The Halo's Ancient Metal-Rich Progenitor Revealed with BHB Stars

TL;DR

This study uses BHB stars from SDSS and Gaia data to reveal that the Milky Way's stellar halo is dominated by metal-rich debris from an ancient collision, with a mixture model showing the Sausage component's varying contribution across radii.

Contribution

The paper introduces a flexible mixture model to analyze the halo's composition, revealing the dominance of the Gaia Sausage debris within 25 kpc and its decline beyond 30 kpc.

Findings

The halo is largely unmixed and cannot be represented by a Gaussian velocity distribution.

The Gaia Sausage contributes at least 50% of the halo within 25 kpc.

The Sausage's contribution drops sharply beyond 30 kpc.

Abstract

Using the data from the Sloan Digital Sky Survey and the Gaia satellite, we assemble a pure sample of $\sim$3000 Blue Horizontal Branch (BHB) stars with 7-D information, including positions, velocities and metallicities. We demonstrate that, as traced with BHBs, the Milky Way's stellar halo is largely unmixed and can not be well represented with a conventional Gaussian velocity distribution. A single-component model fails because the inner portions of the halo are swamped with metal-rich tidal debris from an ancient, head-on collision, known as the "Gaia Sausage". Motivated by the data, we build a flexible mixture model which allows us to track the evolution of the halo make-up across a wide range of radii. It is built from two components, one representing the radially anisotropic Sausage stars with their lobed velocity distribution, the other representing a more metal-poor and more isotropic component built up from minor mergers. We show that inside 25 kpc the "Sausage" contributes at least 50 % of the Galactic halo. The fraction of "Sausage" stars diminishes sharply beyond 30 kpc, which is the long-established break radius of the classical stellar halo.