ShangGu-3: Ancient Chemo-Dynamical Disc Substructure

• ShangGu-3 is a chemo-dynamical substructure in the metal-poor disc, identified through HDBSCAN clustering of ~46,000 stars with [Fe/H] < -1.0.
• It features an ancient, chemically coherent stellar population with a narrow metallicity peak around [Fe/H] ≈ -2.0 and enhanced α-abundances indicative of rapid early enrichment.
• Its fourfold orbital subdivision, including sectors aligned with Nyx and Nyx-2, supports an accretion origin from a disrupted dwarf galaxy progenitor undergoing phase wrapping.

Searching arXiv for the primary paper and closely related work on Nyx, Nyx-2, and merger-origin interpretations. arXiv_search(query="(Hou et al., 21 Jul 2025) ShangGu-3 Nyx Nyx-2 Galactic metal-poor disc", max_results=10, sort_by="relevance") Searching specifically for Nyx-related papers and dwarf-disk merger simulations mentioned in the source material. arXiv_search(query="Nyx stream Necib 2020 Horta 2023 dwarf disk mergers Koppelman Amarante arXiv", max_results=10, sort_by="relevance") Retrieving the primary paper directly and a small set of supporting arXiv records. arXiv_search(query="id:(Hou et al., 21 Jul 2025)", max_results=5, sort_by="relevance") ShangGu-3 is a chemo-dynamical substructure identified within the Galactic metal-poor disc by Hou et al. in their study of metal-poor stars from the LAMOST Survey (Hou et al., 21 Jul 2025). It was isolated among stars with [Fe/H] $< -1.0$ through clustering in integral-of-motion space and is characterized by disc-like azimuthal motion, very large radial excursions, a metallicity distribution concentrated near [Fe/H] $\sim -2.0$, and an ancient, $\alpha$-enhanced stellar population. In the authors’ analysis, ShangGu-3 is not a single stream in the narrow sense, but a four-part orbital pattern, two sectors of which coincide with the previously known Nyx and Nyx-2 structures.

1. Discovery and identification

ShangGu-3 emerged from HDBSCAN clustering in the four-dimensional space of integrals of motion $(E,\;J_\phi,\;J_R,\;J_z)$ applied to $\sim 46{,}000$ stars with [Fe/H] $< -1.0$ drawn from the LAMOST metal-poor catalog (Hou et al., 21 Jul 2025). Among the 28 Dynamically Tagged Groups not matched to any known structure, the object initially labeled “Cluster 6” stood out because it combined disc-like azimuthal motion with very large radial excursions.

Hou et al. gave an explicit kinematic selection for extracting ShangGu-3 directly from the parent LAMOST+Gaia sample. In the Galactic rest frame, the recommended cuts are

$$-240\;{\rm km\,s^{-1}} < V_\phi < -130\;{\rm km\,s^{-1}},$$

$$150\;{\rm km\,s^{-1}} < |V_r| < 240\;{\rm km\,s^{-1}}, \qquad |V_z| < 100\;{\rm km\,s^{-1}},$$

$$|z| < 3\;{\rm kpc}, \qquad E > -1.41\times10^5\;{\rm km^2\,s^{-2}}.$$

These criteria yield 106 ShangGu-3 candidates. All satisfy the global metal-poor selection [Fe/H] $< -1.0$, but the population is concentrated at substantially lower metallicity in practice. This placement in a metal-poor, disc-crossing region of phase space is central to the object’s interpretation: it is dynamically disc-associated in the solar neighborhood, yet chemically and orbitally atypical of canonical in-situ disc populations.

2. Chemical abundance pattern and age distribution

The 106 kinematically selected ShangGu-3 stars have [Fe/H] values ranging from $\sim -2.0$0 to $\sim -2.0$1, with mean $\sim -2.0$2 and median $\sim -2.0$3 (Hou et al., 21 Jul 2025). Their normalized metallicity distribution peaks at $\sim -2.0$4, distinctly below that of the local metal-poor disc. This metallicity regime places ShangGu-3 among the more metal-poor disc-crossing stellar structures identified in the study.

In abundance space, ShangGu-3 follows a single high-$\sim -2.0$5 sequence in [Mg/Fe] versus [Fe/H], with an analogous trend in [Ti/Fe]. Hou et al. report a mild $\sim -2.0$6-knee at [Fe/H] $\sim -2.0$7. They further note that the trend is qualitatively similar to the one seen in Gaia-Sausage-Enceladus at low metallicity, but with somewhat smaller [Mg/Fe] dispersion, $\sim -2.0$8 dex. The combination of low [Fe/H] and enhanced [Mg/Fe] indicates rapid early enrichment dominated by core-collapse supernova yields, rather than the lower-$\sim -2.0$9 chemistry characteristic of more extended star-formation histories.

Age information is available for five of the 106 candidates that fall within the LAMOST–Xiang et al. subgiant sample with precise isochrone ages. Their ages are 12.4, 12.8, 13.1, 13.3, and 13.8 Gyr, giving $\alpha$0 Gyr and $\alpha$1 Gyr. Hou et al. therefore describe ShangGu-3 as unambiguously ancient, coeval with or even older than Gaia-Sausage-Enceladus. The small age spread is an important part of the later argument against a heated in-situ origin.

