We present an analysis of the large-scale structure of the halo of the Andromeda galaxy , based on the Pan-Andromeda Archeological Survey ( PAndAS ) , currently the most complete map of resolved stellar populations in any galactic halo .
Despite the presence of copious substructure , the global halo populations follow closely power law profiles that become steeper with increasing metallicity .
We divide the sample into stream-like populations and a smooth halo component ( defined as the population that can not be resolved into spatially distinct substructure with PAndAS ) .
Fitting a three-dimensional halo model reveals that the most metal-poor populations ( { [ Fe / H ] < -1.7 } ) are distributed approximately spherically ( slightly prolate with ellipticity c / a = 1.09 \pm 0.03 ) , with only a relatively small fraction residing in discernible stream-like structures ( f _ { stream } = 42 \% ) .
The sphericity of the ancient smooth component strongly hints that the dark matter halo is also approximately spherical .
More metal-rich populations contain higher fractions of stars in streams , with f _ { stream } becoming as high as 86 % for { [ Fe / H ] > -0.6 } .
The space density of the smooth metal-poor component has a global power-law slope of \gamma = -3.08 \pm 0.07 , and a non-parametric fit shows that the slope remains nearly constant from 30 { kpc } to \sim 300 { kpc } .
The total stellar mass in the halo at distances beyond 2 ^ { \circ } is \sim 1.1 \times 10 ^ { 10 } { M _ { \odot } } , while that of the smooth component is \sim 3 \times 10 ^ { 9 } { M _ { \odot } } .
Extrapolating into the inner galaxy , the total stellar mass of the smooth halo is plausibly \sim 8 \times 10 ^ { 9 } { M _ { \odot } } .
We detect a substantial metallicity gradient , which declines from \langle [ Fe / H ] \rangle = -0.7 at R = 30 { kpc } to \langle [ Fe / H ] \rangle = -1.5 at R = 150 { kpc } for the full sample , with the smooth halo being \sim 0.2 Â dex more metal poor than the full sample at each radius .
While qualitatively in-line with expectations from cosmological simulations , these observations are of great importance as they provide a prototype template that such simulations must now be able to reproduce in quantitative detail .