Formation of the Galactic bulge from a two-component stellar disc: explaining cylindrical rotation and a vertical metallicity gradient

Recent observational studies have revealed that the Galactic bulge has cylindrical rotation and a steeper vertical metallicity gradient. We adopt two representative models for the bulge formation and thereby investigate whether the two models can explain both the observed cylindrical rotation and vertical metallicity gradient in a self-consistent manner. One is the 'pure disc scenario' (PDS) in which the bulge is formed from a pure thin stellar disc through spontaneous bar instability. The other is the 'two-component disc scenario' (TCDS) in which the bulge is formed from a disc composed of thin and thick discs through bar instability. Our numerical simulations show that although the PDS can reproduce the cylindrical rotation, it shows a rather flatter vertical metallicity gradient that is inconsistent with observations. The derived flatter metallicity gradient is due to the vertical mixing of stars with different initial metallicities by the stellar bar. This result implies that the bulge cannot be simply formed from a pure thin stellar disc. On the other hand, the bulge formed from the two-component disc in the TCDS can explain both the observed cylindrical rotation and vertical metallicity gradient of the Galactic bulge reasonably well. In the TCDS, more metal-poor stars at higher vertical bar z vertical bar (vertical distance), which originate from the already dynamically hotter thick disc, cannot be strongly influenced by vertical mixing of the bar so that they can stay in situ for longer time-scales and thus keep the lower metallicity at higher vertical bar z vertical bar. Consequently, the vertical metallicity gradient of the bulge composed of initially thin and thick disc stars cannot be so flattened, even if the gradient of the thin disc can be flattened significantly by the bar in the TCDS. We therefore suggest that a significant fraction of the present Galactic bulge is composed of stars initially in the inner part of the thick disc and thus that these bulge stars and the thick disc have a common origin. We also suggest that the Galaxy might well have experienced some merger events that could dynamically heat up its inner regions until similar to 10 Gyr ago.