3. Orbital architecture and internal subdivision

Inspection of the three-dimensional velocity distribution $\alpha$2 shows that ShangGu-3 separates into four coherent subgroups, each associated with a quadrant in the $\alpha$3 plane after rotation into the Local Standard of Rest frame (Hou et al., 21 Jul 2025). Hou et al. summarize these groups with eccentricity, inclination, and orbital turning points.

Group	$\alpha$4 sign	$\alpha$5; $\alpha$6; $\alpha$7
A	$\alpha$8	$\alpha$9; $(E,\;J_\phi,\;J_R,\;J_z)$0; $(E,\;J_\phi,\;J_R,\;J_z)$1 kpc
B	$(E,\;J_\phi,\;J_R,\;J_z)$2	$(E,\;J_\phi,\;J_R,\;J_z)$3; $(E,\;J_\phi,\;J_R,\;J_z)$4; $(E,\;J_\phi,\;J_R,\;J_z)$5 kpc
C	$(E,\;J_\phi,\;J_R,\;J_z)$6	$(E,\;J_\phi,\;J_R,\;J_z)$7; $(E,\;J_\phi,\;J_R,\;J_z)$8; $(E,\;J_\phi,\;J_R,\;J_z)$9 kpc
D	$\sim 46{,}000$0	$\sim 46{,}000$1; $\sim 46{,}000$2; $\sim 46{,}000$3 kpc

The eccentricity is defined as

$\sim 46{,}000$4

and the inclination as

$\sim 46{,}000$5

where $\sim 46{,}000$6 and $\sim 46{,}000$7 are the azimuthal and total angular momentum. The apocenter $\sim 46{,}000$8 and pericenter $\sim 46{,}000$9 are medians from Monte Carlo realizations, and the quoted dispersions are half the 16th–84th percentile ranges.

All four subgroups share very high eccentricities, $< -1.0$0–$< -1.0$1, large inclinations of $< -1.0$2–$< -1.0$3, and radial excursions with $< -1.0$4 kpc and $< -1.0$5 kpc. In the authors’ description of orbital direction, Groups A and D are retrograde in $< -1.0$6, whereas Groups B and C are prograde. The key structural result is therefore not merely the existence of one kinematic clump, but a fourfold orbital organization with closely matched chemo-dynamical properties.

4. Relation to Nyx and Nyx-2

Hou et al. identify two of the four ShangGu-3 sectors with the previously known “disk-in-plane” streams Nyx and Nyx-2 (Hou et al., 21 Jul 2025). Nyx aligns with Group A, defined by $< -1.0$7 and $< -1.0$8, and shares Nyx’s characteristic $< -1.0$9 and $$-240\;{\rm km\,s^{-1}} < V_\phi < -130\;{\rm km\,s^{-1}},$$0. Nyx-2 aligns with Group B, defined by $$-240\;{\rm km\,s^{-1}} < V_\phi < -130\;{\rm km\,s^{-1}},$$1 and $$-240\;{\rm km\,s^{-1}} < V_\phi < -130\;{\rm km\,s^{-1}},$$2, with $$-240\;{\rm km\,s^{-1}} < V_\phi < -130\;{\rm km\,s^{-1}},$$3 and $$-240\;{\rm km\,s^{-1}} < V_\phi < -130\;{\rm km\,s^{-1}},$$4.

In both cases, the aligned ShangGu-3 subgroups have the same narrow [Fe/H] distribution, with peak $$-240\;{\rm km\,s^{-1}} < V_\phi < -130\;{\rm km\,s^{-1}},$$5 dex and $$-240\;{\rm km\,s^{-1}} < V_\phi < -130\;{\rm km\,s^{-1}},$$6 dex, together with enhanced [Mg/Fe] $$-240\;{\rm km\,s^{-1}} < V_\phi < -130\;{\rm km\,s^{-1}},$$7 dex. The remaining two subgroups, C and D, have similar chemistry—[Fe/H] $$-240\;{\rm km\,s^{-1}} < V_\phi < -130\;{\rm km\,s^{-1}},$$8 and [Mg/Fe] $$-240\;{\rm km\,s^{-1}} < V_\phi < -130\;{\rm km\,s^{-1}},$$9—but differ in orbital inclination and in the sign of $$150\;{\rm km\,s^{-1}} < |V_r| < 240\;{\rm km\,s^{-1}}, \qquad |V_z| < 100\;{\rm km\,s^{-1}},$$0.

This organization reframes Nyx and Nyx-2 within a larger dynamical context. Rather than representing two isolated streams with unrelated origins, they appear as two sectors of a broader fourfold kinematic pattern. A common misconception is therefore to treat Nyx and Nyx-2 as the complete phenomenon; Hou et al. instead place them inside the more inclusive structure they designate ShangGu-3.

