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

Iulia T. Simion, Juntai Shen, Sergey E. Koposov, Melissa Ness, Kenneth Freeman, Jonathan Bland-Hawthorn, Geraint F. Lewis

Research output: Contribution to journalArticlepeer-review

Abstract / Description of output

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.
Original languageEnglish
Pages (from-to)1740-1752
Number of pages13
JournalMonthly Notices of the Royal Astronomical Society
Volume502
Issue number2
Early online date14 Jan 2021
DOIs
Publication statusPublished - 1 Apr 2021

Keywords / Materials (for Non-textual outputs)

  • astro-ph.GA

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