Understanding the velocity distribution of the Galactic Bulge with APOGEE and Gaia

TL;DR

This study analyzes stellar velocity distributions in the Galactic bulge using APOGEE and Gaia data, revealing correlations and patterns consistent with a Galactic bar, and characterizing high-velocity stars' properties and origins.

Contribution

It applies Gauss-Hermite and Gaussian mixture models to velocity data, providing new insights into the kinematic structure and high-velocity peaks in the Galactic bulge.

Findings

Positive correlation between mean velocity and skewness linked to the Galactic bar

Symmetric trends in secondary component parameters inconsistent with x2 orbital predictions

High-velocity stars are generally old, with properties similar to other bulge stars

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 $|l|\sim6^\circ$. 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 ($V_{\rm R}-V_{\rm T}$), regardless of the existence of a distinct cold HV peak. The observed $V_{\rm R}-V_{\rm T}$ (or $V_{\rm GSR}-\mu_{l}$) 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.