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arxiv: 2605.20959 · v1 · pith:H4SBYQTPnew · submitted 2026-05-20 · 🌌 astro-ph.GA

Dawn of the Milky Way disk: Determination of when a rotationally supported disk appears and dating the spin-up of the disk

Sofia Feltzing , Diane Feuillet , Thomas Bensby This is my paper

Pith reviewed 2026-05-21 03:26 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords Milky Way diskstellar agesgalactic archaeologychemical evolutionkinematicshigh-alpha starslow-alpha starssub-giants
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The pith

The Milky Way disk spun up to rotational support around 12 billion years ago.

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

The paper examines the kinematics of a large sample of sub-giant stars to determine when the Milky Way developed a rotationally supported stellar disk. It identifies the metallicity range where mean circular velocity shifts from halo-like to disk-like values and assigns an age to that change using literature ages for the stars. The work also locates the metallicity where ordered rotation begins to dominate random motions and shows that these changes occur rapidly. High-alpha stars carry the transitions while low-alpha stars appear already in a disk-like state with no transition period.

Core claim

Using 319835 sub-giants from LAMOST with precise literature ages, the spin-up occurs for -1.25 < [Fe/H] < -0.9 and is dated to a mean age of 12.1 +/- 2.8 Gyr (median age 12.4 Gyr). The disk becomes rotationally supported for -1.25 < [Fe/H] < -1. The transition is very rapid in age. This supports that the spin-up traces the motion to a rotationally supported disk. These transitions are traced by the high-alpha stars while the low-alpha stars do not spin-up but start directly at approximately the circular velocity seen for the Sun today. The low-alpha disk is rotationally supported with no transition period in [Fe/H] or in age.

What carries the argument

Tracking mean circular velocity and the dominance of ordered over random motions as functions of metallicity [Fe/H] and stellar age in sub-giant stars.

If this is right

  • The high-alpha population exhibits a clear and rapid shift to rotational support, confirming the spin-up traces a genuine change to disky dynamics.
  • The low-alpha stars form already in a rotationally supported configuration with no detectable transition in metallicity or age.
  • The timing of the spin-up at roughly 12 Gyr provides a chronological anchor for when the Milky Way assembled its ordered disk.

Where Pith is reading between the lines

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

  • The sharp transition may help distinguish between different gas accretion or merger scenarios in models of early disk assembly.
  • Comparable kinematic and age analyses in external galaxies could test whether this rapid spin-up timeline is typical.

Load-bearing premise

The literature ages assigned to the sub-giants are sufficiently accurate and unbiased to resolve a transition whose width is only a few Gyr without kinematic or age-dependent selection effects shifting the apparent transition point.

What would settle it

A new sample with independent age estimates for similar stars that shows the velocity transition occurring at a substantially different age or metallicity range.

Figures

Figures reproduced from arXiv: 2605.20959 by Diane Feuillet, Sofia Feltzing, Thomas Bensby.

Figure 1
Figure 1. Figure 1: a) [α/Fe] as a function of [Fe/H] for the full sample, with Gaia-Enceladus stars removed (see Sect. 2). The underlying data are shown as a 2D histogram with power-law relationship∗ . The coloured boxes define the different subsamples analysed in panels b and c. The number of stars in each bin is indicated in. The dashed line shows Eq. (1) which splits the data into high￾and low-α stars and the dashed line … view at source ↗
Figure 2
Figure 2. Figure 2: Determination of the age of the spin-up. The trends are calculated by rolling over a bin including 1000 data-points along the [Fe/H]-axis. a) Full sample. Age – [Fe/H] relation shown in black and Vϕ – [Fe/H] relation in grey. b) High-α sample. Age – [Fe/H] relation shown in red and Vϕ – [Fe/H] relation in brown. The grey shaded area shows the identified [Fe/H] range of the spin-up. c) Comparison of the Age… view at source ↗
Figure 3
Figure 3. Figure 3: Age-metallicity plots exploring the properties of the stars in Xiang et al. (2025). a) Full sample. b) Shows where the stars with ruwe > 1.4 fall in orange. c) Shows the stars with log g < 3.2 in orange. d) Shows the stars with relative error in parallax > 0.2 in orange. e) Shows the sample when it has been trimmed using ruwe, parallax and log g. f) Shows the trimmed sample with stars with a relative error… view at source ↗
Figure 4
Figure 4. Figure 4: a) Cumulative histograms of the stars in the full sample and the trimmed sample (see Sect. 2 and App. A.1) as a function of hight above the Galactic plane (z). The cut at z = 1.5 kpc is marked with a red dashed line. b) 2D histogram of [Fe/H] as a function of z for the trimmed sample, where the colour shows the number of stars. The cut at z = 1.5 kpc is marked with a red dashed line. The white, horizontal … view at source ↗
Figure 5
Figure 5. Figure 5: a) Age – [Fe/H] relation. Comparison of the rolling median trend using 1000 datapoints and used in this paper with median and mean values calculated in bins of size 0.1 dex. Symbols as given in the legend. The errorbars represent the one σ around the mean values. For clarity we only show the errorbars for the high-α sample. b) Age – [Fe/H] relation for the full sample as well as a 2D histogram of the whole… view at source ↗
read the original abstract

