# Evidence for Two Early Accretion Events That Built the Milky Way Stellar Halo

## TL;DR

This paper presents evidence for a second major accretion event, called the Sequoia, which contributed significantly to the Milky Way's stellar halo, distinct from the previously known Gaia Sausage event.

## Contribution

It provides dynamical and chemical evidence for the Sequoia event as a separate accretion episode, characterizing its properties and its role in galaxy formation.

## Key findings

- Sequoia contributed high-energy retrograde stars and globular clusters.
- Sequoia's stellar mass is about 5×10^7 solar masses, with a total mass around 10^10 solar masses.
- Sequoia stars have lower metallicity and distinct chemo-dynamical signatures compared to the Gaia Sausage.

## 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 6 further globular clusters, including $\omega$~Centauri, as well as many of the retrograde substructures in Myeong et al. (2018), associated with the progenitor dwarf galaxy, named the Sequoia. The stellar mass in the Sequoia galaxy is $\sim 5 \times 10^{7} M_\odot$, whilst the total mass is $\sim 10^{10} M_\odot$, 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 chemo-dynamical signature. The strongly retrograde Sequoia stars have a typical eccentricity of $\sim0.6$, whereas the Sausage stars have no clear net rotation and move on predominantly radial orbits. On average, the Sequoia stars have lower metallicity by $\sim 0.3$ 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.