Insight into the Galactic Bulge Chemodynamical Properties from Gaia DR3

We explore the chemodynamical properties of the Galaxy in the azimuthal velocity V_ϕ and metallicity [Fe/H] space using red giant stars from Gaia Data Release 3. The row-normalized V_ϕ-[Fe/H] maps form a coherent sequence from the bulge to the outer disk, clearly revealing the thin/thick disk and the Splash. The metal-rich stars display bar-like kinematics while the metal-poor stars show dispersion-dominated kinematics. The intermediate-metallicity population (-1<[Fe/H]<-0.4) can be separated into two populations, one that is bar-like, i.e. dynamically cold (σ_V_R∼80 km s^-1) and fast rotating (V_ϕ≳100 km s^-1), and the Splash, which is dynamically hot (σ_V_R∼110 km s^-1) and slow rotating (V_ϕ≲100 km s^-1). We compare the observations in the bulge region with an Auriga simulation where the last major merger event occurred ∼10 Gyr ago: only stars born around the time of the merger reveal a Splash-like feature in the V_ϕ-[Fe/H] space, suggesting that the Splash is likely merger-induced, predominantly made-up of heated disk stars and the starburst associated with the last major merger. Since the Splash formed from the proto-disk, its lower metallicity limit coincides with that of the thick disk. The bar formed later from the dynamically hot disk with [Fe/H] >-1 dex, with the Splash not participating in the bar formation and growth. Moreover, with a set of isolated evolving N-body disk simulations, we confirm that a non-rotating classical bulge can be spun up by the bar and develop cylindrical rotation, consistent with the observation for the metal-poor stars.