Mapping the tilt of the Milky Way bulge velocity ellipsoids with ARGOS and $Gaia$ DR2

TL;DR

This study combines Gaia DR2 proper motions with ARGOS line-of-sight velocities to analyze the Milky Way bulge's velocity ellipsoids, revealing the bulge's non-axisymmetric structure and estimating its orientation angle.

Contribution

It provides the first large-scale kinematic analysis of the MW bulge using combined Gaia and ARGOS data, and measures the bulge's orientation angle independently from stellar kinematics.

Findings

The bulge's orientation angle is approximately 29 degrees.

Metal-rich stars show larger vertex deviations than metal-poor stars.

The velocity ellipsoid tilt indicates a non-axisymmetric, bar-like bulge structure.

Abstract

Until the recent advent of $Gaia$ Data Release 2 (DR2) and deep multi-object spectroscopy, it has been difficult to obtain 6-D phase space information for large numbers of stars beyond 4 kpc, in particular towards the Galactic centre, where dust and crowding effects are significant. In this study we combine line-of-sight velocities from the Abundances and Radial velocity Galactic Origins Survey (ARGOS) spectroscopic survey with proper motions from $Gaia$ DR2, to obtain a sample of $\sim$ 7,000 red clump stars with 3-D velocities. We perform a large scale stellar kinematics study of the Milky Way (MW) bulge to characterize the bulge velocity ellipsoids. We measure the tilt $l_{v}$ of the major-axis of the velocity ellipsoid in the radial-longitudinal velocity plane in 20 fields across the bulge. The tilt or vertex deviation, is characteristic of non-axisymmetric systems and a significant tilt is a robust indicator of non-axisymmetry or bar presence. We compare the observations to the predicted kinematics of an N-body boxy-bulge model formed from dynamical instabilities. In the model, the $l_{v}$ values are strongly correlated with the angle ($\alpha$) between the bulge major-axis and the Sun-Galactic centre line-of-sight. We use a maximum likelihood method to obtain an independent measurement of $\alpha$, from bulge stellar kinematics alone. The most likely value of $\alpha$ given our model is $\alpha = (29 \pm 3)^{\circ}$. In the Baade's window, the metal-rich stars display a larger vertex deviation ($l_{v} = -40^{\circ}$) than the metal-poor stars ($l_{v} = 10^{\circ}$) but we do not detect significant $l_{v}-$metallicity trends in the other fields.