The Origin of the Milky Way's Halo Age Distribution

TL;DR

This study analyzes simulated Milky Way-like stellar halos to understand their age gradients, revealing that accretion of small satellites primarily shapes the observed age distribution and supports hierarchical galaxy formation models.

Contribution

It demonstrates that the Milky Way's halo age distribution can be explained by accretion of small satellite galaxies with minimal in situ star formation, aligning simulations with observations.

Findings

Simulated halos show diverse age gradients, often shallower than observed.

Accreted components exhibit steeper age gradients closer to observational data.

Old stars are concentrated in the halo centers, matching the Ancient Chronographic Sphere.

Abstract

We present an analysis of the radial age gradients for the stellar halos of five Milky Way mass-sized systems simulated as part of the Aquarius Project. The halos show a diversity of age trends, reflecting their different assembly histories. Four of the simulated halos possess clear negative age gradients, ranging from approximately -7 to -19 Myr/kpc , shallower than those determined by recent observational studies of the Milky Way's stellar halo. However, when restricting the analysis to the accreted component alone, all of the stellar halos exhibit a steeper negative age gradient with values ranging from $-$8 to $-$32~Myr/kpc, closer to those observed in the Galaxy. Two of the accretion-dominated simulated halos show a large concentration of old stars in the center, in agreement with the Ancient Chronographic Sphere reported observationally. The stellar halo that best reproduces the current observed characteristics of the age distributions of the Galaxy is that formed principally by the accretion of small satellite galaxies. Our findings suggest that the hierarchical clustering scenario can reproduce the MW's halo age distribution if the stellar halo was assembled from accretion and disruption of satellite galaxies with dynamical masses less than ~10^9.5M_sun, and a minimal in situ contribution.