Spiral galaxies, like the Milky Way, transform at some point in time into a rotationally supported system. Using an extant data-set consisting of 319 835 sub-giants from LAMOST with precise ages from the literature, we determine, for the first time the age when the Milky Way disk spins up, i.e. when the mean circular velocity changes from halo-like to disk-like. We find in concordance previous studies that the spin-up takes place for -1.25 < [Fe/H] <- 0.9 and we can date this transition to a mean age of 12.1 +/- 2.8 Gyr (median age 12.4 Gyr). We further study when the disk became rotationally supported, i.e. when the ordered, disky motion dominates over the random motions. We find that this happens for $-1.25<$[Fe/H]$<-1$. The transition is very rapid in age. This gives support to that the spin-up seen in this and other works genuinely traces the motion to a rotationally supported disk, which has not previously been shown. These transitions are traced by the high-alpha stars. while the low-alpha stars do not spin-up but start directly at approximately the circular velocity seen for the Sun today. The low-alpha disk is rotationally supported with no transition period in [Fe/H] or in age.

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

3 major / 2 minor

Summary. The manuscript analyzes a sample of 319835 LAMOST sub-giant stars assigned literature ages to identify the metallicity interval and corresponding age at which the Milky Way stellar disk undergoes spin-up to rotationally supported kinematics. It reports that this transition occurs for -1.25 < [Fe/H] < -0.9 at a mean age of 12.1 ± 2.8 Gyr (median 12.4 Gyr), with the disk becoming rotationally supported for -1.25 < [Fe/H] < -1; the transition is described as rapid, traced exclusively by high-α stars, while low-α stars exhibit disk-like rotation from the outset with no transition.

Significance. If the literature ages prove unbiased and sufficiently precise across the narrow transition metallicity range, the work supplies a concrete empirical anchor for the epoch of Milky Way disk formation, reinforcing prior high-α sequence results and offering a falsifiable timeline that can be tested against cosmological simulations of disk assembly.

major comments (3)
  1. [Abstract and §3] Abstract and §3 (data description): the central age of 12.1 ± 2.8 Gyr is derived from literature ages assigned to the 319835 sub-giants, yet no quantitative assessment is provided of how age uncertainties or systematic offsets tied to [Fe/H] or α-enhancement propagate into the reported transition width; this is load-bearing because the claimed rapidity of the spin-up rests on the ages resolving a few-Gyr interval without smearing.
  2. [§4] §4 (results on high-α vs low-α): the claim that only high-α stars trace the spin-up while low-α stars start at solar circular velocity requires explicit checks that kinematic or age-dependent selection functions in the LAMOST sub-giant sample do not differ between the sequences; without such tests the separation of the two populations could be an artifact of the data cuts.
  3. [§2] §2 (sample selection): the manuscript states the sample size and metallicity bounds but does not detail the objective criterion used to define the transition metallicities (-1.25 < [Fe/H] < -0.9 and -1.25 < [Fe/H] < -1) or any robustness tests against binning choices, which directly affects the reported concordance with previous studies.
minor comments (2)
  1. [Abstract] Abstract: inconsistent spacing in metallicity inequalities (e.g., <-0.9 versus <-1) should be standardized for clarity.
  2. [Figures] Figure captions (throughout): labels for high-α and low-α sequences should explicitly reference the [α/Fe] threshold adopted.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for their thorough and constructive report. We address each of the major comments below and have revised the manuscript accordingly to strengthen the analysis.

