We map the stellar structure of the Galactic thick disk and halo by applying color-magnitude diagram ( CMD ) fitting to photometric data from the SEGUE survey .
The SEGUE imaging scans allow , for the first time , a comprehensive analysis of Milky Way structure at both high and low latitudes using uniform SDSS photometry .
Incorporating photometry of all relevant stars simultaneously , CMD fitting bypasses the need to choose single tracer populations .
Using old stellar populations of differing metallicities as templates we obtain a sparse three-dimensional map of the stellar mass distribution at |Z| > 1 kpc .
Fitting a smooth Milky Way model comprising exponential thin and thick disks and an axisymmetric power-law halo allows us to constrain the structural parameters of the thick disk and halo .
The thick-disk scale height and length are well constrained at 0.75 \pm 0.07 kpc and 4.1 \pm 0.4 kpc , respectively .
We find a stellar halo flattening within \sim 25 kpc of c / a = 0.88 \pm 0.03 and a power-law index of 2.75 \pm 0.07 ( for 7 kpc \lesssim R _ { GC } \lesssim 30 kpc ) .
The model fits yield thick-disk and stellar halo densities at the solar location of \rho _ { thick, \sun } = 10 ^ { -2.3 \pm 0.1 } M _ { \sun } pc ^ { -3 } and \rho _ { halo, \sun } = 10 ^ { -4.20 \pm 0.05 } M _ { \sun } pc ^ { -3 } , averaging over any substructures .
Our analysis provides the first clear in situ evidence for a radial metallicity gradient in the Milky Way ’ s stellar halo : within R \lesssim 15 kpc the stellar halo has a mean metallicity of [ Fe / H ] \simeq - 1.6 , which shifts to [ Fe / H ] \simeq - 2.2 at larger radii , in line with the two-component halo deduced by ( 11 ) from a local kinematic analysis .
Subtraction of the best-fit smooth and symmetric model from the overall density maps reveals a wealth of substructures at all latitudes , some attributable to known streams and overdensities , and some new .
A simple warp can not account for the low latitude substructure , as overdensities occur simultaneously above and below the Galactic plane .