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

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.