Ancient Very Metal-poor Stars Associated with the Galactic Disk in the H3 Survey
Date
2021
Authors
Carter, Courtney
Conroy, Charlie
Zaritsky, Dennis
Ting, Yuan-Sen
Bonaca, Ana
Naidu, Rohan P
Johnson, Benjamin D
Cargile, Phillip A
Caldwell, Nelson
Speagle, Joshua S
Journal Title
Journal ISSN
Volume Title
Publisher
Institute of Physics Publishing
Abstract
Ancient, very metal-poor (VMP) stars offer a window into the earliest epochs of galaxy formation and assembly. We combine data from the H3 Spectroscopic Survey and Gaia to measure metallicities, abundances of alpha elements, stellar ages, and orbital properties of a sample of 482 VMP ([Fe/H] < -2) stars in order to constrain their origins. This sample is confined to 1 less than or similar to divide Z divide less than or similar to 3 kpc from the Galactic plane. We find that >70% of VMP stars near the disk are on prograde orbits and this fraction increases toward lower metallicities. This result is unexpected if metal-poor stars are predominantly accreted from many small systems with no preferred orientation, as such a scenario would imply a mostly isotropic distribution. Furthermore, we find there is some evidence for higher fractions of prograde orbits among stars with lower [alpha/Fe]. Isochrone-based ages for main-sequence turn-off stars reveal that these VMP stars are uniformly old (12 Gyr) irrespective of the alpha abundance and metallicity, suggesting that the metal-poor population was not born from the same well-mixed gas disk. We speculate that the VMP population has a heterogeneous origin, including both in situ formation in the ancient disk and accretion from a satellite with the same direction of rotation as the ancient disk at early times. Our precisely measured ages for these VMP stars on prograde orbits show that the Galaxy has had a relatively quiescent merging history over most of cosmic time, and implies the angular momentum alignment of the Galaxy has been in place for at least 12 Gyr.
Description
Keywords
Citation
Collections
Source
The Astrophysical Journal
Type
Journal article
Book Title
Entity type
Access Statement
License Rights
Restricted until
2099-12-31
Downloads
File
Description