The Metallicity Gradient and Complex Formation History of the Outermost Halo of the Milky Way

TL;DR

This study analyzes the metallicity distribution of the Milky Way's outer halo, revealing a negative gradient and supporting hierarchical formation, with distinct properties for retrograde and prograde stars linked to different accretion events.

Contribution

It provides the first detailed metallicity gradient measurement of the outer halo using large spectroscopic and photometric samples, supporting hierarchical galaxy formation models.

Findings

Negative metallicity gradient detected beyond 35 kpc

High fraction (~60%) of very metal-poor stars in the outer halo

Distinct metallicity behaviors for retrograde and prograde stars

Abstract

We present an examination of the metallicity distribution function of the outermost stellar halo of the Galaxy based on an analysis of both local (within 4 kpc of the Sun, ~16,500 stars) and non-local (~21,700 stars) samples. These samples were compiled using spectroscopic metallicities from the Sloan Digital Sky Survey and photometric metallicities from the SkyMapper Southern Survey. We detect a negative metallicity gradient in the outermost halo (r > 35 kpc from the Galactic center), and find that the frequency of very metal-poor ([Fe/H] < -2.0) stars in the outer-halo region reaches up to ~60% in our most distant sample, commensurate with previous theoretical predictions. This result provides clear evidence that the outer-halo formed hierarchically. The retrograde stars in the outermost halo exhibit a roughly constant metallicity, which may be linked to the accretion of the Sequoia progenitor. In contrast, prograde stars in the outermost halo exhibit a strong metallicity-distance dependence, indicating that they likely originated from the accretion of galaxies less massive than the Sequoia progenitor galaxy.