5. Origin and formation scenario

Hou et al. interpret ShangGu-3 as the remnant of a single accretion event whose progenitor was on a prograde orbit and was disrupted early in the Milky Way’s history (Hou et al., 21 Jul 2025). The basis for this interpretation is the conjunction of several properties: uniformly ancient ages, metallicity centered near [Fe/H] $$150\;{\rm km\,s^{-1}} < |V_r| < 240\;{\rm km\,s^{-1}}, \qquad |V_z| < 100\;{\rm km\,s^{-1}},$$1, enhanced $$150\;{\rm km\,s^{-1}} < |V_r| < 240\;{\rm km\,s^{-1}}, \qquad |V_z| < 100\;{\rm km\,s^{-1}},$$2-abundances, and the presence of four coherent orbital quadrants.

In this picture, the four subgroups are not evidence for multiple progenitors. Rather, they are taken to reflect phase wrapping of tidal debris along different orbital phases. The argument against a heated in-situ origin is likewise explicit: Hou et al. cite the absence of any appreciable age spread and the cold radial velocity dispersions, $$150\;{\rm km\,s^{-1}} < |V_r| < 240\;{\rm km\,s^{-1}}, \qquad |V_z| < 100\;{\rm km\,s^{-1}},$$3. This suggests that ShangGu-3 is better understood as accreted debris now traversing the solar neighborhood in several phase-separated lanes.

The same section of the study relates ShangGu-3 to numerical simulations of dwarf-disk mergers. Hou et al. state that such simulations suggest that massive $$150\;{\rm km\,s^{-1}} < |V_r| < 240\;{\rm km\,s^{-1}}, \qquad |V_z| < 100\;{\rm km\,s^{-1}},$$4 satellites on nearly polar, eccentric orbits can deposit stars into distinct orbital sectors that later phase-mix into azimuthally separated streams. They further state that ShangGu-3’s metallicity floor, $$150\;{\rm km\,s^{-1}} < |V_r| < 240\;{\rm km\,s^{-1}}, \qquad |V_z| < 100\;{\rm km\,s^{-1}},$$5, and $$150\;{\rm km\,s^{-1}} < |V_r| < 240\;{\rm km\,s^{-1}}, \qquad |V_z| < 100\;{\rm km\,s^{-1}},$$6-enhancement are consistent with a progenitor dwarf of stellar mass $$150\;{\rm km\,s^{-1}} < |V_r| < 240\;{\rm km\,s^{-1}}, \qquad |V_z| < 100\;{\rm km\,s^{-1}},$$7–$$150\;{\rm km\,s^{-1}} < |V_r| < 240\;{\rm km\,s^{-1}}, \qquad |V_z| < 100\;{\rm km\,s^{-1}},$$8, roughly $$150\;{\rm km\,s^{-1}} < |V_r| < 240\;{\rm km\,s^{-1}}, \qquad |V_z| < 100\;{\rm km\,s^{-1}},$$9 the Milky Way’s mass at the time of merger. A plausible implication is that ShangGu-3 preserves not only the presence of an early accretion event, but also some information about the progenitor’s enrichment timescale and mass scale.

6. Place within the Galactic metal-poor disc

ShangGu-3 was identified in a study whose broader aim was to reconstruct the formation history of the Galactic metal-poor disc (Hou et al., 21 Jul 2025). In that larger framework, Hou et al. distinguish four main populations within the metal-poor disc: a primordial disc older than 12 Gyr with [Fe/H] $$|z| < 3\;{\rm kpc}, \qquad E > -1.41\times10^5\;{\rm km^2\,s^{-2}}.$$0; debris stars from the progenitor galaxy of Gaia-Sausage-Enceladus now residing in the Galactic disc; the metal-poor tail of the metal-rich, high-$$|z| < 3\;{\rm kpc}, \qquad E > -1.41\times10^5\;{\rm km^2\,s^{-2}}.$$1 disc formed 10–12 Gyr ago and extending down to [Fe/H] $$|z| < 3\;{\rm kpc}, \qquad E > -1.41\times10^5\;{\rm km^2\,s^{-2}}.$$2; and the metal-poor tail of the metal-rich, low-$$|z| < 3\;{\rm kpc}, \qquad E > -1.41\times10^5\;{\rm km^2\,s^{-2}}.$$3 disc younger than 8 Gyr and reaching [Fe/H] $$|z| < 3\;{\rm kpc}, \qquad E > -1.41\times10^5\;{\rm km^2\,s^{-2}}.$$4.

Within this taxonomy, ShangGu-3 belongs to a set of newly recognized substructures that also includes ShangGu-1 and ShangGu-2. ShangGu-1 is characterized by unusual [Fe/H]–eccentricity correlations, and ShangGu-2 is described as possibly heated disc stars. ShangGu-3 differs from both by the clarity of its fourfold orbital splitting and by its connection to already known local streams.

Its significance lies in the way it combines chemistry, age, and kinematics into a single fossil record. Hou et al. conclude that ShangGu-3 is a coherent, early-accreted substructure revealed by combining LAMOST chemistry and Gaia kinematics, and that its stars now populate four orbital “lanes” in the solar neighborhood. This suggests that the metal-poor disc is not only a mixture of in-situ disc populations and major-merger debris, but also a repository of finer-grained ancient accretion structures whose full morphology becomes visible only in joint chemo-dynamical analyses.