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

摘要

Recent studies using Gaia DR2 have identified a massive merger in the early history of the Milky Way (MW) whose debris is dominated by radial and counterrotating orbits. This event, dubbed 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 cold dark matter. From similar to 150 MW analogues, similar to 80 per cent have experienced at least one merger of similar mass and infall time as the GE/GS. Within this sample, 37 have debris as radial as the GE/GS, which we dub the ancient radial mergers (ARMs). Counterrotation is not rare among ARMs, with 43 per cent having > 40 per cent of their debris in counterrotating orbits. However, the compactness inferred for the GE/GS debris given its large radial orbital anisotropy, beta, and its substantial contribution to the stellar halo are difficult to reproduce. The median radius of ARM debris is r(*,deb) similar or equal to 45 kpc, while GE/GS is thought to be mostly contained within r similar to 30 kpc. For most MW analogues, few mergers are required to build the inner stellar halo, and ARM debris only accounts for (median) similar to 12 per cent of inner accreted stars. Encouragingly, we find one ARM that is both compact and dominates the inner halo of its central, making it our best GE/GS analogue. Interestingly, this merger deposits a significant number of stars (M* similar or equal to 1.5 x 10(9) M-circle dot) in the outer halo, suggesting that an undiscovered section of GE/GS may await detection.