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

Abstract 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…

• ES
  European Space Agency
• RS
  Royal Society
• SF
  Seventh Framework Programme

  Award: FP/2007–2013
• SA
  Science and Technology Facilities Council

  Award: ST/N000927/1
• ER
  European Research Council

  Award: 308024