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

Although there is consensus that metal-rich stars in the Milky Way 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 metal-poor 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.