2020ApJ...897..119W

By combining LAMOST DR4 and Gaia DR2 common red clump stars with age and proper motion, we analyze the amplitude evolution of the stellar warp independently of any assumption with a simple model. The greatest height of the warp disk increases with galactocentric distance in different populations and is dependent on age: the younger stellar populations exhibit stronger warp features than the old ones, accompanied by the warp amplitude γ (age) decreasing with age, and its first derivative {dot}γ(age) is different from zero. The azimuth of the line of nodes ph_w is stable at -5° without clear time evolution, which perfectly confirms some previous works. All of this self-consistent evidence supports that our Galactic warp should most likely be a long-lived but nonsteady structure and not a transient one, which is supporting that the warp originated from gas infall onto the disk or other hypotheses that suppose that the warp mainly affects the gas, and consequently, younger populations tracing the gas are stronger than older ones. In other words, the Galactic warp is induced by the nongravitational interaction over the disk models.