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

• Published in: The Astrophysical journal Vol. 908; no. 1; pp. 21 - 37
• Main Authors: Zhou, Yingying, Li, Zhao-Yu, Simion, Iulia T., Shen, Juntai, Mao, Shude, Liu, Chao, Jian, Mingjie, Fernández-Trincado, José G.
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01.02.2021; IOP Publishing
• Subjects: Astrophysics; Galactic bulge; Galactic evolution; Hermite polynomials; Milky Way; Milky Way dynamics; Probabilistic models; Stars; Stellar age; Velocity; Velocity distribution
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/abd181

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" (h3) of the velocity distribution, possibly caused by the Galactic bar. The n = 2 GMM fitting reveals a symmetric longitudinal trend of 2 and 2 (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- 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.