Oral Presentation

The global structure of the Milky Way's stellar halo based on the orbits of local metal-poor stars

Author(s): Genta Sato (Tohoku University)

Presenter: Genta Sato (Tohoku University)

We analyze the global structure of the Milky Way (MW)'s stellar halo including its dominant subcomponent, Gaia-Sausage-Enceladus (GSE). The method to reconstruct the global distribution of this old stellar component is to employ the superposition of the orbits covering over the large MW's space, where each of the orbit-weighting factor is assigned following the probability that the star is located at its currently observed position. The selected local, metal-poor sample with [Fe/H] < -1 using Gaia EDR3 and SDSS DR16 shows that the global shape of the halo is systematically rounder at all radii in more metal-poor ranges, such that an axial ratio is nearly 1 for [Fe/H] < -2.2 and 0.7 for -1.4 < [Fe/H] < -1.0. It is also found that a halo in relatively metal-rich range of [Fe/H] > -1.8 actually shows a boxy/peanut-like shape. For the subsample of stars showing GSE-like kinematics and at [Fe/H] > -1.8, its global density distribution has the more spherical axial ratio about 0.9 than the general halo sample, with an outer ridge at 20 kpc. This spherical shape is consistent with the feature of accreted halo components and the ridge suggests that the orbit of GSE's progenitor has an apocenter of 20 kpc. Implications for the formation of the stellar halo are also presented.