Gaia-Sausage/Enceladus Stars: Milky Way Merger Debris

Updated 18 August 2025

Gaia-Sausage/Enceladus stars are debris from an accreted dwarf galaxy, characterized by highly radial orbits and a narrow metallicity peak near [Fe/H] ∼ −1.2.
Chemical evolution models and surveys reveal their low star formation efficiency and delayed enrichment from Type Ia supernovae and r-process events.
Cosmological simulations and kinematic studies indicate that the GSE merger reshaped the Milky Way’s inner halo and thick disk while seeding diverse stellar subpopulations.

The Gaia-Sausage/Enceladus (GSE) stars constitute the debris of a massive dwarf galaxy accreted by the Milky Way in its last major merger event. Identified via their distinctive chemo-dynamical signatures, GSE stars dominate the local inner stellar halo and have been pivotal in reshaping the chemical and structural evolution of the Milky Way. Comprehensive chemical evolution models and cosmological simulations, along with photometric and spectroscopic surveys, have established GSE as a benchmark for understanding galaxy assembly, star formation quenching, and the interplay of nucleosynthetic processes in dwarf galaxies and their host halos.

1. Defining Properties and Selection of Gaia-Sausage/Enceladus Stars

GSE stars are recognized primarily by their highly radial orbits (eccentricities $e \gtrsim 0.8$ ), near-zero net angular momentum ( $L_z \approx 0$ ), and a narrow metallicity distribution peaking at [Fe/H] $\sim -1.2$ (Feuillet et al., 2020, Feuillet et al., 2021, Carrillo et al., 2023). They exhibit an elongated distribution in action–angular momentum space—specifically, in $(\sqrt{J_R}, L_z)$ —and can be isolated using cuts such as:

$30 \leq \sqrt{J_R} \leq 50$ (kpc km s $^{-1})^{1/2}$
$-500 \leq L_z \leq 500$ kpc km s $^{-1}$

Chemical tagging further refines membership via uniformly low [ $\alpha$ /Fe] at high [Fe/H], low $\sim-0.25$ $\sim - 0.25$ dex" title="" rel="nofollow" data-turbo="false" class="assistant-link">Al/Fe, and tight [Mg/Fe] vs. [Fe/H] sequences exhibiting well-defined knees. Purity and completeness of GSE samples depend on selection strategy: the Jr–Lz method provides maximal sample purity, while simple high-eccentricity cuts maximize completeness but elevate contamination from in-situ and other accreted systems (Carrillo et al., 2023).

2. Star Formation History and Chemical Evolution

Detailed chemical evolution modeling indicates that the GSE progenitor formed via rapidly collapsing, exponentially infalling primordial gas—with infall timescale $\tau_{\mathrm{inf}}$ = 0.24 Gyr and total infall mass $M_{\mathrm{infall}} = 10^{10} M_\odot$ —but maintained a low star formation efficiency (SFE $\approx$ 0.42 Gyr $^{-1}$ ) (Vincenzo et al., 2019). This low efficiency delayed gas metal enrichment, allowing prompt Type Ia supernovae to reduce [ $\alpha$ /Fe] at low [Fe/H]. Models reproduce the observed metallicity distribution function ( $\mathrm{median}~[\mathrm{Fe/H}] \sim -1.2$ ) and [ $\alpha$ /Fe] trends, with outflows parametrized as $O(t) = w \times \mathrm{SFR}(t)$ , where the best fit is $w = 0.5$ (mild galactic winds).

Elemental abundance ratios serve as chemical clocks:

[Fe/Mg] traces the onset and efficiency of Type Ia SN enrichment.
[Ba/Mg] and [Ba/Eu] constrain the timescales of $s$ - and $r$ -process nucleosynthesis, indicating the delayed rise of AGB and neutron star merger contributions.
[Eu/Mg] increases with [Mg/H], signaling delayed $r$ -process sources (see Section 5).

