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The Shape and Profile of the Milky Way Halo as Seen by the CFHT Legacy Survey

We use Canada-France-Hawaii Telescope Legacy Survey data for 170 deg^2, recalibrated and transformed to the Sloan Digital Sky Survey ugri photometric system, to study the distribution of near-turnoff main-sequence stars in the Galactic halo along four lines of sight to heliocentric distances of ~35 kpc. We find that the halo stellar number density profile becomes steeper at Galactocentric distances greater than R_{gal}~28 kpc, with the power law index changing from n_{inner}=-2.62+-0.04 to n_{outer}=-3.8+-0.1. In particular, we test a series of single power law models and find them to be strongly disfavored by the data. The parameters for the best-fit Einasto profile are n=2.2+-0.2 and R_e=22.2+-0.4 kpc. We measure the oblateness of the halo to be q=c/a=0.70+-0.01 and detect no evidence of it changing across the range of probed distances. The Sagittarius stream is detected in the l=173 and b=-62 direction as an overdensity of [Fe/H] ~ -1.5 dex stars at R_{gal}~32 kpc, providing a new constraint for the Sagittarius stream and dark matter halo models. We also detect the Monoceros stream as an overdensity of [Fe/H] > -1.5 dex stars in the l=232 and b=26 direction at R_{gal}<25 kpc. In the two sightlines where we do not detect significant substructure, the median metallicity is found to be independent of distance within systematic uncertainties ([Fe/H] ~ -1.5+-0.1 dex).