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 ( ) and the “skewness” ( h 3 ) of the velocity distribution, possibly caused by the Galactic bar. The n = 2 GMM fitting reveals a symmetric longitudinal trend of ∣ μ 2 ∣ and σ 2 (the mean velocity and the standard deviation of the secondary component), which is inconsistent with the x 2 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.