The age structure of the Milky Way's halo

TL;DR

This paper creates a detailed age map of the Milky Way's halo using 130,000 BHB stars, revealing an older central region and a gradient of decreasing stellar ages outward, consistent with cosmological models.

Contribution

It provides the first high-resolution age map of the Milky Way's halo based on BHB stars, highlighting the dual halo structure and associated substructures.

Findings

Inner halo stars are older than 12 Gyr.

Stellar ages decrease with distance from the Galactic center.

Detected over-densities and streams support a dual halo model.

Abstract

We present a new, high-resolution chronographic (age) map of the Milky Way's halo, based on the inferred ages of ~130,000 field blue horizontal-branch (BHB) stars with photometry from the Sloan Digital Sky Survey. Our map exhibits a strong central concentration of BHB stars with ages greater than 12 Gyr, extending up to ~15 kpc from the Galactic center (reaching close to the solar vicinity), and a decrease in the mean ages of field stars with distance by 1-1.5 Gyr out to ~45-50 kpc, along with an apparent increase of the dispersion of stellar ages, and numerous known (and previously unknown) resolved over-densities and debris streams, including the Sagittarius Stream. These results agree with expectations from modern LambdaCDM cosmological simulations, and support the existence of a dual (inner/outer) halo system, punctuated by the presence of over-densities and debris streams that have not yet completely phase-space mixed.