Real substructure or artefact? Untested anomalies in the LAMOST–Gaia chemo-kinematic decomposition of the Milky Way's disc and halo: the three-peaked $|Z|_{\max}$ distribution of Gaia-Enceladus/Sausage and the $[\alpha/\mathrm{Fe}]$ scatter of metal-poor thin-disc stars

A 2024 study of the Milky Way's thin disc, thick disc, and halo substructures using about five million LAMOST spectroscopic targets cross-matched with Gaia astrometry (Deepak 2024, J. Astrophys. Astr. 45, 25) reported two anomalous signals it explicitly left unresolved: (i) the kinematically selected Gaia-Enceladus/Sausage (GE/S) sample shows three peaks in the distribution of maximum orbital height above the Galactic plane, $|Z|_{\max}\approx1.5$, $6.5$, and $14$ kpc, of unknown reality; and (ii) kinematically selected metal-poor thin-disc stars show a large scatter in $[\alpha/\mathrm{Fe}]$ whose origin — poorly mixed star-forming gas, bulge or thick-disc contamination, or measurement uncertainties in low-resolution spectra — is undetermined. The same analysis also noted, as a limitation of its purely kinematic selection rather than as an explicit open question, residual contamination between overlapping substructures, most notably the in-situ Splash and low-energy GE/S. Independent works report internal GE/S substructure (apocentric chevrons; two to four chemically distinct components), operational probabilistic chemical classifiers, and measured intrinsic abundance scatter at low metallicity. However, neither of the two specific questions has been directly answered. Each question therefore survives as an open core, unified by one objective: deciding whether these signals are real astrophysical substructure or artefacts of selection, contamination, and measurement.

Background

The Milky Way's stellar component is organized into a cold, thin, rotating disc; a hotter, thicker, older disc; and a diffuse stellar halo built largely from accreted dwarf galaxies. Modern maps of this structure come from combining Gaia astrometry (precise positions, distances, and space velocities) with large ground-based spectroscopic surveys such as LAMOST (the Large Sky Area Multi-Object Fiber Spectroscopic Telescope in China), which supplies line-of-sight velocities, metallicities $[\mathrm{Fe/H}]$ (iron abundance relative to the Sun), and coarse $[\alpha/\mathrm{Fe}]$ (abundance of alpha-process elements such as Mg, Si, Ca, Ti relative to iron). Because core-collapse (Type II) supernovae produce alpha elements quickly while Type Ia supernovae dominate iron production after a delay, $[\alpha/\mathrm{Fe}]$ acts as a chronometer of chemical enrichment, and the alpha-rich thick disc is chemically distinct from the alpha-poor thin disc. Stars are commonly classified by integrals of motion — orbital energy $E$ and angular momentum $L_z$ — and by orbital quantities such as $|Z|_{\max}$, the maximum height a star reaches above the Galactic plane. In the $E$–$L_z$ plane the halo reveals accreted substructures, most prominently Gaia-Enceladus/Sausage (GE/S), the radially anisotropic debris of the Milky Way's last major merger about 10 Gyr ago (Helmi et al. 2018), and the Splash, a prograde, high-eccentricity population of in-situ disc stars dynamically heated by that same merger (Belokurov et al. 2020), alongside smaller retrograde structures (Thamnos, Sequoia, the Helmi streams). 'Chemical tagging' is the program of identifying each star's birth environment from its detailed multi-element abundance pattern, including neutron-capture elements synthesized in slower channels. Deepak (2024) performed a purely kinematic selection of these populations in LAMOST DR5 plus Gaia, deriving ages, metallicities, and $[\alpha/\mathrm{Fe}]$ for the discs and six halo substructures, and concluded that GE/S, Sequoia, and the Helmi stream had chemical histories distinct from the Milky Way's own thick disc. Within that analysis, two observations were explicitly flagged as requiring further study: the three peaks of increasing width (about 1.5, 6.5, and 14 kpc) in the GE/S $|Z|_{\max}$ distribution, for which the author asked whether they reflect real kinematic substructures within the GE/S sample; and the largest $[\alpha/\mathrm{Fe}]$ scatter in the whole sample among kinematically selected thin-disc stars at low $[\mathrm{Fe/H}]$, which the author read as indicating poorly mixed gas at the time these stars formed, while allowing that some of them may instead have originated in the Galactic bulge. The author requested further studies of light and neutron-capture elements to find the contributions of these two causes to that scatter, and noted in closing that LAMOST's low resolution ($R\approx1800$) is not adequate to resolve the small compositional differences present in these stellar populations, motivating follow-up with high-resolution spectroscopic surveys. The analysis also noted, descriptively, that GE/S overlaps with the Splash in the $E$–$L_z$ plane at lower energies (below the adopted GE/S cut) — a selection limitation relevant to the contamination side of both questions. Adjacent results frame why these matter: near-radial mergers are expected to pile GE/S debris up at discrete apocenters (producing chevron-like phase-space structures), contamination and selection functions can imprint spurious peaks, and genuinely unmixed early interstellar gas would leave a measurable intrinsic abundance-ratio scatter in the oldest disc stars.

Problem Statement

For the chemo-kinematic decomposition of the Milky Way's disc and halo based on LAMOST and Gaia data, determine — with quantitative attribution — whether two reported anomalous signals are real astrophysical substructure or artefacts of selection, contamination, and measurement:

(1) The three peaks at $|Z|_{\max}\approx1.5$, $6.5$, and $14$ kpc (with widths increasing with peak height) in the $|Z|_{\max}$ distribution of the kinematically selected Gaia-Enceladus/Sausage sample: establish whether each peak reflects real kinematic substructure within the GE/S debris (and if so, its dynamical origin, e.g. apocentric pile-ups or distinct merger episodes) or arises from selection and analysis artefacts such as the LAMOST survey footprint and distance selection, disc-star contamination at low $|Z|_{\max}$, or orbital-integration systematics.

(2) The large scatter in $[\alpha/\mathrm{Fe}]$ among kinematically selected metal-poor thin-disc stars: partition it into (a) intrinsic scatter from poorly mixed star-forming gas at early times, (b) contamination from thick-disc, bulge, or prograde-halo stars entering the kinematic thin-disc selection, and (c) measurement uncertainties.

The attribution for each of the two signals bears on the inferred formation and merger history of the Galactic disc and halo (for example, whether GE/S is a single or composite merger and whether the oldest thin disc retains a record of an unmixed interstellar medium).

References

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