Insights into the Galactic Bulge Chemodynamical Properties from Gaia Data Release 3

Abstract

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 ( ${\sigma }_{{V}_{R}}\sim 80$ km s^‑1) and fast-rotating (V _ϕ ≳ 100 km s^‑1), and the Splash, which is dynamically hot ( ${\sigma }_{{V}_{R}}\sim 110$ 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 nonrotating classical bulge can be spun up by the bar and develop cylindrical rotation, consistent with the observations for the metal-poor stars.