Evidence for two early accretion events that built the Milky Way stellar halo

By G. Myeong, Eugene O. Vasiliev, G. Iorio, N. Evans II, V. B. IoA, Cambridge, L. Institute, Moscow. Monthly notices of the Royal Astronomical Society, 2019

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The Gaia Sausage is the major accretion event that built the stellar halo of the Milky Way galaxy. Here, we provide dynamical and chemical evidence for a second substantial accretion episode, distinct from the Gaia Sausage. The Sequoia Event provided the bulk of the high-energy retrograde stars in the stellar halo, as well as the recently discovered globular cluster FSR 1758. There are up to six further globular clusters, including ω Centauri, as well as many of the retrograde substructures in Myeong et al., associated with the progenitor dwarf galaxy, named the Sequoia. The stellar mass in the Sequoia galaxy is ∼5 × 10 M⊙ , whilst the total mass is ∼1010 M⊙ , as judged from abundance matching or from the total sum of the globular cluster mass. Although clearly less massive than the Sausage, the Sequoia has a distinct chemodynamical signature. The strongly retrograde Sequoia stars have a typical eccentricity of ∼0.6, whereas the Sausage stars have no clear net rotation and move on predominantly radial orbits. On average, the Sequoia stars have lower metallicity by ∼0.3 dex and higher abundance ratios as compared to the Sausage. We conjecture that the Sausage and the Sequoia galaxies may have been associated and accreted at a comparable epoch.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/a0487e1ec9180009817123819c555c308e51c2c3

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Citations in the corpus

Evidence from the H3 Survey That the Stellar Halo Is Entirely Comprised of Substructure (2020, methodology): “Debris of the Sequoia dwarf galaxy (Myeong et al. 2018c, 2019; Matsuno et al. 2019) dominates the local retrograde halo at higher energies while Thamnos (Koppelman et al. 2019a) resides at lower energy.”
The biggest splash (2019, background): “At large negative velocities, i.e. vφ <−100 km s−1, an overdensity with −2 < [Fe/H] < −1.3 is visible, corresponding most likely to the debris from the so-called Sequoia event (see Myeong et al. 2019).”