Searching for a kinematic signature of the moderately metal-poor stars in the Milky Way bulge using N-body simulations

Abstract

Although there is consensus that metal-rich stars in the MilkyWay bulge are formed via secular evolution of the thin disc, the origin of their metal-poor counterparts is still under debate. Two different origins have been invoked for metal-poor stars: they might be classical bulge stars or stars formed via internal evolution of a massive thick disc.We use N-body simulations to calculate the kinematic signature given by the difference in the mean Galactocentric radial velocity (δVGC) between metal-rich stars ([Fe/H] ≥ 0) and moderately metalpoor stars (-1.0 ≤ [Fe/H] < 0) in two models, one containing a thin disc and a small classical bulge (B/D = 0.1), and the other containing a thin disc and a massive centrally concentrated thick disc.We reasonably assume that thin-disk stars in each model may be considered as a proxy of metal-rich stars. Similarly, bulge stars and thick-disc stars may be considered as a proxy of metal-poor stars.We calculate δVGC at different latitudes (b = 0°, -2°, -4°, -6°, -8° and -10°) and longitudes (l = 0°, ±5°, ±10° and ±15°) and show that the δVGC trends predicted by the two models are different. We compare the predicted results with ARGOS data and APOGEE DR13 data and show that moderately metal-poor stars are well reproduced with the co-spatial stellar discs model, which has a massive thick disc. Our results give more evidence against the scenario that most of the metal-poor stars are classical bulge stars. If classical bulge stars exists, most of them probably have metallicities [Fe/H] < -1 dex, and their contribution to the mass of the bulge should be a small percentage of the total bulge mass.