Three‐dimensional Simulations of the Chemical and Dynamical Evolution of the Galactic Bulge

Abstract

A three-dimensional, hydrodynamical, N-body model for the formation of the Galaxy is presented, with special attention paid to the formation of the bulge component. Since none of the previous numerical models for the Galaxy's formation have a proper treatment of the chemical evolution and/or sufficient spatial resolutions, we have constructed a detailed model of the chemical and dynamical evolution of the Galaxy using our GRAPE smoothed particle hydrodynamics (SPH) code. Our SPH code includes various physical processes related to the formation of stellar systems. Starting with cosmologically motivated initial conditions, we obtain a stellar system qualitatively similar to the Galaxy. Then we analyze the chemical and kinematic properties of the bulge stars in our model and find qualitative agreement with observational data. The early evolution of our model has revealed that most bulge stars form during the subgalactic merger (merger component of the bulge stars). Because of the strong starburst induced by the merger, the metallicity distribution function of such stars becomes as wide as observed. We find that another group of the bulge stars forms later in the inner region of the disk (nonmerger component of the bulge stars). Because of the difference in the formation epoch, the main source of iron for this group of stars is different from that for the merger component. Iron in the merger and nonmerger components comes mainly from Type II and Type Ia supernovae, respectively. Since a Type Ia supernova ejects ~10 times more iron than a Type II supernova, [Fe/H] of the nonmerger component tends to be higher than that of the merger component, which widens the metallicity distribution function. From these results, we suggest that the Galactic bulge consists of two chemically different components; one has formed quickly through the subgalactic clump merger in the proto-Galaxy, and the other has formed gradually in the inner disk.