Evidence from the H3 Survey that the Stellar Halo is Entirely Comprised of Substructure

Rohan P. Naidu; Charlie Conroy; Ana Bonaca; Benjamin D. Johnson; Yuan-Sen Ting; Nelson Caldwell; Dennis Zaritsky; Phillip A. Cargile

doi:10.3847/1538-4357/abaef4

paper

Evidence from the H3 Survey that the Stellar Halo is Entirely Comprised of Substructure

arXiv:2006.08625 · doi:10.3847/1538-4357/abaef4

Abstract

In the $Λ$ CDM paradigm the Galactic stellar halo is predicted to harbor the accreted debris of smaller systems. To identify these systems, the H3 Spectroscopic Survey, combined with $G aia$ , is gathering 6D phase-space and chemical information in the distant Galaxy. Here we present a comprehensive inventory of structure within 50 kpc from the Galactic center using a sample of 5684 giants at $∣ b ∣ > 4 0^{\circ}$ and $∣ Z ∣ > 2$ kpc. We identify known structures including the high- $α$ disk, the in-situ halo (disk stars heated to eccentric orbits), Sagittarius (Sgr), $G aia$ -Sausage-Enceladus (GSE), the Helmi Streams, Sequoia, and Thamnos. Additionally, we identify the following new structures: (i) Aleph ([Fe/H] $= - 0.5$ ), a low eccentricity structure that rises a surprising 10 kpc off the plane, (ii, iii) Arjuna ([Fe/H] $= - 1.2$ ) and I'itoi ([Fe/H] $< - 2$ ), which comprise the high-energy retrograde halo along with Sequoia, and (iv) Wukong ([Fe/H] $= - 1.6$ ), a prograde phase-space overdensity chemically distinct from GSE. For each structure we provide [Fe/H], [ $α$ /Fe], and orbital parameters. Stars born within the Galaxy are a major component at $∣ Z ∣ \sim$ 2 kpc ( $\approx$ 60 $%$ ), but their relative fraction declines sharply to $≲$ 5 $%$ past 15 kpc. Beyond 15 kpc, $>$ 80 $%$ of the halo is built by two massive ( $M_{⋆} \sim 1 0^{8} - 1 0^{9} M_{⊙}$ ) accreted dwarfs: GSE ([Fe/H] $= - 1.2$ ) within 25 kpc, and Sgr ([Fe/H] $= - 1.0$ ) beyond 25 kpc. This explains the relatively high overall metallicity of the halo ([Fe/H] $\approx - 1.2$ ). We attribute $≳$ 95 $%$ of the sample to one of the listed structures, pointing to a halo built entirely from accreted dwarfs and heating of the disk.

Submitted to ApJ. Key results in Figures 18-21. Summary of individual structures in Sec. 3.3 and Table 1. Comments very welcome!

Evidence from the H3 Survey that the Stellar Halo is Entirely Comprised of Substructure

Abstract

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