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

Authors :
Dietz, Sarah E.
Yoon, Jinmi
Beers, Timothy C.
Placco, Vinicius M.
Source :
Astrophysical Journal; May2020, Vol. 894 Issue 1, p1-10, 10p
Publication Year :
2020

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. [ABSTRACT FROM AUTHOR]

Details

Language :
English
ISSN :
0004637X
Volume :
894
Issue :
1
Database :
Complementary Index
Journal :
Astrophysical Journal
Publication Type :
Academic Journal
Accession number :
143141921
Full Text :
https://doi.org/10.3847/1538-4357/ab7fa4