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

TL;DR

This study uses N-body simulations to analyze the kinematic signatures of metal-poor stars in the Milky Way bulge, providing evidence that they are more likely associated with a thick disc rather than a classical bulge.

Contribution

It introduces a comparative analysis of kinematic signatures between different bulge formation models using N-body simulations and observational data.

Findings

Moderately metal-poor stars are consistent with a thick disc origin.

Kinematic signatures differ significantly between models with classical bulge and thick disc.

Observational data favor the thick disc model over the classical bulge scenario.

Abstract

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 ($\Delta\rm V_{\rm GC}$) between metal-rich stars ([Fe/H] $\ge$ 0) and moderately metal-poor stars (-1.0 $\le$ [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 $\Delta\rm V_{\rm GC}$ at different latitudes ($b=0^\circ$, $-2^\circ$, $-4^\circ$,$-6^\circ$, $-8^\circ$ and $-10^\circ$) and longitudes ($l=0^\circ$, $\pm5^\circ$, $\pm10^\circ$ and $\pm15^\circ$) and show that the $\Delta\rm V_{\rm GC}$ 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.