Gaia-Enceladus-Sausage (GES) Members

• GES members are the accreted halo stars, defined by high orbital eccentricities, specific [Fe/H] ranges, and low [Al/Fe] signatures via dynamical and chemical selection.
• They exhibit highly radial orbits with pericenters around 3–5 kpc and apocenters of 10–30 kpc, consistent with a major dwarf-galaxy merger 8–12 Gyr ago.
• Age and abundance trends in both field stars and associated globular clusters trace bursty star formation episodes, refining our reconstruction of the Milky Way’s merger history.

Gaia-Enceladus-Sausage (GES) members constitute the dominant, highly radial, metal-intermediate accreted debris population in the Galactic halo, originating from one or more major dwarf-galaxy mergers 8–12 Gyr ago. Robust identification and characterization of GES members have been central to reconstructing the Milky Way’s merger history, with recent studies revealing subtle chemical, dynamical, and chronological features, as well as a population of associated globular clusters.

1. Dynamical and Chemodynamical Selection of GES Members

GES member identification relies on integrals-of-motion (IoM), action/energy space, and chemical tagging. Canonical dynamical identifiers include:

• Integrals of motion: Orbital energy $$E = \frac{1}{2}(v_R^2 + v_\phi^2 + v_z^2) + \Phi(R)$$, vertical angular momentum $L_z = R\,v_\phi$, and $L_\perp = \sqrt{L_x^2 + L_y^2}$.
• Action space cuts: $$\sqrt{J_R} \in [30,50]\ \mathrm{(kpc\,km\,s^{-1})^{1/2}}$$ and $|L_z|\leq 500$ kpc km s$^{-1}$ select the high-eccentricity, low-rotation population (Feuillet et al., 2020, González-Koda et al., 27 Feb 2025).
• Orbital eccentricity: $e > 0.7$, typically $e>0.8$ for the "sausage" component (Perottoni et al., 2022, Liu et al., 9 Oct 2024).
• Energy and angular momentum box: $-2.0 \times 10^5 < E < -1.2 \times 10^5$ km$^2$/s$^2$, $|L_z| < 500$–1500 kpc km s$^{-1}$ for both field stars and globular clusters (Aguado-Agelet et al., 27 Feb 2025, Perottoni et al., 2022).

Chemodynamical selection is further refined by:

• Metallicity: $-2.0 < [\mathrm{Fe/H}] < -0.8$; median values span $-1.20$ to $-1.15$ dex, with observed dispersions $0.2$–$0.3$ dex (Myeong et al., 2022, Feuillet et al., 2021).
• Alpha and odd-Z elements: GSE has uniformly low $[\mathrm{Al}/\mathrm{Fe}]$ (typically $-0.25$ dex), a distinct $[\mathrm{Mg/Fe}]$–$[\mathrm{Fe/H}]$ track with a "knee" at $[\mathrm{Fe/H}] \sim -1.3$, and $[\mathrm{Mg/Mn}] > 0.4$ (Feuillet et al., 2021, Myeong et al., 2022).
• Probabilistic mixture modeling: Gaussian Mixture Models and unsupervised algorithms applied to abundance and orbital parameters assign GSE memberships with high purity (Myeong et al., 2022, Perottoni et al., 2022).

Recent studies employing HDBSCAN clustering in $(E,L_z,L_\perp)$ space have yielded member lists of over 1,300 stars from Gaia DR3, with eccentricity and angular momentum cuts consistent with previous methodology (Liu et al., 9 Oct 2024).

2. Kinematic, Spatial, and Orbital Properties

GES members are defined by highly radial orbits and low net angular momentum:

• Eccentricity: Median $e \gtrsim 0.8$, majority of orbits satisfy $e > 0.7$; for RR Lyrae, GSE-fraction stars have $e>0.85$ (Kunder et al., 15 Jul 2025, Liu et al., 9 Oct 2024).
• Pericenter and apocenter: $r_\mathrm{peri} \lesssim 3$–5 kpc; $r_\mathrm{apo} \sim 10$–30 kpc (Perottoni et al., 2022, González-Koda et al., 27 Feb 2025).
• Spatial extent: Members are distributed across the halo with a density profile $\nu(r)\propto r^{-4.9\pm0.1}$ and flattening axis ratio $q(r)$ increasing from $\sim0.5$ at $r\sim10$ kpc to $\sim0.8$ at $r\sim30$ kpc (Wu et al., 2022).
• Phase-space signature: Vertical bar in $L_z$–$E$, "sausage"-shaped locus in $V_r$–$V_\phi$, and spread in Galactocentric radii to at least 20 kpc (Ding et al., 9 Sep 2025).
• Bulk rotation: $V_\phi$ near zero or slight retrograde; $\langle L_z\rangle\sim0$ kpc km/s.

