Gaia-Sausage/Enceladus Accretion Remnant

• Gaia-Sausage/Enceladus is a phase-mixed, radially anisotropic stellar population from a massive dwarf galaxy merger around 10 Gyr ago.
• It is defined by distinct chemo-dynamical signatures, including a narrow metallicity distribution, low [Al/Fe], and high orbital eccentricity.
• Dynamical models and chemical tagging techniques reveal its significant role in quenching Milky Way star formation and sculpting halo structure.

The Gaia-Sausage/Enceladus (GS/E) accretion remnant is a prominent phase-mixed stellar population in the Milky Way’s inner halo, associated with the last significant merger the Galaxy experienced. Its identification rests on the highly radially anisotropic kinematics, distinct chemical abundance signatures, and a well-defined age–metallicity relation seen in both globular clusters and field stars. The progenitor was a massive (stellar mass $\sim 10^8$–$10^9\,M_\odot$, total mass up to $5\times10^{10}\,M_\odot$) dwarf galaxy on a strongly radial orbit accreted at $z\lesssim3$, approximately 10 Gyr ago. The GS/E debris dominates the intermediate-metallicity inner halo, and its dynamical and chemical fingerprints have enabled reconstruction of its mass, star formation history, and impact on the Milky Way’s morphological evolution.

1. Progenitor Properties, Merger Epoch, and Dynamical Identification

The GS/E progenitor is inferred to have had a total mass $M\approx 5\times10^{10}\,M_\odot$ and was accreted on a highly radial orbit with little angular momentum, merging with the Galactic halo at redshift $z\lesssim3$ (lookback time $\sim$10 Gyr) (Myeong et al., 2018). Kinematic analyses, particularly the placement of globular clusters (GCs) and stars in energy–action space, reveal a sharp separation between in situ and accreted populations. This is quantified via a characteristic threshold, $E_\mathrm{crit}$, above which GCs are interpreted as accreted: GCs with $E>E_\mathrm{crit}$ cluster in vertical and azimuthal actions and are strung out at high radial action ($J_R$), congruent with highly radial, eccentric orbits ($e\gtrsim0.80$) and extreme velocity anisotropy ($\beta\sim0.95$) (Myeong et al., 2018).

The remnant’s present-day spatial structure is markedly triaxial, with axis ratios 1:0.55:0.45 and its major axis oriented $\sim$80° from the Sun–Galactic center line and 16° from the plane (Lane et al., 2023). The density profile is shallow within 15–25 kpc and steepens at larger radii, yielding a mass estimate for the GS/E stellar debris of $2.3^{+0.95}_{-0.63} \times 10^8\,M_\odot$ once radially varying anisotropy is accounted for (Lane et al., 4 Sep 2025).

2. Chemo-Dynamical Properties and Population Tagging

GS/E stars are identified by their unique combination of dynamical and chemical traits. Dynamically, they occupy a region characterized by low to near-zero $L_z$ (azimuthal angular momentum), high $J_R$, and very high orbital eccentricities. Chemically, a distinctive sequence in abundance patterns is observed:

• Narrow MDF peaking at [Fe/H] $\sim-1.15$ with a relatively small spread; the metal-weak tail ([Fe/H]$<-2$) is deficient relative to the Milky Way’s general halo (Bonifacio et al., 2021).
• Uniformly low $\sim-0.25$ dex" title="" rel="nofollow" data-turbo="false" class="assistant-link">Al/Fe and a pronounced “knee” in [Mg/Fe] at [Fe/H] $\sim-1.4$ (Feuillet et al., 2021, Myeong et al., 2022).
• Outlier abundances in multiple elements, especially lower [Na/Fe], [Mn/Fe], [Ni/Fe], [Cu/Fe], and lower $\alpha$-elements compared to the “Splash”/“Aurora” in situ halo populations (Buder et al., 2021, Myeong et al., 2022).
• Systematic chemical tagging using Gaussian Mixture Models in abundance–abundance space (e.g., [Na/Fe] vs [Mg/Mn]) robustly isolates GS/E debris (Buder et al., 2021).

The GS/E population is old, with most stars formed $\sim$10–12 Gyr ago. A clear age–metallicity relation and episodic star formation are evident both in field stars and associated GCs (González-Koda et al., 27 Feb 2025).

3. Star Formation History, Chemical Evolution, and Internal Structure

A multi-epoch star formation history is recovered through age–metallicity and neutron-capture element analyses (González-Koda et al., 27 Feb 2025, Ernandes et al., 10 May 2025). CMD-fitting and elemental abundance ratios ([Eu/Mg], [Ba/Mg], [Eu/Ba]) point to a prolonged ($\gtrsim2$ Gyr) period of slow, inefficient star formation in GSE, terminated abruptly at the time of accretion (Ernandes et al., 10 May 2025). This is reflected in the gradual rise of neutron-capture elements with metallicity and the absence of a downturn in [Eu/Ba], supporting rapid quenching following the merger.

The GS/E progenitor exhibited a negative metallicity gradient prior to disruption, preserved as a weak but significant radial [Fe/H] gradient in the phase-mixed debris ($\approx -0.014$ dex/kpc), inferred to correspond to an original progenitor gradient of $\approx -0.1$ dex/kpc (Khoperskov et al., 2023). The age–metallicity relation is nearly linear for the main populations, indicating efficient in situ enrichment up to the time of merger, with a small fraction of stars possibly formed in the aftermath or as contaminant populations.

