We characterize the Gaia -Sausage kinematic structure recently discovered in the Galactic halo using photometric metallicities from the SkyMapper survey , and kinematics from Gaia radial velocities measurements .
By examining the metallicity distribution functions ( MDFs ) of stars binned in kinematic/action spaces , we find that the \sqrt { J _ { R } } vs L _ { z } space allows for the cleanest selection of Sausage stars with minimal contamination from disc or halo stars formed in situ or in other past mergers .
Stars with 30 \leq \sqrt { J _ { R } } \leq 50 ( kpc km s ^ { -1 } ) ^ { 1 / 2 } and -500 \leq L _ { z } \leq 500 kpc km s ^ { -1 } have a narrow MDF centered at [ Fe/H ] = -1.17 dex with a dispersion of 0.34 dex .
This [ Fe/H ] estimate is more metal-rich than literature estimates by 0.1 - 0.3 dex .
Based on the MDFs , we find that selection of Sausage stars in other kinematic/action spaces without additional population information leads to contaminated samples .
The clean Sausage sample selected according to our criteria is slightly retrograde and lies along the blue sequence of the high V _ { T } halo CMD dual sequence .
Using a galaxy mass-metallicity relation derived from cosmological simulations and assuming a mean stellar age of 10 Gyr we estimate the mass of the Sausage progenitor satellite to be 10 ^ { 8.85 - 9.85 } M _ { \odot } , which is consistent with literature estimates based on disc dynamic and simulations .
Additional information on detailed abundances and ages would be needed for a more sophisticated selection of purely Sausage stars .