ShangGu-1: Ancient Metal-Poor Galactic Substructure

• ShangGu-1 is a chemo-dynamically defined cluster of 24 metal-poor stars identified using LAMOST DR9, Gaia DR3, and StarHorse distances.
• It exhibits a unique positive correlation between metallicity and orbital eccentricity, suggesting internal chemical enrichment before disruption.
• Its distinct dynamical signature and estimated age of over 12 Gyr imply it is the remnant of an early accretion event from a low-mass dwarf galaxy.

ShangGu-1 is a chemo-dynamically identified stellar substructure in the Galactic metal-poor disc, introduced in Hou et al. as part of a broader reconstruction of the formation history of metal-poor stars in the Milky Way (Hou et al., 21 Jul 2025). Within that study, ShangGu-1 corresponds to “Cluster 4,” a compact group of 24 stars selected from a LAMOST DR9 metal-poor sample cross-matched with Gaia DR3 astrometry and StarHorse distances. It is characterized by low metallicity, strong prograde motion, unusually large vertical velocities, and a reported positive correlation between $[{\rm Fe/H}]$ and orbital eccentricity. These properties distinguish it from the primordial disc, Gaia-Sausage-Enceladus (GSE), Wukong, heated disc populations, and the metal-poor tail of GSE, leading to the interpretation that it may be the remnant of a previously unrecognized early accretion event.

1. Identification within the metal-poor disc sample

ShangGu-1 was identified in a parent sample drawn from the LAMOST DR9 metal-poor catalog with $[{\rm Fe/H}] < -1.0$, supplemented by Gaia DR3 astrometry and StarHorse distances. The phase-space selection applied quality cuts requiring distance error $< 20\%$, Gaia RUWE $< 1.4$, $J_r, J_z > 0$, and relative errors $E_{\rm err}/E$, $J_{\phi,{\rm err}}/J_\phi$, $J_{r,{\rm err}}/J_r$, and $J_{z,{\rm err}}/J_z < 0.3$. The resulting sample contains 46,178 stars with full 6D phase-space information and multi-element abundances; 2,517 of these are subgiants with precise ages (Hou et al., 21 Jul 2025).

Dynamical tagging was performed with HDBSCAN in $(E, J_\phi, J_r, J_z)$ space using min_cluster_size = 15, min_samples = 6, the Euclidean metric, and “leaf” cluster selection. This procedure yielded 222 Dynamically Tagged Groups (DTGs), of which 194 were associated with known structures and 28 remained unassigned. The unassigned DTGs were then grouped by eye into six higher-level “clusters.” ShangGu-1 is the designation assigned to Cluster 4, represented by hexagons in Figures 4 and 14 of the source study.

The naming of ShangGu-1 follows directly from the claim that its kinematics and metallicities do not match any known structure. In that sense, its definition is not purely morphological but comparative: it is the subset of unassigned DTGs whose location in kinematic and action space remains distinct from previously catalogued merger debris or disc-related populations.

2. Observational definition and basic properties

The defining observational parameters of ShangGu-1 are concise enough to summarize directly.

Property	Reported value	Context
Number of stars	24	Cluster 4
Azimuthal velocity $[{\rm Fe/H}] < -1.0$0	$[{\rm Fe/H}] < -1.0$1	Between GSE at $[{\rm Fe/H}] < -1.0$2 and hot disc at $[{\rm Fe/H}] < -1.0$3
Vertical velocity $[{\rm Fe/H}] < -1.0$4	$[{\rm Fe/H}] < -1.0$5	Above cold and hot disc limits of $[{\rm Fe/H}] < -1.0$6–$[{\rm Fe/H}] < -1.0$7
Eccentricity $[{\rm Fe/H}] < -1.0$8	$[{\rm Fe/H}] < -1.0$9–$< 20\%$0	Reported from Fig. 15b
Mean metallicity	$< 20\%$1	$< 20\%$2
Median metallicity	$< 20\%$3	Fig. 15a

The metallicity distribution is predominantly $< 20\%$4, with a tail extending to $< 20\%$5 dex. This places ShangGu-1 among the more metal-poor substructures discussed in the study. Its combination of substantial prograde rotation and very large vertical motion is central to why it is not grouped with GSE-like debris, despite its low metallicity, nor with disc tails, despite its prograde character (Hou et al., 21 Jul 2025).

