Insight into the Galactic Bulge Chemodynamical Properties from Gaia DR3

Abstract

We explore the chemodynamical properties of the Galaxy in the azimuthal velocity $V_{\ddot{I}}•$ and metallicity [Fe/H] space using red giant stars from Gaia Data Release 3. The row-normalized $V_{\ddot{I}}•$-[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 ($\ddot{I}{ƒ}_{V_R}\sim 80$ $\mathrm{km}{\mathrm{s}}^{-1}$) and fast rotating ($V_{\ddot{I}}•\gtrsim 100$ $\mathrm{km}{\mathrm{s}}^{-1}$), and the Splash, which is dynamically hot ($\ddot{I}{ƒ}_{V_R}\sim 110$ $\mathrm{km}{\mathrm{s}}^{-1}$) and slow rotating ($V_{\ddot{I}}•\lesssim 100$ $\mathrm{km}{\mathrm{s}}^{-1}$). We compare the observations in the bulge region with an Auriga simulation where the last major merger event occurred $\sim 10$ Gyr ago: only stars born around the time of the merger reveal a Splash-like feature in the $V_{\ddot{I}}•$-[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.

ApJ accepted, 20 pages, 15 figures, comments welcome