That's so Retro: The Gaia-Sausage-Enceladus Merger Trajectory as the Origin of the Chemical Abundance Bimodality in the Milky Way Disk

TL;DR

This work addresses the origin of the Milky Way's alpha abundance bimodality by testing whether the retrograde Gaia-Sausage-Enceladus (GSE) merger can drive inward gas flows and alter the disk's chemical evolution. Using multi-zone Galactic chemical evolution models with angular-momentum-diluted radial gas flows (AMD) and a GSE accretion episode, the authors compare prograde, radial, and retrograde merger trajectories, incorporating SN yields and a delayed-time distribution for SNe Ia. They find that radial and retrograde trajectories produce centrally concentrated gas, reduced outer-disk gas, and a faster decline in $[ ext{Mg}/ ext{Fe}]$, which can yield a bimodal $[ ext{Mg}/ ext{Fe}]-[ ext{Fe}/ ext{H}]$ distribution provided there is a substantial high-$[ ext{Mg}/ ext{Fe}]$ population formed early. The results emphasize gas dynamics in GCE during major mergers, challenge dilution-based explanations, and offer a framework that can be tested against external galaxies like M31 and future surveys.

Abstract

The Milky Way (MW) is thought to have experienced a $\sim$3:1 mass-ratio merger event near redshift $z\sim2$ with a significantly retrograde trajectory. This now-disrupted dwarf galaxy is commonly known as the Gaia-Sausage-Enceladus (GSE). In this paper, we investigate the impact of the GSE merger trajectory on metal abundances in the MW disk. We construct numerical models of Galactic chemical evolution (GCE) incorporating radial gas flows to account for angular momentum transport during the merger event. Unlike prograde trajectories, radial and retrograde mergers are generally accompanied by a major sinking event in which much of the interstellar medium falls toward the Galactic center. This effect leads to a net decrease in surface density across much of the disk. Ongoing Type Ia supernova explosions then drive a rapid decline in [$α$/Fe] due to the lowered gas supply. Consequently, radial and retrograde trajectories increase (decrease) the number of low (high) [$α$/Fe] stellar populations relative to prograde trajectories. If high [$α$/Fe] stars form in sufficient numbers through other mechanisms, the effect of the retrograde trajectory can produce a bimodal [$α$/Fe] distribution at fixed [Fe/H], as observed in the MW. In models dominated by low [$α$/Fe] stellar populations, a bimodality does not arise because the retrograde trajectory cannot increase the number of high [$α$/Fe] stars. More broadly, our results highlight the importance of gas dynamics in GCE models featuring major merger events.

Figures (9)

• Figure 1: Accretion and star formation histories in our GCE models with (solid) and without (dashed) a $\sim$3:1 mass-ratio merger $\sim$10 Gyr ago, broadly consistent with a GSE-like event. Left: The surface density of accretion at Galactocentric radii of $R = 3$, $5$, $7$, $9$, $11$, and $13$ kpc, color coded according to the colorbar. Right: The total accretion (blue) and star formation (red) histories across the entire Galactic disk. Summary: Each model with a GSE merger has the same accretion history and similar total SFHs, but the gas dynamics and where stars form during the merger differs between models based on the GSE merger trajectory.
• Figure 2: Radial profiles in the stellar surface density (left, solid), gas surface density (left, dashed), and median stellar age (right). Red and blue lines differentiate between the base model with no GSE merger and our retrograde merger model with $\beta_{\phi,\text{GSE}} = -0.8$. The observed surface density profiles are taken from the literature (gas: Kalberla2009; stars: Bland-Hawthorn2016). The median stellar age measurements are taken from our previous work Johnson2025. Summary: The models with and without GSE-like merger events each predict disk galaxies with surface denstiy and age profiles that are a reasonable match to the MW.
• Figure 3: The radial profile in the ISM surface density immediately following the GSE merger. Solid lines show the surface density for prograde (blue; $\beta_{\phi,\text{GSE}} = +0.8$), radial (black; $\beta_{\phi,\text{GSE}} = 0$), and retrograde (red; $\beta_{\phi,\text{GSE}} = -0.8$) merger trajectories. The gray dashed line shows the base model with no GSE merger. Summary: Radial and retrograde mergers lead to significantly more centrally concentrated gas distributions, which is a consequence of angular momentum conservation.
• Figure 4: A comparison of the evolution and chemical enrichment for GSE-like mergers with different infall trajectories. Shades of red and blue show models that are retrograde and prograde, respectively, color coded according to the colorbar. We mark the perfectly radial merger scenario ($\beta_{\phi,\text{GSE}} = 0$) as a black dashed line. Top: The radial velocity in the ISM (left), the surface density of gas (middle), and the [Mg/Fe] ratio in the ISM at $R = 8$ kpc as functions of lookback time. Bottom Left: The evolution of the ISM in the [Mg/Fe]-[Fe/H] plane at $R = 8$ kpc. Bottom Right: The [Mg/Fe] distribution for stars with [Fe/H] between $-0.4$ and $-0.2$ in the solar annulus ($R = 7 - 9$ kpc; $\left|z\right| \leq 0.5$ kpc) at the present day. Summary: Retrograde mergers around redshift $z \sim 2$ facilitate an early transition from high [Mg/Fe] to low [Mg/Fe], resulting in more stars at $\sim$solar [Mg/Fe] than otherwise.
• Figure 5: The distribution of stars in the [Mg/Fe]-[Fe/H] plane in different Galactic regions. Following Figure 4 of Hayden2015, columns represent different ranges of Galactocentric radius $R$ (noted at the top), and rows denote different ranges of mid-plane distance $\left|z\right|$ (noted on the right). The grey-scale distribution shows the observed number counts of stars in SDSS-V MWM. Colored points show a random subsample of 500 stars in each region from our prograde ($\beta_{\phi,\text{GSE}} = +0.8$) and retrograde ($\beta_{\phi,\text{GSE}} = -0.8$) GSE merger scenarios. Summary: The data are more readily explained by the retrograde merger scenario, which predicts more stars near solar [Mg/Fe] at sub-solar [Fe/H] and fewer stars on the high-alpha sequence in the outer disk than the prograde merger scenario.