Evidence from the H3 Survey that the Stellar Halo is Entirely Comprised of Substructure (2006.08625v1)

Abstract: In the $\Lambda$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 $Gaia$, 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 $|b|>40^{\circ}$ and $|Z|>2$ kpc. We identify known structures including the high-$\alpha$ disk, the in-situ halo (disk stars heated to eccentric orbits), Sagittarius (Sgr), $Gaia$-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], [$\alpha$/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 $\lesssim$5$\%$ past 15 kpc. Beyond 15 kpc, $>$80$\%$ of the halo is built by two massive ($M_{\star}\sim10^{8}-10^{9}M_{\odot}$) 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 $\gtrsim$95$\%$ of the sample to one of the listed structures, pointing to a halo built entirely from accreted dwarfs and heating of the disk.

Overview of "The H3 Survey: Halo Substructure" by Naidu et al.

The paper "Evidence from the H3 Survey that the Stellar Halo is Entirely Comprised of Substructure" explores the intricate composition of the Milky Way's stellar halo, utilizing data from the H3 Spectroscopic Survey integrated with Gaia mission insights. The primary focus of the paper is to assess and categorize the distinct substructures within the stellar halo, refining our understanding of its assembly history.

Methodology and Data

The researchers employ data from the H3 Spectroscopic Survey, which provides detailed 6D phase-space information and chemical abundances for stars in the galaxy. The survey is aimed at high-latitude stars, specifically targeting giant stars beyond a distance of 2 kpc from the Earth. This approach mitigates the biases prevalent in other surveys which often rely on color selection that may preferentially target certain metallicities. The dataset comprises 5684 giant stars located at high Galactic latitudes, which are analyzed to decipher substructures within 50 kpc of the Galactic center.

Identification of Substructures

The paper identifies multiple known and novel substructures within the Galactic halo:

1. Known Structures:
   • The paper reconfirms the presence of several known streams and structures including the Sagittarius Stream (Sgr), Gaia-Sausage-Enceladus (GSE), Helmi Streams, Sequoia, Thamnos, and components of the disk such as the high-alpha disk and the in-situ halo.
2. Novel Discoveries:
   • Four new structures are introduced: Aleph, Arjuna, I'itoi, and Wukong. Each of these is characterized by unique kinematic and chemical signatures, with metallicities ranging from [Fe/H] = -0.5 (Aleph) to [Fe/H] < -2 (I'itoi).
3. Substructure Characteristics:
   • Aleph is a low-eccentricity structure extending up to 10 kpc off the plane, predominantly composed of stars with [Fe/H]= -0.5.
   • Arjuna, Sequoia, I'itoi, and Wukong are described as parts of the high-energy retrograde halo with distinct chemical compositions.

Implications and Composition Analysis

The research claims that the halo is almost entirely composed of accreted structures and heated disk components. A significant portion of the stellar halo within 15 kpc is found to be formed in-situ (~60%), but this declines drastically to less than 5% beyond 15 kpc, where accreted materials dominate. The findings challenge the traditional dual-halo concept, suggesting the halo's assembly results from the accretion of two substantial dwarf galaxies, GSE and Sgr, alongside numerous smaller systems.

Conclusions and Future Perspectives

The paper provides an extensive inventory of Galactic substructures, supporting the hypothesis that the halo is largely built from accreted systems. These findings have profound implications on the theories of galaxy formation and evolution, particularly in understanding the role of hierarchical assembly in shaping the halo. Speculatively, future developments might focus on further characterizing these substructures' origins and fully utilizing the H3 data in conjunction with forthcoming Gaia data releases to improve the spatial and kinematic mapping of the outer halo regions.

By meticulously mapping and analyzing these components, the paper enriches our comprehension of the Milky Way's formation narrative and sets a robust foundation for future explorations into the Galactic archeology and kinematic history.