Understand
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 $\it{Gaia}$ that the nearby stellar halo is dominated by a massive accretion event -- referred to as $\it{Gaia-Enceladus}$ 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 $\mathrm{[Mg/Fe]}$ populations have $\alpha \sim 4$, whereas low [Mg/Fe] MAPs are more extended with shallow slopes, $\alpha \sim 2$.