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

Abstract

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 α 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 |Z| 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 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 [α/Fe]. Isochrone-based ages for main sequence turn-off stars reveal that these VMP stars are uniformly old (≈ 12 Gyr) irrespective of the α 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.

The origin of metal-poor stars on prograde disk orbits in FIRE simulations of Milky Way-mass galaxies

“…By selecting stars at higher vertical distance from the disk and without imposing a selection on 𝐽 𝑧 , we now select stars with orbits farther from the disk, which leads to a somewhat smaller prograde bias. Again, our simulation suite broadly agrees with MW observations from Carter et al (2020), although less well than with the results of Sestito et al (2020). At −2.5 < [Fe/H] < −1, the observations are consistent with being constant, but they show an upward trend at lower [Fe/H].…”

“…Recently, the H3 Spectroscopic Survey (Conroy et al 2019) also examined the orbits of metal-poor ([Fe/H] < −2) stars in the MW's halo (out of the disk; Carter et al 2020). They also found a strong prograde bias, with nearly 70 per cent of their stars on prograde orbits (defined as 𝐽 𝜙 > 0), in general agreement with (Sestito et al 2020), though Carter et al (2020) found some evidence that the prograde bias increases at lower [Fe/H] as well. Figure 3 (crosses with uncertainties) shows their results.…”

“…Figure 3 also shows the results from our simulation suite when applying a selection window and prograde bias metric similar to Carter et al (2020). Specifically, we select star particles above the disk, at 1 < |𝑍 | < 3 kpc, and at high Galactic latitude, |𝑏| > 30 • .…”

“…Specifically, we select star particles above the disk, at 1 < |𝑍 | < 3 kpc, and at high Galactic latitude, |𝑏| > 30 • . Following Carter et al (2020), we calculate the prograde bias as simply the ratio of the mass of star particles with 𝐽 𝜙 > 0 to those with 𝐽 𝜙 < 0. As in Figure 2, Figure 3 shows each of our hosts separately, and the thick black line shows the median.…”

“…Thus we investigate how the prograde bias at 𝑧 = 0 depends on stellar age, with no selection based on metallicity. We once again select stars as in Section 3.1.1, following the approach of Sestito et al ( 2020 broadly similar results following the methodology of Carter et al (2020) from the H3 Survey.…”