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 in the range 15 < RGC < 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 ̃4000 deg2 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 parametrize 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 ± 0.02, with an inner slope of γ = 4.24 ± 0.08. This is much steeper than the preferred outer profile that has a slope of β = 3.21 ± 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