3. Chemical Abundance Patterns and Metallicity Distribution

GES members show distinctive chemical evolution:

• Metallicity distribution: Median $[\mathrm{Fe/H}]= -1.20$ in APOGEE+Gaia, $\sim-1.15$ from APOGEE-based samples, $-1.17$ from SkyMapper–Gaia; MDF dispersions $0.23$–$0.34$ dex (Myeong et al., 2022, Feuillet et al., 2021).
• Alpha–elements: $[\alpha/\mathrm{Fe}]\sim+0.18$, declining from a plateau at low $[\mathrm{Fe/H}]$ to lower values at higher metallicity ("knee" at $[\mathrm{Fe/H}]\sim-1.3$) (Myeong et al., 2022).
• Odd-Z and Fe-peak elements: $[\mathrm{Al/Fe}]$ is systematically low (–0.24 to –0.25 dex), $[\mathrm{Mg/Mn}]$ enhanced ($\sim0.49$), and $[\mathrm{Mn/Fe}]$ subsolar (–0.3 dex) (Myeong et al., 2022, Feuillet et al., 2021).
• r-process enrichment: GSE is unusually rich in r-process elements; $[\mathrm{Eu/Fe}]\sim+0.5$ and $[\mathrm{Eu/Mg}]\sim0$ (Myeong et al., 2022, Aguado-Agelet et al., 27 Feb 2025).

4. Age Distributions, Star Formation History, and the Age–Metallicity Relation

GES member ages and SFH have been characterized using isochrone fitting and CMD-based methods:

• Dominant age: Mean/median stellar age $10$–$12$ Gyr, with the main population spanning $10$–$13$ Gyr and full width at half maximum $\sim2$ Gyr (Feuillet et al., 2021, González-Koda et al., 27 Feb 2025).
• SFH and AMR: Multiple studies identify two major episodes of star formation:
  • "Isolated" evolution phase at $t\sim13.5$ Gyr, with $[\mathrm{M/H}]\sim-1.6$.
  • "Merger-induced" phase at $t\sim11.6$–$10.5$ Gyr, with $[\mathrm{M/H}]$ increasing to $-0.8$ (González-Koda et al., 27 Feb 2025).
• Recent population: A fourth, younger subpopulation at $t\sim8.5$ Gyr and $[\mathrm{M/H}]\sim-0.4$ is observed, but its association with GSE is unresolved (González-Koda et al., 27 Feb 2025).
• AMR functional form: For globular clusters,

$$\mathrm{Age\ (Gyr)} = (8.17 \pm 0.20) - (3.08 \pm 0.30)\, [\mathrm{Fe/H}],$$

with 1$\sigma$ scatter of 0.15 Gyr (Aguado-Agelet et al., 27 Feb 2025).

• Cluster and field AMR: Field star and globular cluster AMRs coincide, confirming that cluster formation tracks global GSE chemical evolution (González-Koda et al., 27 Feb 2025, Aguado-Agelet et al., 27 Feb 2025).

5. GSE-Associated Globular Clusters

Selection of globular clusters (GCs) as GSE members is based on integrals-of-motion, specifically:

• Selection box: $-2.0\times10^5 < E < -1.2\times10^5$ km$^2$/s$^2$, $|L_z| < 500$ kpc km s$^{-1}$ (Aguado-Agelet et al., 27 Feb 2025).
• Outliers: Four clusters (NGC 288, NGC 6205, NGC 5286, NGC 7099) are dynamical outliers or show chemical/age deviation; e.g., NGC 288 and NGC 6205 are likely in-situ "Splash" clusters, NGC 7099 is younger and retrograde (Sequoia member), NGC 5286 is older with high Eu/Si (Aguado-Agelet et al., 27 Feb 2025).
• Two formation epochs: Nine bona-fide GSE clusters exhibit bimodal cluster ages:
  • First cohort formed at $t_1=13.2\pm0.3$ Gyr (NGC 2298, 5897, 6341, 6779, 7089),
  • Second at $t_2=11.4\pm0.3$ Gyr (NGC 362, 1261, 1851, 2808).
  • Duration of both episodes is short ($\sim$0.3 Gyr each), separation $\sim$2 Gyr, with the timing attributed to pericentric passage and final coalescence (Aguado-Agelet et al., 27 Feb 2025).
• Age–Si–Eu relation: $[\mathrm{Eu/Si}]$ rises with decreasing age, while $[\mathrm{Eu/Fe}]$ declines and $[\mathrm{Si/Fe}]$ decreases, tracing the onset of r-process production and SNIa enrichment; these trends provide additional membership discrimination (Aguado-Agelet et al., 27 Feb 2025).