Synthesized constraints require three SFH phases (Ernandes et al., 22 May 2024, Ernandes et al., 10 May 2025):

Gradual star formation onset (modest SFR, slow initial chemical enrichment).
Star formation extended over $>2$ Gyr, allowing delayed enrichment from SNIa, AGB stars, and NS mergers.
Abrupt quenching around [Fe/H] $\sim -0.5$ , most likely coinciding with GSE's infall and merger with the Milky Way.

3. Kinematics, Spatial Distribution, and Dynamical Imprints

Kinematically, GSE stars form a radially anisotropic halo population with little net rotation (orbital circularity $\epsilon \sim 0$ ) (Grand et al., 2020). Their orbits, characterized by high $J_R$ and low $|L_z|$ , are interpretable as debris from a near head-on (highly radial) major merger. This debris dominates regions of the stellar halo out to $>30$ kpc and produces detectable overdensities at apocentric shells 15–90 kpc from the Galactic center (Chandra et al., 2022). In the inner regions (e.g., $R<10$ kpc), the GSE fraction among RR Lyrae is $6$–$9$\%, lower than local halo fractions, as confirmed by the Auriga cosmological zoom-in simulations (Kunder et al., 15 Jul 2025).

Cosmological MHD simulations affirm that the gas-rich, radially plunging GSE merger not only deposited field stars (ex-situ) but also dynamically heated the pre-merger proto-disc, generating both a "splash" halo component and seeding the formation of a centrally concentrated, thick-disc-like population via a starburst (Grand et al., 2020).

4. Chemical and Nucleosynthetic Signatures

GSE stars display unique chemical evolution features distinct from both in-situ stars and other accreted substructures (e.g., Sequoia):

[ $\alpha$ /Fe]: high and nearly constant at low [Fe/H] with a “knee” at $[\mathrm{Fe/H}] \sim -1.4$ – $-1.6$ where SNIa Fe enrichment begins to dominate, leading to a negative slope in [ $\alpha$ /Fe] beyond this metallicity (Feuillet et al., 2021, Xie et al., 9 Jun 2025).
[ $\mathrm{Al}/\mathrm{Fe}]$ is uniformly low, confirming low SFE and a massive progenitor (Feuillet et al., 2021).
Iron-peak elements such as Zn and Ni decrease with increasing [Fe/H] after the knee, in contrast to the more constant patterns seen in in-situ stars (Xie et al., 9 Jun 2025).
[Ba/Mg], [Eu/Mg], and [Eu/Ba] all rise with [Fe/H], tracking the delayed contributions from AGB stars and NS mergers (Ernandes et al., 22 May 2024, Ernandes et al., 10 May 2025).
The r-process pattern in GSE r-II stars (e.g., J0722) generally matches the solar system r-process, except for some anomalies such as enhanced Pr (Xie et al., 9 Jun 2025).

Actinide-boost phenomena (elevated log( $\epsilon$ (Th/Eu))) have been identified in very metal-poor GSE stars (e.g., LAMOST J0804+5740 with [Fe/H] = –2.38, [Eu/Fe] = 0.8, log( $\epsilon$ (Th/Eu)) = –0.22), supporting scenarios where GSE progenitor hosted magnetorotationally-driven supernovae that produced extreme r-process enhancements (Lin et al., 12 May 2025).

5. Reconstruction of Star Formation and Evolutionary Phases

Color-magnitude diagram (CMD) fitting of Gaia DR3 stars reveals the existence of multiple evolutionary sequences within GSE:

GSE0: Oldest, most metal-poor population ( $\sim$ 13.5 Gyr, [M/H] $\sim -1.6$ )
Population A: 12–11.5 Gyr, [M/H] $\sim -1$
Population B: 10–10.5 Gyr, [M/H] $\sim -0.8$
Population C: A minority, younger (8.5 Gyr), more metal-rich population ([M/H] $\sim -0.4$ ), possibly reflecting residual GSE star formation or contamination (González-Koda et al., 27 Feb 2025).

