The merger that led to the formation of the Milky Way’s inner stellar halo and thick disk

By A. Helmi, C. Babusiaux, H. Koppelman, D. Massari, J. Veljanoski, Anthony G. A. Brown. Nature, 2018

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The assembly of our Galaxy can be reconstructed using the motions and chemistry of individual stars1,2. Chemo-dynamical studies of the stellar halo near the Sun have indicated the presence of multiple components3, such as streams4 and clumps5, as well as correlations between the stars’ chemical abundances and orbital parameters6–8. Recently, analyses of two large stellar surveys9,10 revealed the presence of a well populated elemental abundance sequence7,11, two distinct sequences in the colour–magnitude diagram12 and a prominent, slightly retrograde kinematic structure13,14 in the halo near the Sun, which may trace an important accretion event experienced by the Galaxy15. However, the link between these observations and their implications for Galactic history is not well understood. Here we report an analysis of the kinematics, chemistry, age and spatial distribution of stars that are mainly linked to two major Galactic components: the thick disk and the stellar halo. We demonstrate that the inner halo is dominated by debris from an object that at infall was slightly more massive than the Small Magellanic Cloud, and which we refer to as Gaia–Enceladus. The stars that originate in Gaia–Enceladus cover nearly the full sky, and their motions reveal the presence of streams and slightly retrograde and elongated trajectories. With an estimated mass ratio of four to one, the merger of the Milky Way with Gaia–Enceladus must have led to the dynamical heating of the precursor of the Galactic thick disk, thus contributing to the formation of this component approximately ten billion years ago. These findings are in line with the results of galaxy formation simulations, which predict that the inner stellar halo should be dominated by debris from only a few massive progenitors2,16. A galaxy slightly larger than the Small Magellanic Cloud merged with the Milky Way about 10 billion years ago, thickening the ancient disk and forging the Galaxy’s inner halo.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/32f14e79d51a1125ce5b2196fa3f24e0ddebc370

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Citations in the corpus

Photo-astrometric distances, extinctions, and astrophysical parameters for Gaia DR2 stars brighter than G = 18 (2019, background): “…Zari et al. 2018; Baumgardt et al. 2019; Bossini et al. 2019; de Boer et al. 2019; Meingast & Alves 2019), the Galactic star-formation history (Helmi et al. 2018; Mor et al. 2019), hyper-velocity stars (e.g. Bromley et al. 2018; Scholz 2018; Shen et al. 2018; Boubert et al. 2018, 2019; Erkal…”
Evidence from the H3 Survey That the Stellar Halo Is Entirely Comprised of Substructure (2020, methodology): “Our estimate of GSE’s metallicity ([Fe/H]=- - +1.15 0.33 0.24, weighted) is ≈0.1–0.2 dex higher than most of the literature (e.g., Helmi et al. 2018; Mackereth et al. 2019; Matsuno et al. 2019; Sahlholdt et al. 2019; Vincenzo et al. 2019) and more in line with the recent [Fe/H]=−1.17±0.34 estimate…”
Proper motions and dynamics of the Milky Way globular cluster system fromGaiaDR2 (2018, background): “Gaia collaboration (Helmi et al. 2018, hereafter H18) determined the mean proper motions (PM) for 75 galactic globular clusters – roughly a half of the known population listed in the Harris (1996, 2010) catalogue, mostly lying within a heliocentric distance of 15 kpc and having a sufficient number…”
The biggest splash (2019, methodology): “Only at −0.9 < [Fe/H] < −0.7, the median [α/Fe] of the GS starts to decrease, diminishing slightly to 0.2– 0.25 in agreement with Helmi et al. (2018) and Mackereth et al. (2019).”
Evidence for two early accretion events that built the Milky Way stellar halo (2019, methodology): “Our hypothesis is distinct from the argumentation in Helmi et al. (2018), who made a broad selection based on the azimuthal action −1500 kpc km s−1 < Jφ < 150 kpc km s−1 only, as opposed to using additional integrals of motion.”