Formation of the Metal-Poor Disk in a Milky Way-like Galaxy

We explore the properties of a metal-poor disk in a simulation of the Milky Way (MW) to gain insight into current observations. We first performed a cosmological zoom-in simulation of a MW-like galaxy using N-body/hydrodynamic simulation code ASURA with gas mass resolution of ~10⁴ M_Sun. Our analysis of rotation velocity (V_Phi) versus metallicity ([Fe/H]) showed that beyond ~6 kpc from the Galactic center, the disk-like distributions extend into the very metal-poor region ([Fe/H] < -2.5), consistent with the metal-weak thick-disk (MWTD) population in the MW. Our analysis of the age distributions show that although the most metal-poor stars are very old, those with V_Phi > 150 kms^-1 have a bimodal age distribution, including young (< 6 Gyr) as well as very old (> 12 Gyr) stars. By comparing Toomre diagrams for stars of differing age and V_Phi, we found disk-like structures for both old and young stars. These results indicate that the MWTD includes stars that formed in the early epochs of disk formation, supplemented by stars that were accreted from dwarf satellites with extended star formation.