# The intermediate age population of the Galactic halo

## TL;DR

This paper investigates the age composition of the Milky Way's inner halo using Gaia and other photometric data, revealing the absence of a billion-year-old stellar population and setting constraints on the halo's formation history.

## Contribution

It introduces a method to constrain the age distribution of the galactic halo using Gaia AGB stars and photometry, providing new insights into the halo's merger history.

## Key findings

- Inner halo lacks a billion-year-old stellar population.
- Gaia AGB stars effectively trace the age distribution.
- Constraints on the timing of past merger events.

## Abstract

We have learned recently that the inner halo of the Milky Way contains a kinematically coherent component (Gaia-Enceladus) from a significant merger 10 Gyrs ago. By contrast the inner halo (defined to exclude the Magellanic Stream) contains no similar intruder stellar population of billion year age. The tracer we use to set the corresponding upper limit is Gaia asymptotic giant branch stars, rather than Gaia kinematics. The primary sample is drawn from Gaia DR2 with SkyMapper photometry. This is supplemented with PanSTARRS and 2MASS photometry. As the Gaia mission proceeds, a star formation history in the galactic halo should emerge.