A Gaia-Enceladus Analog in the EAGLE Simulation: Insights into the Early Evolution of the Milky Way

Abstract

We identify a simulated Milky Way analog in the eagle suite of cosmological hydrodynamical simulations. This galaxy not only shares similar global properties as the Milky Way, but was specifically selected because its merger history resembles that currently known for the Milky Way. In particular we find that this Milky Way analog has experienced its last significant merger (with a stellar mass ratio ∼ 0.2) at z ∼ 1.2. We show that this merger affected both the dynamical properties of the stars present at the time, contributing to the formation of a thick disk, and also leading to a significant increase in the star formation rate of the host. This object is thus particularly suitable for understanding the early evolutionary history of the Milky Way. It is also an ideal candidate for re-simulation with much higher resolution as this would allow addressing a plethora of interesting questions such as, for example, the specific distribution of dark matter near the Sun.

Cosmological insights into the assembly of the radial and compact stellar halo of the Milky Way

“…If GE/GS stars could be found beyond r ∼ 25 kpc, our simulations predict that their age and metallicities should be comparable to the section already identified of GS/GE, a conclusion that needs confirmation from higher resolution experiments and a more detailed ISM treatment than in our simulations (along the lines of work proposed by Bignone et al (2019)). Our sample has more predictive power for dynamical quantities instead.…”

“…If these estimates are correct, the GE/GS event should have had important consequences for the present-day structure of the MW. For instance, the merger with a massive GE/GS would stir up the proto-disk of the MW, perhaps triggering dynamical heating and the posterior build up of the old thick disk in our Galaxy (Helmi et al 2018;Belokurov et al 2019;Haywood et al 2018); a hypothesis confirmed by cosmological simulations identifying GE/GS analogs (Bignone et al 2019;Grand et al 2020). These studies also suggest that the gas-rich nature of high-redshift mergers would simultaneously imply an enhancement of the MW's star formation rate as the GE/GS coalesces into the central regions, a prediction for which there is already observational evidence (Kruijssen et al 2019).…”

Predicting accreted satellite galaxy masses and accretion redshifts based on globular cluster orbits in the E-MOSAICS simulations

“…The Gaia Sausage/Enceladus accretion event has been suggested to have a stellar mass ∼10 9 M and accreted ∼9-10 Gyr ago or later (Belokurov et al 2018;Haywood et al 2018;Helmi et al 2018;Myeong et al 2018dMyeong et al , 2019Bignone et al 2019;Conroy et al 2019;Mackereth et al 2019), though in Kruijssen et al (2020) we find a mass M * ≈ 10 8.4 M . The median apocentre of G-E GCs (18 kpc) does not itself place a constraint on the galaxy mass, since such apocentres are achievable (Table 1) through both early, low-mass mergers (z M > 2, M * < 10 7.5 M ) and late, major mergers (z M < 0.5, M * > 10 9.5 M ).…”

“…Belokurov et al 2018Belokurov et al , 2020Deason et al 2018;Haywood et al 2018;Helmi et al 2018;Myeong et al 2018a,c,b,d Koppelman & Helmi 2019;Necib et al 2019Necib et al , 2020Vasiliev 2019). A major outcome of these works is that the outer stellar halo (>10 kpc) of the Milky Way appears to be dominated by the debris from a single merged satellite, the Gaia Sausage/Enceladus (G-E), which was accreted ∼9-10 Gyr ago (Belokurov et al 2018;Haywood et al 2018;Helmi et al 2018;Myeong et al 2018dMyeong et al , 2019Bignone, Helmi & Tissera 2019;Conroy et al 2019;Mackereth et al 2019;Kruijssen et al 2020). Along with G-E, the analysis of Gaia DR2 data has resulted in the discovery of a number of less massive substructures and the characterization of their progenitor galaxies (e.g.…”

The dual origin of the Galactic thick disc and halo from the gas-rich Gaia–Enceladus Sausage merger

“…They found also that the age of the Splash component is as old as the other more metal-poor halo stars, which are thought to have formed in smaller accreted galaxies, including the GES. Belokurov et al (2020) compared their observational data with the state-of-the-art cosmological numerical simulations of the Milky Way-like galaxies, AURIGA (Grand et al 2017(Grand et al , 2018b and LATTE (Sanderson et al 2020), and found evidence that the Splash component is produced from a merger that dynamically heats stars from the Milky Way proto-disc on to halo-like orbits (also Zolotov et al 2009;Font et al 2011;McCarthy et al 2012;Bignone, Helmi & Tissera 2019;Monachesi et al 2019).…”