A useful clarification concerns sign conventions. The study reports $< 20\%$6 while also describing ShangGu-1 as prograde and listing $< 20\%$7. The intended point is the dynamical classification: ShangGu-1 rotates in the prograde sense, but more slowly than the canonical disc populations referenced in the comparison set.

3. Chemical characterization

The principal chemical diagnostic explicitly presented for ShangGu-1 is its $< 20\%$8 distribution. The group has $< 20\%$9, a median of $< 1.4$0, and a low-metallicity tail reaching $< 1.4$1 dex (Hou et al., 21 Jul 2025). No other individual abundance ratios, such as $< 1.4$2 or $< 1.4$3, are singled out specifically for ShangGu-1 in the source study.

The abundance notation used in the study follows the standard logarithmic definition

$< 1.4$4

This provides the formal basis for comparison with other Galactic populations, even when only a subset of elemental ratios is discussed explicitly.

The paper does not show a dedicated $< 1.4$5–$< 1.4$6 sequence for ShangGu-1. It states, however, that because its members lie at $< 1.4$7 and well above the thick-disc knee, they likely have elevated $< 1.4$8–$< 1.4$9, similar to halo debris. This is an interpretation rather than a directly plotted measurement for the substructure itself. A plausible implication is that ShangGu-1 occupies a chemical regime more typical of old accreted or early-formed populations than of the younger low-$J_r, J_z > 0$0 disc.

4. Dynamics, actions, and orbital structure

ShangGu-1 is defined as much by its dynamical compactness as by its metallicity. In velocity space, it lies at $J_r, J_z > 0$1 and $J_r, J_z > 0$2. The latter is especially important: the study emphasizes that these vertical motions lie well above the cold and hot disc limits of $J_r, J_z > 0$3–$J_r, J_z > 0$4, separating ShangGu-1 from ordinary disc-like kinematics (Hou et al., 21 Jul 2025).

In orbital terms, its eccentricities span $J_r, J_z > 0$5–$J_r, J_z > 0$6. The study further reports a striking positive correlation between $J_r, J_z > 0$7 and $J_r, J_z > 0$8: more metal-rich members are on more eccentric orbits. This trend is singled out as unusual and is not reported for the comparison substructures in the same way. Although the paper gives the eccentricity in terms of apocentre and pericentre radii, the main empirical statement is the range and the correlation pattern rather than a new formalism for $J_r, J_z > 0$9.

In action space, ShangGu-1 forms a clear dense knot in $E_{\rm err}/E$0. Its location is described as intermediate in binding energy, between the high-$E_{\rm err}/E$1, low-$E_{\rm err}/E$2 region occupied by GSE and the kinematically hot disc. The reported actions include $E_{\rm err}/E$3, with both $E_{\rm err}/E$4 and $E_{\rm err}/E$5 large. The study computes actions with Agama in the McMillan (2017) Milky Way potential and uses

$E_{\rm err}/E$6

These dynamical properties jointly establish ShangGu-1 as neither a cold disc extension nor a canonical GSE-like radialized halo component. Its phase-space placement is therefore central to its interpretation as a separate accretion relic.

5. Age constraints and formation scenario

The age information for ShangGu-1 is limited but consequential. Only one member falls within the subgiant age subset derived from isochrone fitting following Xiang and Rix, constructed by cross-matching to 2,517 stars with age errors $E_{\rm err}/E$7 Gyr and relative age error $E_{\rm err}/E$8. That star has an asteroseismic/isochrone age of $E_{\rm err}/E$9 Gyr; the study notes that this is likely an overestimate but still interprets it as indicating an age $J_{\phi,{\rm err}}/J_\phi$0 Gyr (Hou et al., 21 Jul 2025).

Because the remaining members lack similarly precise ages, the classification of ShangGu-1 as ancient is based on a combination of the one dated star, its low metallicity, and its high eccentricities. The source study therefore interprets ShangGu-1 as an old structure, with an age plausibly exceeding 12 Gyr.

