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

Understand

Recent studies using Gaia DR2 have identified a massive merger in the history of the Milky Way (MW) whose debris is markedly radial and counterrotating.

• This event, known as the Gaia-Enceladus/Gaia-Sausage (GE/GS), is also hypothesized to have built the majority of the inner stellar halo.
• We use the cosmological hydrodynamic simulation Illustris to place this merger in the context of galaxy assembly within $\Lambda$CDM.
• From $\sim$150 MW analogs, $\sim 80 \%$ have experienced at least one merger of similar mass and infall time as GE/GS.