The Galactic disc 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 $V_{\Phi }$ and $V_R$ in the vertical position-velocity plane $Z$-$V_Z$ across the Galactic disc of $6.34\lesssim R\lesssim 12.34$\,kpc and $|\Phi |\lesssim 7.5^{\circ }$ 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 $V_{\Phi }$ become more and more relaxed as $R$ increases. The spiral patterns of $V_{\Phi }$ and $V_R$ and the quadrupole patterns of $V_R$ are strongest at $-2^{\circ }<\Phi <2^{\circ }$ but negligible at $4^{\circ }<\Phi <6^{\circ }$ and $-6^{\circ }<\Phi <-4^{\circ }$. 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 towards 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 $V_{\Phi }$ and $V_R$.

arXiv comment6pages, 4 figures, Accepted to ApJL

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