The proposed origin scenario is explicitly exclusionary before becoming constructive. ShangGu-1 is stated not to be part of the primordial in-situ disc because it is too metal poor. It is also stated not to be debris from GSE or Wukong because its kinematics and $J_{\phi,{\rm err}}/J_\phi$1 differ. The preferred interpretation is that ShangGu-1 is a relic of a previously unrecognized low-mass dwarf galaxy that merged more than 12 Gyr ago. The reported positive $J_{\phi,{\rm err}}/J_\phi$2–$J_{\phi,{\rm err}}/J_\phi$3 correlation could reflect internal chemical enrichment before disruption. This last statement is interpretive rather than directly demonstrated, but it is presented in the source as the most plausible reading of the correlation.

6. Relation to ShangGu-2, ShangGu-3, Nyx, and known debris

The distinctiveness of ShangGu-1 is sharpened by comparison with the other newly identified substructures in the same analysis (Hou et al., 21 Jul 2025).

ShangGu-2, corresponding to Cluster 5, shows prograde disc-like $J_{\phi,{\rm err}}/J_\phi$4 but also $J_{\phi,{\rm err}}/J_\phi$5. Its $J_{\phi,{\rm err}}/J_\phi$6–$J_{\phi,{\rm err}}/J_\phi$7 distribution exhibits two branches like the cold-disc sample, and it is interpreted as heated metal-poor disc stars. ShangGu-3, corresponding to Cluster 6, occupies $J_{\phi,{\rm err}}/J_\phi$8 to $J_{\phi,{\rm err}}/J_\phi$9, has $J_{r,{\rm err}}/J_r$0, and remains at $J_{r,{\rm err}}/J_r$1. Its $J_{r,{\rm err}}/J_r$2–$J_{r,{\rm err}}/J_r$3 and $J_{r,{\rm err}}/J_r$4–$J_{r,{\rm err}}/J_r$5 patterns resemble GSE, leading to the interpretation that it is likely the very metal-poor tail of the GSE progenitor. Nyx and Nyx-2 are treated as subcomponents of ShangGu-3 rather than independent mergers.

Against this background, ShangGu-1 is described as unique in four respects. First, it has much lower $J_{r,{\rm err}}/J_r$6 than the disc tails while retaining strong prograde motion, unlike GSE’s near-zero rotation. Second, its very large $J_{r,{\rm err}}/J_r$7 separates it from both the heated-disc interpretation of ShangGu-2 and the GSE-like tail represented by ShangGu-3. Third, it shows a positive $J_{r,{\rm err}}/J_r$8–$J_{r,{\rm err}}/J_r$9 trend not observed in the other substructures. Fourth, its mean metallicity of about $J_{z,{\rm err}}/J_z < 0.3$0 is lower than ShangGu-2 and ShangGu-3/GSE, which are described as being at approximately $J_{z,{\rm err}}/J_z < 0.3$1 to $J_{z,{\rm err}}/J_z < 0.3$2.

A common misconception would be to treat all prograde metal-poor structures in the Galactic disc region as variants of heated disc stars or GSE-related debris. The source analysis argues against that simplification for ShangGu-1 specifically.

7. Significance and current uncertainties

Within the broader framework of the Galactic metal-poor disc, ShangGu-1 functions as evidence that not all metal-poor stars in disc-crossing or disc-resident orbits can be assigned to the primordial disc, high-$J_{z,{\rm err}}/J_z < 0.3$3 or low-$J_{z,{\rm err}}/J_z < 0.3$4 disc tails, GSE, or Wukong (Hou et al., 21 Jul 2025). Its existence supports the view that the early Milky Way retained the dynamical signatures of multiple accretion events, including at least one component not readily matched to previously named substructures.

At the same time, several aspects remain provisional. The age inference rests directly on only one star with a precise age. The $J_{z,{\rm err}}/J_z < 0.3$5 characterization is stated as likely rather than demonstrated by a dedicated ShangGu-1 abundance sequence. The physical interpretation of the positive $J_{z,{\rm err}}/J_z < 0.3$6–$J_{z,{\rm err}}/J_z < 0.3$7 relation as internal enrichment prior to disruption is plausible but not uniquely established by the reported observables. These caveats do not negate the identification of ShangGu-1 as a coherent chemo-dynamical structure; they delimit how strongly one can specify its progenitor and enrichment history from the present data alone.

In the terminology of the source study, ShangGu-1 is therefore best described as a newly identified, ancient, metal-poor, prograde substructure with unusually large vertical motions and an unusual eccentricity–metallicity relation, currently interpreted as the remnant of an as-yet uncharted early dwarf-galaxy accretion.