Ancient Very Metal-Poor Stars Associated With the Galactic Disk in the H3 Survey

Ancient, very metal-poor 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 very metal-poor (VMP; [Fe/H]$<-2$) stars in order to constrain their origins. This sample is confined to $1\lesssim |Z| \lesssim3$ 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 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 amongst stars with lower [$\alpha$/Fe]. Isochrone-based ages for main sequence turn-off stars reveal that these VMP stars are uniformly old ($\approx12$ 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.
Comment: 9 pages, 4 figures, accepted to ApJ