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

Abstract

The assembly process of our Galaxy can be retrieved using the motions and chemistry of individual stars. 1, 2 Chemo-dynamical studies of the nearby halo have long hinted at the presence of multiple components such as streams, 3 clumps, 4 duality 5 and correlations between the stars' chemical abundances and orbital parameters. 6,7,8 More recently, the analysis of two large stellar surveys 9, 10 have revealed the presence of a well-populated chemical elemental abundance sequence, 7, 11 of two distinct sequences in the colour-magnitude diagram, 12 and of a prominent slightly retrograde kinematic structure 13, 14 all in the nearby halo, which may trace an important accretion event experienced by the Galaxy. 15 Here report an analysis of the kinematics, chemistry, age and spatial distribution of stars in a relatively large volume around the Sun 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 which at infall was slightly more massive than the Small Magellanic Cloud, and which we refer to as Gaia-Enceladus. The stars originating in Gaia-Enceladus cover nearly the full sky, their motions reveal the presence of streams and slightly retrograde and elongated trajectories. Hundreds of RR Lyrae stars and thirteen globular clusters following a consistent age-metallicity relation can be associated to Gaia-Enceladus on the basis of their orbits. With an estimated 4:1 mass-ratio, the merger with Gaia-Enceladus must have led to the dynamical heating of the precursor of the Galactic thick disk and therefore contributed to the formation of this component approximately 10 Gyr ago. These findings are in line with simulations of galaxy formation, which predict that the inner stellar halo should be dominated by debris from just a few massive progenitors. 2,16 The sharp view provided by the second data release (DR2) of the Gaia mission, 17 has recently revealed 14 that, besides a few tight streams, a significant fraction of the halo stars

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

“…We conclude that, for this sample of MW-like analogs, GE/GS-like mergers seem to provide a more minor contribution to the stellar halo than that inferred for the case of the MW. Our theoretical results are therefore better aligned with other estimates that place the GS as a non-dominant contributor (perhaps lower end of the modeling in Helmi et al 2018;Mackereth & Bovy 2019).…”

“…6 shows a wide range of metallicities associated to stars in a single satellite. This range overlaps well with the estimated metallicity of GE/GS, log([Fe/H]) ∼ −1.5 (Helmi et al 2018). We find no significant segregation with present-day distance of the stars (see color coding as in Fig.…”

“…4 makes a direct comparison of the debris of our four selected ARMs in Fig. 3, using the same identification space of the GE (top, Helmi et al 2018) and the GS (bottom, Belokurov et al 2018). All stars brought in by satellites S1-S4 are colored by their galactocentric distance today.…”

“…The inner regions of our halo, on the other hand, are characterized by shorter dynamical times, ≤ 100 Myr, requiring full phase-space information to identify the debris of those once coherent structures . The availability of superb data from Gaia DR2 (Gaia Collaboration et al 2018), including positions and velocities for millions of stars, unveiled the remnant of one of the largest accretion events in the history of our Galaxy, the Gaia Enceladus (GE, Helmi et al 2018) or Gaia Sausage (GS, Belokurov et al 2018) event.…”

“…It is nonetheless evident that there is a large degree of overlap in the stars belonging to GS and GE (Myeong et al 2019;Matsuno et al 2019;Koppelman et al 2019). The motion of GE/GS stars is largely eccentric, with radial anisotropy 0.8 < β < 0.9 , as well as counterrotating (Helmi et al 2018), both characteristics enhancing the profile of GE/GS as the result of a past accretion event.…”