The Global Structure of the Milky Way’s Stellar Halo Based on the Orbits of Local Metal-poor Stars

Sato, Genta; Chiba, Masashi

doi:10.3847/1538-4357/ac47fb

The Global Structure of the Milky Way’s Stellar Halo Based on the Orbits of Local Metal-poor Stars

Journal Article · Thu Mar 10 00:00:00 EST 2022 · The Astrophysical Journal

DOI:https://doi.org/10.3847/1538-4357/ac47fb· OSTI ID:1983226

^[1]; ^[1]

Tohoku Univ., Sendai (Japan)

We analyze the global structure of the Milky Way (MW)'s stellar halo, including its dominant subcomponent, Gaia-Sausage-Enceladus (GSE). The method for reconstructing the global distribution of this old stellar component is to employ the superposition of the orbits covering the large MW's space, where each of the orbit-weighting factors 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 Early Data Release 3 and Sloan Digital Sky Survey Data Release 16, shows that the global shape of the halo is systematically rounder at all radii in more metal-poor ranges, such that an axial ratio, q, 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 the relatively metal-rich range of [Fe/H] >–1.8 actually shows a boxy/peanut-like shape, suggesting a major merger event. The distribution of azimuthal velocities shows a disk-like flattened structure at –1.4 < [Fe/H] < –1.0, which is thought to be the metal-weak thick disk. For the subsample of stars showing GSE-like kinematics, at [Fe/H] >–1.8, its global density distribution has an axis ratio of 0.9, which is more spherical than the general halo sample, and an outer ridge at r ~ 20 kpc. This spherical shape is consistent with the features of accreted halo components, and the ridge suggests that the orbit of GSE's progenitor had an apocenter of ~20 kpc. Implications for the formation of the stellar halo are also presented.

View Accepted Manuscript (DOE)

Research Organization:: US Department of Energy (USDOE), Washington, DC (United States). Office of Science, Sloan Digital Sky Survey (SDSS)

Sponsoring Organization:: Japan Society for the Promotion of Science (JSPS); USDOE Office of Science (SC)

OSTI ID:: 1983226

Journal Information:: The Astrophysical Journal, Journal Name: The Astrophysical Journal Journal Issue: 2 Vol. 927; ISSN 0004-637X

Publisher:: IOP PublishingCopyright Statement

Country of Publication:: United States

Language:: English

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