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 APOGEE DR16 and {\it 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 polynomial and Gaussian mixture model (GMM). The result of the fit using Gauss-Hermite polynomials reveals a positive correlation between the mean velocity ($\bar{V}$) 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 $|{\mu }_2|$ and ${\sigma }_2$ (the mean velocity and the standard deviation of the secondary component), which is inconsistent to the $x_2$ orbital family predictions. Cold secondary peaks could be seen at $|l|\sim 6^{\circ }$. However, with the additional tangential information from {\it Gaia}, we find that the HV stars in the bulge show similar patterns in the radial-tangential velocity distribution ($V_{\mathrm{R}}-V_{\mathrm{T}}$), regardless of the existence of a distinct cold HV peak. The observed $V_{\mathrm{R}}-V_{\mathrm{T}}$ (or $V_{\mathrm{GSR}}-{\mu }_l$) distributions are consistent with the predictions of a simple MW 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.

17 pages,13 figures

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• Towards Understanding the Milky Way's Typicality: Assessing the Chemodynamics of M31's Bulge & Bar, Thick & Thin Discs
• Self-consistent modelling of the Milky Way's Nuclear Stellar Disc