The Galactic Disk Phase Spirals at Different Galactic Positions Revealed by Gaia and LAMOST Data

Abstract

We have investigated the distributions of stellar azimuthal and radial velocity components V Φ and V R in the vertical position–velocity plane Z – V Z across the Galactic disk of 6.34 ≲  R  ≲ 12.34 kpc and using a Gaia and Gaia -LAMOST sample of stars. As found in previous works, the distributions exhibit significant spiral patterns. The V R distributions also show clear quadrupole patterns, which are the consequence of the well-known tilt of the velocity ellipsoid. The observed spiral and quadrupole patterns in the phase space plane vary strongly with radial and azimuthal positions. The phase spirals of V Φ become more and more relaxed as R increases. The spiral patterns of V Φ and V R and the quadrupole patterns of V R are strongest at −2° < Φ < 2° but negligible at 4° < Φ < 6° and −6° < Φ < −4°. Our results suggest an external origin of the phase spirals. In this scenario, the intruder, most likely the previously well-known Sagittarius dwarf galaxy, passed through the Galactic plane in the direction toward either Galactic center or anti-center. The azimuthal variations of the phase spirals also help us constrain the passage duration of the intruder. A detailed model is required to reproduce the observed radial and azimuthal variations of the phase spirals of V Φ and V R .