The Spatial Distribution of the Galactic First Stars. II. Smoothed Particle Hydrodynamics Approach

We use cosmological, chemodynamical, SPH simulations of Milky Way-analog galaxies to find the expected present-day distributions of both metal-free stars that formed from primordial gas and the oldest star populations. We find that metal-free stars continue to form until z ~ 4 in halos that are chemically isolated and located far away from the biggest progenitor of the final system. As a result, if the Population III initial mass function allows stars with low enough mass to survive until z = 0 (<0.8 M☉), they would be distributed throughout the Galactic halo. On the other hand, the oldest stars form in halos that collapsed close to the highest density peak of the final system, and at z = 0 they are located preferentially in the central region of the Galaxy, i.e., in the bulge. According to our models, these trends are not sensitive to the merger histories of the disk galaxies or the implementation of supernova feedback. Furthermore, these full hydrodynamics results are consistent with our N-body results in Paper I and lend further weight to the conclusion that surveys of low-metallicity stars in the Galactic halo can be used to directly constrain the properties of primordial stars. In particular, they suggest that the current lack of detections of metal-free stars implies that their lifetimes were shorter than a Hubble time, placing constraints on the metal-free initial mass function.