Resolving the Metallicity Distribution of the Stellar Halo with the H3 Survey

TL;DR

The H3 Survey provides an unbiased, detailed view of the Galactic stellar halo's metallicity distribution, revealing a structured halo with distinct components and supporting an accretion-based formation history.

Contribution

This study presents the first nearly unbiased, high-resolution metallicity and orbital analysis of the stellar halo out to 100 kpc using the H3 Survey data.

Findings

The stellar halo has a mean metallicity of [Fe/H]=-1.2 with no gradient from 6 to 100 kpc.

The halo's metallicity distribution is highly structured with distinct metal-rich and metal-poor components.

Most of the halo formed through accretion and tidal disruption of dwarf galaxies.

Abstract

The Galactic stellar halo is predicted to have formed at least partially from the tidal disruption of accreted dwarf galaxies. This assembly history should be detectable in the orbital and chemical properties of stars. The H3 Survey is obtaining spectra for 200,000 stars, and, when combined with Gaia data, is providing detailed orbital and chemical properties of Galactic halo stars. Unlike previous surveys of the halo, the H3 target selection is based solely on magnitude and Gaia parallax; the survey therefore provides a nearly unbiased view of the entire stellar halo at high latitudes. In this paper we present the distribution of stellar metallicities as a function of Galactocentric distance and orbital properties for a sample of 4232 kinematically-selected halo giants to 100 kpc. The stellar halo is relatively metal-rich, [Fe/H]=-1.2, and there is no discernable metallicity gradient over the range $6<R_{\rm gal}<100$ kpc. However, the halo metallicity distribution is highly structured including distinct metal-rich and metal-poor components at $R_{\rm gal}<10$ kpc and $R_{\rm gal}>30$ kpc, respectively. Metal-poor stars with [Fe/H]$<-2$ are a small population of the halo at all distances and orbital categories. We associate the "in-situ" stellar halo with stars displaying thick-disk chemistry on halo-like orbits; such stars are confined to $|z|<10$ kpc. The majority of the stellar halo is resolved into discrete features in orbital-chemical space, suggesting that the bulk of the stellar halo formed from the accretion and tidal disruption of dwarf galaxies. (ABRIDGED)