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 ∼ 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 lv 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 lv values are strongly correlated with the angle
(α) 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 α, from bulge stellar kinematics alone. The most likely
value of α given our model is α=(29±3)∘. In the
Baade's window, the metal-rich stars display a larger vertex deviation (lv=−40∘) than the metal-poor stars (lv=10∘) but we do not
detect significant lv−metallicity trends in the other fields.Comment: 13 pages, 11 figure