Mapping the Milky Way with LAMOST II: the stellar halo

Understand

The radial number density and flattening of the Milky Way's stellar halo is measured with $\mathrm{5351}$ metal-poor ([Fe/H]$<-1$) K giants from LAMOST DR3, using a nonparametric 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, 0.96 at $r=15$, 20 and 30 kpc (where $r=\sqrt{R^2+\left[Z/q\left(r\right)\right]^2}$), respectively.