The Tilt of the Velocity Ellipsoid in the Milky Way Disk with GALAH–Gaia Data

We investigate the metallicity dependence of the tilt of the velocity ellipsoid using a sample of over 572,000 stars from GALAH and Gaia. We find that the tilt angles of disk stars can be well described by the relation α = α 0 arctan ( Z / R ) , and the α 0 differ significantly among different disk populations, with α 0 are 0.78 ± 0.05, 0.86 ± 0.07, and 0.77 ± 0.07 for the whole sample, the low-[ α /Fe] and the high-[ α /Fe] populations, respectively. These results indicate that Galactic disk populations are globally tilted toward cylindrical alignment, implying that the gravitational potential of the Galactic baryonic disk plays a dominant role in shaping the kinematics of dynamically relaxed stars. Our detailed analysis further reveals a metallicity dependence of α 0 for various populations, with the low-[ α /Fe] and the high-[ α /Fe] populations are respectively α 0 = 0.12 sin (3.72 [Fe/H] + 1.09) + 0.75 and α 0 = 0.20 sin (2.99 [Fe/H] + 1.90) + 0.71. These results indicate that the metallicity dependence observed in the low-[ α /Fe] population may be primarily associated with resonances induced by the Galactic bar, while the behavior of the high-[ α /Fe] population may also have been affected by additional dynamical and evolutionary processes, such as violent heating associated with mergers, accretion, and/or the formation of these stars in an environment shaped by mergers of gas-rich systems and/or a turbulent ISM.