Mapping the Milky Way with LAMOST - II. The stellar halo

The radial number density and flattening of the Milky Way's stellar halo is measured with 5351 metal-poor ([Fe/H] < -1) K giants from The Large sky Area Multi-Object fiber Spectroscopic Telescope Data Release 3 (LAMOST DR3), using a non-parametric method, which is model independent and largely avoids the influence of halo substucture. The number density profile is well described by a single power law with index 5.03(-0.64)(+0.64) and flattening that varies with radius. The stellar halo traced by LAMOST K giants is more flattened at smaller radii, and becomes nearly spherical at larger radii. The flattening, q, is about 0.64, 0.8 and 0.96 at r = 15, 20 and 30 kpc (where r = root R-2 + [Z/q (r)](2)), respectively. Moreover, the leading arm of the Sagittarius dwarf galaxy tidal stream in the north, and the trailing arm in the south, is significant in the residual map of density distribution. In addition, an unknown overdensity is identified in the residual map at (R, Z) = (30, 15) kpc.