Reliability and limitations of inferring birth radii in the Milky Way disk

TL;DR

This study assesses the reliability of inferring stars' birth radii in the Milky Way using metallicity relations, finding high accuracy only during early disk formation and increased uncertainties due to galactic dynamics.

Contribution

It tests the validity of a common assumption in Galactic Archaeology using cosmological simulations, revealing when and how the method is reliable.

Findings

Precise birth radii can be inferred only during early disk formation (~10 Gyr ago).

The metallicity-radius correlation strengthens over time as stellar motions become more ordered.

Galactic features like bars and mergers increase uncertainty in birth radius estimates.

Abstract

Recovering the birth radii of observed stars in the Milky Way is one of the ultimate goals of Galactic Archaeology. One method to infer the birth radius and the evolution of the ISM metallicity assumes a linear relation between the ISM metallicity with radius at any given look-back time. Here we test the reliability of this assumption by using 4 zoom-in cosmological hydrodynamic simulations from the NIHAO-UHD project. We find that one can infer precise birth radii only when the stellar disk starts to form, which for our modeled galaxies happens ~ 10 Gyr ago, in agreement with recent estimates for the Milky Way. At later times the linear correlation between the ISM metallicity and radius increases, as stellar motions become more ordered and the azimuthal variations of the ISM metallicity start to drop. The formation of a central bar and perturbations from mergers can increase this uncertainty in the inner and outer disk, respectively.