4. Orbital Anisotropy and Dynamical Modeling

The GS/E remnant exhibits a strongly radially biased velocity distribution with anisotropy parameter $\beta\sim0.9$ at $r\gtrsim8$ kpc, dropping to $\beta\sim0.4$ within $\sim2$ kpc (Lane et al., 4 Sep 2025). A single constant-$\beta$ model is inadequate; instead, a two-component Osipkov–Merritt DF, characterized by scale radii $r_\text{a,1}=2$ kpc and $r_\text{a,2}=547$ kpc with mixture fraction $k_\text{om}=0.88$, accurately reproduces both the flat high-anisotropy outer profile and the central isotropy (Lane et al., 17 May 2024, Lane et al., 4 Sep 2025). This functional form is motivated by both simulation results and observational data, which consistently show declining anisotropy toward the Galactic center.

These sophisticated DF-based models capture the unique phase-space density, velocity dispersions, and density breaks (steepening beyond $\sim$20 kpc) documented in the GS/E structure. Triaxiality is an essential feature, and dynamic and chemical selections tend to have limited overlap, emphasizing the need for multi-dimensional approaches (Buder et al., 2021, Lane et al., 2023).

5. Impact on the Milky Way: Quenching, Disc Formation, and Overdensities

The GS/E merger is a critical event in the Milky Way’s formation narrative, influencing both the halo and disc components:

• The merger is implicated in halting gas accretion onto the Galactic disc and temporarily quenching star formation, a hiatus required by chemical evolution models to explain the observed thick/thin disc [$\alpha$/Fe]–[Fe/H] bimodality (Vincenzo et al., 2019).
• The thick–to–thin disc transition, marked by a minimum in disc scale length at $\sim10$ Gyr ago, coincides with the end of the GS/E-driven starburst. Action-based quasi-isothermal DF fits to APOGEE+Gaia data show a scale-length dip (R$_\text{disc}\sim$1.7 kpc) followed by inside-out thin disc growth, reproduced in Auriga simulations by gas disc shrinking due to a shift from cold- to hot-mode accretion (Funakoshi et al., 30 Jul 2025).
• Major inner-halo overdensities, notably the Hercules–Aquila Cloud and Virgo Overdensity, are chemically and dynamically indistinguishable from GS/E debris, pointing to their common origin in the merger (Perottoni et al., 2022).
• The GS/E remnant comprises only $\sim$15–25% of the total halo stellar mass (halo mass $\sim$6.7–8.4$\times10^8\,M_\odot$), making the merger dynamically important but not monolithic (Lane et al., 2023). Its metal-weak tail suggests that extremely metal-poor halo stars predominantly originate from lower-mass accreted dwarfs (Bonifacio et al., 2021).

6. Progenitor Mass, Black Hole Content, and Connection to $\omega$ Centauri

GS/E’s progenitor stellar mass is now constrained to $2.3^{+0.95}_{-0.63}\times10^8\,M_\odot$ (GS/E field stars only; higher, $\sim10^9\,M_\odot$, if including all globular clusters). The velocity dispersion and the observed scaling of the likely surviving nuclear star cluster ($\omega$ Centauri) imply the existence of an intermediate-mass black hole (IMBH) with mass $M_{\rm BH}\sim4,000-21,000\,M_\odot$, lying squarely on extrapolated $M_{\rm BH}$–$M_\star$ and $M_{\rm BH}$–$\sigma_{\star}$ relations of higher-mass galaxies. This suggests that the mechanisms of black hole seeding and co-evolution found in larger systems apply down to the dwarf galaxy regime, and that GSE represents a rare example of direct linking of an IMBH to its accretion debris (Limberg, 18 Nov 2024).

Two plausible formation channels for the central BH are considered: heavy “direct collapse” seeds with minimal subsequent accretion, or lighter Population III remnants with extended but still modest accretion histories.

7. Assembly History: Multiple Accretion Events and Cosmological Context

There is growing evidence from both stellar kinematics and cosmological simulations that the inner halo’s “sausage”-like signature, while often attributed to a single dominant merger, could in a significant fraction of Milky Way analogues be the composite result of two (or more) radially biased accretion events (“RA pairs”) (Folsom et al., 5 Aug 2024, Kim et al., 2021). Distinguishing single-merger from double-merger debris is difficult in kinematic space but more tractable using chemodynamical diagnostics and star-formation histories; the merger timescales, ages, and enrichment patterns of the GSE candidates provide discriminants, with single mergers typically accreted later (median infall $\sim6$ Gyr ago) and double-mergers earlier ($\sim11$ Gyr ago).

Selection methods—kinematic or chemical—affect inferences of GS/E mass and properties. High-purity (but often incomplete) kinematic selections yield lower mass estimates than more inclusive or chemistry-based schemes, underscoring the need for careful control of sample purity, completeness, and contamination when interpreting the assembly history of the inner halo (Carrillo et al., 2023, Lane et al., 2023).

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In summary, the Gaia-Sausage/Enceladus accretion remnant is a foundational component of the Milky Way’s hierarchical assembly, linking a chemically and dynamically distinct, radially anisotropic population to both field halo stars and globular clusters, anchoring a key episode in the Galaxy’s morphological transformation. Advances in DF-based dynamical modeling, chemical tagging, and cross-comparisons with cosmological simulations and resolved star formation histories have led to robust, quantitative constraints on its progenitor mass, assembly timescale, internal gradients, and wider impact on the Milky Way’s structural and star-forming evolution.