  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.