6. Multiple Progenitor Events and Substructure

Several analyses support a composite origin for the classical GSE structure:

• Multiple accretion events: Action-inclination decomposition reveals at least three distinct major merger components—low-inclination prograde and retrograde "Sausage" events, and a high-inclination, nearly radial "Enceladus" component—plus additional minor outer-halo retrograde dwarfs (Kim et al., 2021).
• Distinct metallicity peaks: Kinematic/chemical separation finds $[\mathrm{Fe/H}]$ peaks at $-1.5$ (LOI, radial), $-1.7$ (LOI, tangential), $-1.3$ (HOI, radial), $-1.9$ (HOI, tangential) that map onto different mass progenitors (Kim et al., 2021).
• Simulation results: TNG50 and VINTERGATAN-GM cosmological analogues show that both single- and double-merger GSE analogues yield similar present-day chemical–dynamical signatures; explicit mass ratios and star-formation burst features can only be teased apart with additional chronochemical constraints (Folsom et al., 5 Aug 2024, Rey et al., 2022).

7. Summary Table: Core Properties of GES Members

Property (Field Stars)	Value or Range	Source(s)
Median $[\mathrm{Fe/H}]$	–1.20 to –1.15 (APOGEE), –1.17 (SM)	(Myeong et al., 2022, Feuillet et al., 2021, Feuillet et al., 2020)
Dispersion $\sigma_{[\mathrm{Fe/H}]}$	0.23–0.34 dex	(Myeong et al., 2022, Feuillet et al., 2021)
Median $[\alpha/\mathrm{Fe}]$	0.18 ($\pm0.05$)	(Myeong et al., 2022)
Median $[\mathrm{Al}/\mathrm{Fe}]$	–0.24 to –0.25 ($\pm0.07$)	(Myeong et al., 2022, Feuillet et al., 2021)
Median Age	10–12 Gyr	(Feuillet et al., 2021, González-Koda et al., 27 Feb 2025)
Eccentricity $e$	$>$0.8 (majority), selection $e>0.7$	(Perottoni et al., 2022, Liu et al., 9 Oct 2024)
Pericenter $r_\mathrm{peri}$	$\lesssim$3–5 kpc	(Perottoni et al., 2022, Perottoni et al., 2022)
Apocenter $r_\mathrm{apo}$	$\sim$10–30 kpc	(Perottoni et al., 2022, Perottoni et al., 2022)
$L_z$ Range	$|L_z|\leq500$ or $|L_z|<1500$ kpc km/s	(Feuillet et al., 2020, Perottoni et al., 2022)

*SM: SkyMapper *Full membership selection and additional covariance information are provided in the referenced works.

8. Implications and Prospects

GES membership definitions have matured from broad orbital cuts to multi-dimensional probabilistic models incorporating kinematics, actions, and precise chemical tagging. The population encompasses a tightly correlated system of field stars and globular clusters, tracing a bursty, merger-driven star-formation history that dominates the local stellar halo. Detailed paper of age–abundance trends (e.g., [Eu/Fe], [Eu/Si]), chemodynamical substructure, and the full distribution of metallicity and age enables separation of genuine GSE members from in-situ contaminants and allows for nuanced distinctions between single versus multi-progenitor origin scenarios. Simulation analogues confirm that similar present-day properties can originate from a range of early assembly pathways, underscoring the need for high-precision ages and multi-element abundance diagnostics to fully reconstruct the Milky Way’s merger sequence (Rey et al., 2022, Folsom et al., 5 Aug 2024, Aguado-Agelet et al., 27 Feb 2025, González-Koda et al., 27 Feb 2025).