We characterize the Gaia-Enceladus-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 Gaia-Enceladus-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 Gaia-Enceladus-Sausage stars in other kinematic/action spaces without additional population information leads to contaminated samples. The clean Gaia-Enceladus-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 Gaia-Enceladus-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 Gaia-Enceladus-Sausage stars.

中文翻译：

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盖亚-土卫二-香肠的 SkyMapper-Gaia RVS 视图——对银河系最后一次重大合并的金属丰度和质量的调查

我们使用来自 SkyMapper 调查的光度金属丰度和来自 Gaia 径向速度测量的运动学来表征最近在银河晕中发现的 Gaia-Enceladus-Sausage 运动学结构。通过检查运动学/动作空间中的恒星金属丰度分布函数 (MDF)，我们发现 $\sqrt{J_R}$ 与 $L_z$ 空间允许最干净地选择 Gaia-Enceladus-Sausage 恒星，同时将来自在原位或其他过去的合并中形成的圆盘或晕星。星级 $30 \leq \sqrt{J_R} \leq 50$ (kpc km s$^{-1})^{1/2}$ 和 $-500 \leq L_z \leq 500$ kpc km s$^{- 1}$ 具有以 [Fe/H] $= -1.17$ dex 为中心的窄 MDF，分散度为 0.34 dex。这个 [Fe/H] 估计比文献估计更富含金属 0.1-0.3 美元 dex。基于 MDF，我们发现在其他运动学/动作空间中选择 Gaia-Enceladus-Sausage 星而没有额外的种群信息会导致样本受到污染。根据我们的标准选择的干净 Gaia-Enceladus-Sausage 样本略微逆行，位于高 $V_T$ halo CMD 双序列的蓝色序列。使用源自宇宙学模拟的星系质量-金属丰度关系并假设平均恒星年龄为 10 Gyr，我们估计 Gaia-Enceladus-Sausage 祖先卫星的质量为 $10^{8.85-9.85}$M$_{\odot} $，这与基于光盘动态和模拟的文献估计一致。为了更复杂地选择纯盖亚-土卫二-香肠恒星，需要更多关于详细丰度和年龄的信息。