Mapping the tilt of the Milky Way bulge velocity ellipsoids with ARGOS and Gaia DR2
Abstract
Until the recent advent of Gaia Data Release 2 (DR2) and deep multi-object spectroscopy, it has been difficult to obtain 6D phase space information for large numbers of stars beyond 4 kpc, in particular towards the Galactic Centre, where dust and crowding are significant. We combine line-of-sight velocities from the Abundances and Radial velocity Galactic Origins Survey (ARGOS) with proper motions from Gaia DR2 to obtain a sample of ~7000 red clump stars with 3D velocities. We perform a large-scale stellar kinematics study of the Milky Way bulge to characterize the bulge velocity ellipsoids in 20 fields. The tilt of the major-axis of the velocity ellipsoid in the radial-longitudinal velocity plane, 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, performing a robust error analysis. The most likely value of α given our model is α = (29 ± 3)○, with an additional systematic uncertainty due to comparison with one specific model. In 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.
- Publication:
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Monthly Notices of the Royal Astronomical Society
- Pub Date:
- April 2021
- DOI:
- arXiv:
- arXiv:2011.13905
- Bibcode:
- 2021MNRAS.502.1740S
- Keywords:
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- Galaxy: bulge;
- Galaxy: kinematics and dynamics;
- Galaxy: structure;
- galaxies: individual: Milky Way;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 13 pages, 11 figures