Constraints on the Galactic Inner Halo Assembly History from the Age
  Gradient of Blue Horizontal-branch Stars
  We present an analysis of the relative age distribution of the Milky Way
halo, based on samples of blue horizontal-branch (BHB) stars obtained from the
Panoramic Survey Telescope and Rapid Response System and \textit{Galaxy
Evolution Explorer} photometry, as well a Sloan Digital Sky Survey
spectroscopic sample. A machine-learning approach to the selection of BHB stars
is developed, using support vector classification, with which we produce
chronographic age maps of the Milky Way halo out to 40\,kpc from the Galactic
center. We identify a characteristic break in the relative age profiles of our
BHB samples, corresponding to a Galactocentric radius of $R_{\rm{GC}} \sim
14$\,kpc. Within the break radius, we find an age gradient of $-63.4 \pm 8.2$
Myr kpc$^{-1}$, which is significantly steeper than obtained by previous
studies that did not discern between the inner- and outer-halo regions. The
gradient in the relative age profile and the break radius signatures persist
after correcting for the influence of metallicity on our spectroscopic
calibration sample. We conclude that neither are due to the previously
recognized metallicity gradient in the halo, as one passes from the inner-halo
to the outer-halo region. Our results are consistent with a dissipational
formation of the inner-halo population, involving a few relatively massive
progenitor satellites, such as those proposed to account for the assembly of
\textit{Gaia}-Enceladus, which then merged with the inner halo of the Milky
Way.
