Chronography of the Milky Way's Halo System with Field Blue
  Horizontal-Branch Stars
  In a pioneering effort, Preston et al. reported that the colors of blue
horizontal-branch (BHB) stars in the halo of the Galaxy shift with distance,
from regions near the Galactic center to about 12 kpc away, and interpreted
this as a correlated variation in the ages of halo stars, from older to
younger, spanning a range of a few Gyrs. We have applied this approach to a
sample of some 4700 spectroscopically confirmed BHB stars selected from the
Sloan Digital Sky Survey to produce the first "chronographic map" of the halo
of the Galaxy. We demonstrate that the mean de-reddened g$-$r color,
<(g$-$r)o>, increases outward in the Galaxy from $-$0.22 to $-$0.08 (over a
color window spanning [$-$0.3:0.0]) from regions close to the Galactic center
to ~40 kpc, independent of the metallicity of the stars. Models of the expected
shift in the color of the field BHB stars based on modern stellar evolutionary
codes confirm that this color gradient can be associated with an age difference
of roughly 2-2.5 Gyrs, with the oldest stars concentrated in the central ~15
kpc of the Galaxy. Within this central region, the age difference spans a mean
color range of about 0.05 mag (~0.8 Gyrs). Furthermore, we show that
chronographic maps can be used to identify individual substructures, such as
the Sagittarius Stream, and overdensities in the direction of Virgo and
Monoceros, based on the observed contrast in their mean BHB colors with respect
to the foreground/background field population.