The linear age–metallicity relation, punctuated by these populations, suggests two main epochs: (1) evolution in isolation and (2) a merger-driven burst, with the merger event halting GSE's star formation. CMD fitting methods demonstrate robustness against sample variance and reddening assumptions, reinforcing the temporal sequence.

6. Impact of the GSE Merger on the Milky Way

The GSE merger induced far-reaching structural and chemical effects on the Milky Way:

Triggered a central starburst (the Great Galactic Starburst) and formed the metal-rich thick disc.
Increased the total halo mass, accelerating the transition from cold- to hot-mode gas accretion, leading to temporary gas disc shrinking and a dip in the scale length of stars formed $\sim$ 10 Gyr ago (Funakoshi et al., 30 Jul 2025).
Inhibited or delayed further gas accretion onto the disc, causing a quenching phase, which is necessary to explain the [α/Fe]–[Fe/H] bimodality observed in thick vs. thin disk stars (Vincenzo et al., 2019).
Kinematically distinct in-situ heated and ex-situ debris subpopulations ("splash" and merger debris) are the dual products of the merger, shaping the inner halo, thick disk, and chemically distinct low-α, high-Na "LAHN" stars generated in compact, merger-fueled starbursts (An et al., 15 May 2025).

Tables in key studies report that the inferred GSE progenitor mass is $M_\star \sim (4-5)\times 10^8\, M_\odot$ (median at merger, robust across sample selections), with a corresponding halo mass $10^{10.5}$ – $10^{11.1}\, M_\odot$ (Carrillo et al., 2023). Star formation in GSE is directly linked to the observed abundance patterns and the chemical signature left in the Milky Way’s stellar halo.

7. Open Questions and Chemo-Dynamical Diversity

Current cosmological simulations show that GSE-like debris in Milky Way analogues can result from either a single dominant merger or combinations of two smaller mergers. The former typically have more extended SFHs and later quenching, while the latter quench earlier and are more metal-poor and α-enhanced (Folsom et al., 5 Aug 2024). Differences in chemical abundances and stellar ages are more robust diagnostics of the merger scenario than kinematics alone.

The presence of r-process–enhanced and actinide-boost stars among GSE debris provides direct evidence for rare and extreme nucleosynthetic environments in accreted dwarfs. Continued chemical and dynamical mapping, particularly for outer-halo and metal-poor GSE stars, is expected to further constrain the formation and assembly history of the Milky Way and probe the diversity of merger-driven evolutionary pathways.

Summary Table: Key Parameters of GSE Stars

Parameter	Typical Value / Diagnostic	Context
Fe/H	$-1.2$	Metallicity peak of MDF
$\alpha$ /Fe	At $[\mathrm{Fe/H}] \sim -1.4$ – $-1.6$	SNIa onset
[Al/Fe]	$\sim -0.25$	Accreted (low SFE) signature
Age (median at merger)	$12.3^{+0.9}_{-1.4}$ Gyr	Accretion epoch
$M_\star$ (progenitor)	$(4-5)\times10^8\,M_\odot$ up to $10^9\,M_\odot$	Stellar mass at infall
Gas fraction at merger	$\sim0.67$	Gaseous at infall
Kinematic signature	High-eccentricity $e \sim 0.8$ –$0.9$, $L_z \approx 0$	Radial inner halo orbits
[Ba/Mg], [Eu/Mg], [Eu/Ba]	All increase with [Fe/H]	Delayed $r$ / $s$ -process input
RR Lyrae halo fraction	$6$– $9\%$ (inner regions)	Decreases toward GC
Spatial distribution	Inner halo dominant, apocentric shells (15–90 kpc)	Shells/streams

Gaia-Sausage/Enceladus stars constitute the most prominent accreted component of the Milky Way’s stellar halo, encoding a detailed record of the Galaxy’s last major merger, and providing critical constraints for models of Galactic structure, star formation history, and nucleosynthetic enrichment.