read point-by-point responses

  1. Referee: [Abstract and §3] Abstract and §3 (data description): the central age of 12.1 ± 2.8 Gyr is derived from literature ages assigned to the 319835 sub-giants, yet no quantitative assessment is provided of how age uncertainties or systematic offsets tied to [Fe/H] or α-enhancement propagate into the reported transition width; this is load-bearing because the claimed rapidity of the spin-up rests on the ages resolving a few-Gyr interval without smearing.

    Authors: We agree that a quantitative assessment of age uncertainties is important for supporting the rapidity of the transition. Although the manuscript references the literature ages and their typical uncertainties, we did not include a propagation analysis. In the revised version, we will add a discussion in §3 with a Monte Carlo resampling of the ages within their uncertainties to demonstrate that the observed transition width is robust and not due to smearing. This will be presented as a new figure showing the velocity distribution before and after perturbation. revision: yes

  2. Referee: [§4] §4 (results on high-α vs low-α): the claim that only high-α stars trace the spin-up while low-α stars start at solar circular velocity requires explicit checks that kinematic or age-dependent selection functions in the LAMOST sub-giant sample do not differ between the sequences; without such tests the separation of the two populations could be an artifact of the data cuts.

    Authors: This is a valid concern. The original manuscript did not include explicit comparisons of selection functions between the high-α and low-α populations. We will revise §4 to include such checks, comparing the distributions of apparent magnitudes, distances, and other selection-related quantities for stars in the transition metallicity range. Preliminary analysis indicates no substantial differences, but we will document this fully in the revision. revision: yes

  3. Referee: [§2] §2 (sample selection): the manuscript states the sample size and metallicity bounds but does not detail the objective criterion used to define the transition metallicities (-1.25 < [Fe/H] < -0.9 and -1.25 < [Fe/H] < -1) or any robustness tests against binning choices, which directly affects the reported concordance with previous studies.

    Authors: The metallicity intervals were selected to align with those in prior studies where the spin-up has been noted, and based on where the mean rotational velocity shows a clear change from halo-like to disk-like values. To address the referee's point, we will add in §2 an objective definition, for example using the metallicity where the velocity dispersion drops below a threshold or where the mean velocity reaches 150 km/s, and perform robustness tests by shifting the bins by 0.1 dex and showing the transition age remains consistent within uncertainties. revision: yes

Circularity Check

0 steps flagged

Observational dating of disk spin-up uses independent literature ages and direct velocity data

full rationale

The paper's central result dates the Milky Way disk spin-up transition by binning 319835 LAMOST sub-giants in [Fe/H], computing mean circular velocities from the survey data, and reading off the mean literature age at the metallicity where velocities shift from halo-like to disk-like. No equation or fit defines the reported transition age (12.1 ± 2.8 Gyr) as a function of the velocity measurements themselves, nor does any step rename a fitted parameter as a prediction. The ages are external literature values and the kinematics are measured directly; the derivation therefore remains self-contained against external benchmarks with no reduction by construction.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The central claims rest on the accuracy of pre-existing age estimates for sub-giants and on the assumption that the LAMOST sample selection does not introduce systematic biases in age or kinematics at the transition metallicities. No new free parameters are introduced beyond the data-driven identification of the metallicity intervals; no new physical entities are postulated.

axioms (2)
  • domain assumption Literature ages for LAMOST sub-giants are accurate to within a few Gyr and free of large systematic offsets that would shift the apparent transition age.
    The reported mean age of 12.1 Gyr and its uncertainty directly inherit from these external age determinations.
  • domain assumption The selected sub-giant sample is representative of the underlying stellar population across the metallicity range -1.25 to -0.9 without kinematic or age-dependent selection biases.
    Any bias here would move the location or sharpness of the reported spin-up and rotational-support transitions.

pith-pipeline@v0.9.0 · 5794 in / 1866 out tokens · 83170 ms · 2026-05-21T03:26:28.229526+00:00 · methodology

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