Understanding the Velocity Distribution of the Galactic Bulge with APOGEE and Gaia

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 ( $\,\overline{V}$ ) 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 (V_R-V_T), regardless of the existence of a distinct cold HV peak. The observed V_R-V_T (or V_GSR-μ_l) 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.