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We characterize the Gaia-Enceladus-Sausage kinematic structure recentlydiscovered in the Galactic halo using photometric metallicities from theSkyMapper survey, and kinematics from Gaia radial velocities measurements. Byexamining the metallicity distribution functions (MDFs) of stars binned inkinematic/action spaces, we find that the $\sqrt{J_R}$ vs $L_z$ space allowsfor the cleanest selection of Gaia-Enceladus-Sausage stars with minimalcontamination 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 $-500 \leqL_z \leq 500$ 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 thanliterature estimates by $0.1-0.3$ dex. Based on the MDFs, we find thatselection of Gaia-Enceladus-Sausage stars in other kinematic/action spaceswithout additional population information leads to contaminated samples. Theclean Gaia-Enceladus-Sausage sample selected according to our criteria isslightly retrograde and lies along the blue sequence of the high $V_T$ halo CMDdual sequence. Using a galaxy mass-metallicity relation derived fromcosmological simulations and assuming a mean stellar age of 10 Gyr we estimatethe mass of the Gaia-Enceladus-Sausage progenitor satellite to be$10^{8.85-9.85}$ M$_{\odot}$, which is consistent with literature estimatesbased on disc dynamic and simulations. Additional information on detailedabundances and ages would be needed for a more sophisticated selection ofpurely Gaia-Enceladus-Sausage stars.

The SkyMapper-Gaia RVS view of the Gaia-Enceladus-Sausage -- an investigation of the metallicity and mass of the Milky Way's last major merger