A-type stars in the Canada-France Imaging Survey I. The stellar halo of
  the Milky Way traced to large radius by blue horizontal branch stars
  We present the stellar density profile of the outer halo of the Galaxy traced
over a range of Galactocentric radii from $15< R_{GC} < 220$ kpc by blue
horizontal branch (BHB) stars. These stars are identified photometrically using
deep $u-$band imaging from the new Canada-France-Imaging-Survey (CFIS) that
reaches 24.5 mag. This is combined with $griz$ bands from Pan-STARRS 1 and
covers a total of $\sim4000$ deg$^2$ of the northern sky. We present a new
method to select BHB stars that has low contamination from blue stragglers and
high completeness. We use this sample to measure and parameterize the three
dimensional density profile of the outer stellar halo. We fit the profile using
(i) a simple power-law with a constant flattening (ii) a flattening that varies
as a function of Galactocentric radius (iii) a broken power law profile. We
find that outer stellar halo traced by the BHB is well modelled by a broken
power law with a constant flattening of $q=0.86 \pm 0.02$, with an inner slope
of $\gamma=4.24 \pm 0.08$. This is much steeper than the preferred outer
profile that has a slope of $\beta=3.21\pm 0.07$ after a break radius of
$r_b=41.4^{+2.5}_{-2.4}$ kpc. The outer profile of the stellar halo trace by
BHB stars is shallower than that recently measured using RR Lyrae, a surprising
result given the broad similarity of the ages of these stellar populations.
