapping with the final stage of the GSE-induced starburst, thick-to-thin disc transition and the contraction of the star-forming gas disc. If NSD star formation traces bar formation, these coincidences suggest a possible scenario in which the GSE seeded the bar formation, while fresh gas supplied by the GSE and outer disc gas driven inward by its tidal torques, increased the baryonic dominance of the inner Galaxy.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#sentencerolethe authors' own workstates the main conclusionstrength
s0 Whether the formation of the Galactic bar was connected to the Gaia-Sausage-Enceladus (GSE) merger, the last significant merger experienced by the Milky Way, remains an open question. gap 0.82 0.03 0.03 unclear 0.57
s1 The bar formation epoch has been inferred from the star formation history of the nuclear stellar disc (NSD) traced by Mira variables, and the GSE merger epoch from the age-metallicity relation of red giants. background 0.99 0.10 0.03 asserted 0.98
s2 However, these epochs rest on different stellar tracers with different age scales and cannot be compared directly. gap 0.93 0.09 0.02 asserted 1.00
s3 We recalibrate the Mira period-age relation onto the age scale of APOGEE red giants, with ages from a Bayesian neural network trained on Kepler asteroseismic ages. method 0.87 0.95 0.03 asserted 1.00
s4 We fit the Gaia DR3 proper motions of 2394 O-rich disc Mira variables with an action-based quasi-isothermal distribution function whose parameters vary with period, and derive the period-age relation by matching their vertical velocity dispersion as a function of period to that of the APOGEE red giants as a function of age in Funakoshi et al. (2025). method 1.00 0.94 0.03 asserted 1.00
s5 This places the NSD star-formation rise and peak at 8.5-10.5 Gyr ago on the red giant age scale, overlapping with the final stage of the GSE-induced starburst, thick-to-thin disc transition and the contraction of the star-forming gas disc. result 1.00 0.88 0.60 asserted 0.99
s6 If NSD star formation traces bar formation, these coincidences suggest a possible scenario in which the GSE seeded the bar formation, while fresh gas supplied by the GSE and outer disc gas driven inward by its tidal torques, increased the baryonic dominance of the inner Galaxy. interpretation 1.00 0.23 0.73 hedged possibility 1.00

What kind of paper the classifier reports this is: empirical study 0.88 · evidence from analysis of existing data 1.00.

What was examined, and what was not

sources read
  • arXiv listing abstract v1
not examined
  • full text
  • figures and tables
  • references and citing sentences
  • relations to other papers
levels run
  • P abstract
not run
  • S sections
  • T body sentences
  • C citations
  • R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

Figures
Figure 1. Spatial distribution of the Mira variable stars used in our study. The upper panel shows the face-on view of the stellar density distribution. The lower panel presents the distribution in Galactocentric radius and verti￾cal height. In both panels, colour denotes stellar pulsation period. The po￾sition of the Sun is indicated by a black cross in both panels, located at (𝑋, 𝑌)=(−8.275,0) kpc in the upper panel and (𝑅, 𝑧)= (8.275,0.0208) kpc in the lower panel. MNRAS 000, 1–21 (2026)
Figure 2. Posterior distributions of spline models describing the period dependence of the quasi-isothermal distribution-function parameters for Mira variables. Panels (a), (b), (c), (d) and (e) show the distribution-function parameters 𝑅disc, 𝜎𝑅,0, 𝜎𝑧,0, 𝑅𝜎,𝑅 and 𝑅𝜎,𝑧 , respectively, while panel (f) shows the stellar period distribution. The blue solid lines indicate the median of the fitting results, while the shaded blue regions represent the 1𝜎 confidence intervals. The grey dotted lines show the parameters used as proposal distribution function, 𝑓 ′ 𝑃 . Crosses denote the position of…
Figure 3. Corner plot of the posterior distributions of the period–age rela￾tion parameters, 𝜏0 and 𝑃𝑠. The diagonal panels show the one-dimensional posterior distributions with dashed vertical lines showing the 16th, 50th and 84th percentiles, while the bottom left panel shows the joint posterior prob￾ability distributions of the two parameters. Contours enclose 39.3%, 86.5% and 98.9% of the posterior probability, corresponding approximately to 1𝜎, 2𝜎 and 3𝜎 regions for a two-dimensional Gaussian distribution. The labels above the diagonal panels indicate the median values with the 16th and…
Figure 4. Comparison between the red giants’ and Miras’ age–velocity-dispersion relations after applying the inferred period–age relation to the Miras’ period– velocity-dispersion relations. The left panel shows 𝜎𝑅,0, and the right panel shows 𝜎𝑧,0 as a function of the age. The black curves show the median APOGEE red giant relations from Funakoshi et al. (2025), and the grey shaded regions indicate their 16th-84th percentile ranges. The coloured curves show 100 posterior realisations of the Mira velocity-dispersion relations as a function of age. Each curve combines a posterior spline draw…
Figure 5. Inferred Miras’ period–age relation. The black curve shows the posterior median relation, and the grey shaded region indicates the 16th-84th percentile range. The vertical dashed lines mark the reference periods of bar formation, 𝑃 = 280 and 330 days, and the red hatched region shows their corresponding age range 8.5 < 𝜏 < 10.5 Gyr under our inferred period– age relation. The blue curve and shaded region show the period-age relation reported by Zhang & Sanders (2023), together with its 1𝜎 uncertainty. This relation was calibrated using isochrone ages in Yu & Liu (2018) and is…
Figure 6. A comparison of the bar formation epoch inferred in this work and the age-dependent spline posterior distributions of the best-fit disc distribution function model parameters of the APOGEE red giants obtained in Funakoshi et al. (2025). Panels (a), (b), (c), (d) and (e) show the age-dependent distribution￾function parameters 𝑅disc, 𝜎𝑅,0, 𝜎𝑧,0, 𝑅𝜎,𝑅 and 𝑅𝜎,𝑧 , respectively. The blue solid lines indicate the median of the fitting results, while the shaded blue regions represent the 1𝜎 confidence intervals. Crosses denote the position of knots for the fitted spline function. Panel (f)…
Figure 7. The age–metallicity relation of the APOGEE red giants described in Section 2.1, which were used in Funakoshi et al. (2025), coloured by [Mg/Fe]. The region shaded in grey shows the same age range shown in
Figure 8. Distribution of the APOGEE red giants in the [Fe/H]-[Mg/Fe] plane, using the sample described in Section 2.1. The background hexbin map shows the stellar number density, with the greyscale indicating the base-10 logarithm of the number of stars in each bin. The contours show the approximate 1𝜎 regions of stars in different age bins, with the mean age indicated by the vertical colour bar. The dashed thick contours highlight the onset of GGS (𝜏 ∼ 12 Gyr) and the end of GGS which is at a similar epoch to the gas disc shrinking (𝜏 ∼ 10 Gyr). The solid thick contours show the earliest (𝜏…

Figures reproduced from arXiv:2610.30237 by the authors.

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