Gaia kinematics reveal a complex lopsided and twisted Galactic disc warp

Abstract

Context. There are few warp kinematic models of the Galaxy able to characterise both structure and kinematics, since these require high accuracy at large distances. These models are necessary to shed light on the lopsidedness of the warp and the twisting of the line-of-nodes of the stellar warp already seen in gas and dust. Aims. We use the vertical information coming from the Gaia Data Release 2 astrometric data up to G = 20 mag to characterise the structure of the Galactic warp, the related vertical motions, and the dependency of Galactic warp on age. Methods. We analyse two populations up to Galactocentric distances of 16 kpc: a young bright sample mainly formed by OB stars and an older one of red giant branch (RGB) stars. We use two methods (the pole count maps of great circle bands and Galactic longitude – proper motion in latitude lines) based on the Gaia observables, together with 2D projections of the positions and proper motions in the Galactic plane. Results. This work confirms the age dependency of the Galactic warp, both in position and kinematics, the height of the Galactic warp being of the order of 0.2 kpc for the OB sample and 1.0 kpc for the RGB at a Galactocentric distance of 14 kpc. Both methods find that the onset radius of the warp is 12 ∼ 13 kpc for the OB sample and 10 ∼ 11 kpc for the RGB. From the RGB sample, we find from Galactocentric distances larger than 10 kpc that the line-of-nodes twists away from the Sun-anticentre line towards Galactic azimuths ≈180−200° increasing with radius, though possibly influenced by extinction. Also, the RGB sample reveals a slightly lopsided stellar warp with ≈250 pc difference between the up and down sides. The line of maximum of proper motions in latitude is systematically offset from the line-of-nodes estimated from the spatial data, which our warp models predict as a kinematic signature of lopsidedness. We also show a prominent wave-like pattern of a bending mode different in the OB and RGB samples. Both positions and kinematics also reveal substructures that might not be related to the large-scale Galactic warp or to the bending mode. Conclusions. Gaia Data Release 2 data reveals a high degree of complexity in terms of both position and velocity that triggers the need for complex kinematic models flexible enough to combine both wave-like patterns and an S-shaped lopsided warp.