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 H3 Spectroscopic Survey Gaia to measure metallicities, abundances $\alpha$ elements, stellar ages, orbital properties sample 482 (VMP; [Fe/H]$<-2$) in order constrain their origins. This is confined $1\lesssim |Z| \lesssim3$ kpc Galactic plane. find that >70% VMP near disk are on prograde orbits this fraction increases toward lower metallicities. result unexpected if predominantly accreted many small systems with no preferred orientation, as such scenario would imply mostly isotropic distribution. Furthermore, we there some evidence for higher fractions amongst [$\alpha$/Fe]. Isochrone-based ages main sequence turn-off reveal these uniformly old ($\approx12$ Gyr) irrespective abundance metallicity, suggesting population was not born same well-mixed gas disk. speculate has heterogeneous origin, including both in-situ ancient accretion satellite direction rotation at early times. Our precisely measured show Galaxy had relatively quiescent merging history over most cosmic time, implies angular momentum alignment been place least 12 Gyr.