Abstract

We revisit the stellar velocity distribution in the Galactic bulge/bar region with Apache Point Observatory Galactic Evolution Experiment DR16 and Gaia DR2, focusing in particular on the possible high-velocity (HV) peaks and their physical origin. We fit the velocity distributions with two different models, namely with Gauss-Hermite polynomials and Gaussian mixture models (GMMs). The result of the fit using Gauss-Hermite polynomials reveals a positive correlation between the mean velocity ( V ) and the "skewness"(h3) of the velocity distribution, possibly caused by the Galactic bar. The n = 2 GMM fitting reveals a symmetric longitudinal trend of |μ2| and σ2 (the mean velocity and the standard deviation of the secondary component), which is inconsistent with the x2 orbital family predictions. Cold secondary peaks could be seen at |l| ~ 6°. However, with the additional tangential information from Gaia, we find that the HV stars in the bulge show similar patterns in the radial-tangential velocity distribution (VR-VT), regardless of the existence of a distinct cold HV peak. The observed VR-VT (or VGSR-μl) distributions are consistent with the predictions of a simple Milky Way bar model. The chemical abundances and ages inferred from ASPCAP and CANNON suggest that the HV stars in the bulge/bar are generally as old as, if not older than, the other stars in the bulge/bar region.