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

In hierarchical structure formation, metal-poor stars in and around the Milky Way (MW) originate primarily from mergers of lower mass galaxies. A common expectation is therefore that metal-poor stars should have isotropic, dispersion-dominated orbits that do not correlate strongly with the MW disc. However, recent observations of stars in the MW show that metal-poor ([Fe/H] less than or similar to -2) stars are preferentially on prograde orbits with respect to the disc. Using the Feedback In Realistic Environments 2 (FIRE-2) suite of cosmological zoom-in simulations of MW/M31-mass galaxies, we investigate the prevalence and origin of prograde metal-poor stars. Almost all (11 of 12) of our simulations have metal-poor stars on preferentially prograde orbits today and throughout most of their history: we thus predict that this is a generic feature of MW/M31-mass galaxies. The typical prograde-to-retrograde ratio is similar to 2:1, which depends weakly on stellar metallicity at [Fe/H] less than or similar to -1. These trends predicted by our simulations agree well with MW observations. Prograde metal-poor stars originate largely from a single Large/Small Magellanic Cloud (LMC/SMC)-mass gas-rich merger ago, which deposited existing metal-poor stars and significant gas on an orbital vector that sparked the formation of and/or shaped the orientation of a long-lived stellar disc, giving rise to a prograde bias for all low-metallicity stars. We find subdominant contributions from in situ stars formed in the host galaxy before this merger, and in some cases, additional massive mergers. We find few clear correlations between any properties of our MW/M31-mass galaxies at z = 0 and the degree of this prograde bias as a result of diverse merger scenarios.

Automated summary

Metal-poor stars in and around the Milky Way are found to preferentially have prograde orbits, likely originating from a previous gas-rich merger involving the Large/Small Magellanic Clouds. This prograde bias is predicted to be a common feature of MW/M31-mass galaxies, as observed in cosmological simulations. The prograde metal-poor stars are believed to have been influenced by the gas deposition from the merger event, shaping the orientation of a long-lived stellar disc within the galaxies.