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 \sim 4000 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 .