The stellar mass in the halo of the Milky Way is notoriously difficult to determine , owing to the paucity of its stars in the solar neighbourhood .
With tentative evidence from Gaia that the nearby stellar halo is dominated by a massive accretion event – referred to as Gaia-Enceladus or Sausage – these constraints are now increasingly urgent .
We measure the mass in kinematically selected mono-abundance populations ( MAPs ) of the stellar halo between -3 < [ Fe/H ] < -1 and 0.0 < [ Mg/Fe ] < 0.4 using red giant star counts from APOGEE DR14 .
We find that MAPs are well fit by single power laws on triaxial ellipsoidal surfaces , and we show that that the power law slope \alpha changes such that high [ Mg/Fe ] populations have \alpha \sim 4 , whereas low [ Mg/Fe ] MAPs are more extended with shallow slopes , \alpha \sim 2 .
We estimate the total stellar mass to be M _ { *, \mathrm { tot } } = 1.3 ^ { +0.3 } _ { -0.2 } \times 10 ^ { 9 } \mathrm { M _ { \odot } } , of which we estimate \sim 0.9 ^ { +0.2 } _ { -0.1 } \times 10 ^ { 9 } \mathrm { M _ { \odot } } to be accreted .
We estimate that the mass of accreted stars with e > 0.7 is M _ { *, \mathrm { accreted } ,e > 0.7 } = 3 \pm 1 \mathrm { ( stat . ) } \pm 1 \mathrm { ( syst . ) } % \times 10 ^ { 8 } \mathrm { M _ { \odot } } , or \sim 30 - 50 \% of the accreted halo mass .
If the majority of these stars are the progeny of a massive accreted dwarf , this places an upper limit on its stellar mass , and implies a halo mass for the progenitor of \sim 10 ^ { 10.2 \pm 0.2 } \mathrm { M _ { \odot } } .
This constraint not only shows that the Gaia-Enceladus /Sausage progenitor may not be as massive as originally suggested , but that the majority of the Milky Way stellar halo was accreted .
These measurements are an important step towards fully reconstructing the assembly history of the Milky Way .