Rigid rotation of the line of nodes and the Galactic warp

Ascertain whether the evolution of the line of nodes and of the entire Milky Way warp is characterized by rigid rotation by obtaining unbiased measurements of the pattern speed and amplitude evolution of the m = 2 (and higher) Fourier modes of the warp, which are currently not reliably constrained with the Cepheid data due to selection-function-induced biases.

Background

The authors measure a prograde pattern speed for the dominant m = 1 mode but cannot robustly recover the evolutionary terms for m = 2 because of selection function effects and azimuthal coverage limitations, particularly in velocity. Since the m = 2 mode significantly influences the line of nodes and the kinematic line of maximum vertical velocity, establishing the global rotational behavior (rigid vs. differential/complex) of the warp requires reliable m = 2 evolution.

Without accurate m = 2 measurements, the authors cannot confirm whether the line of nodes (and the warp as a whole) rotates rigidly, prompting the explicit unresolved question.