2019MNRAS.490.5414F

We present a newly found wave-like pattern in mean Galactocentric radial velocity _ g_ versus guiding centre radius R_g or angular momentum L_z of stars in the RV subsample of Gaia DR2. The short-wave pattern has a wavelength of order 1.2 kpc in R_g or 285 kpc km s^–1 in L_z. The pattern shows only weak changes with Galactocentric radius R and little change in strength in particular with the vertical energy E_z of the stars or the distance to the Galactic plane |z|. The pattern is to first order symmetric around the plane, i.e. has no significant odd terms in z. There is a weak phase shift with the pattern moving towards slightly lower L_z (i.e. trailing) with |z| and E_z. However, we observe a highly significant phase shift in Galactic azimuth ph, which is different for different peaks. The peak around L_z∼2100 kpc km s^–1 only shows a weak change with ph, while the rest of the pattern shows a clearly detectable shift of d L_z/ d φ= (200 ±22) kpc km s^–1 rad^–1. If we consider all peaks to belong to the same pattern, this would suggest a wavenumber m = 4. We further find that the wave-like pattern in U_g appears to be related to the  versus L_z pattern detected in Gaia DR1. A comparison of the _ g_–Lz_ wave pattern with changes of _ g_ versus R, which have been previously discussed, suggests that the latter can be understood as just the _ g_–Lz_ pattern washed out by blurring (i.e. orbital excursions around their guiding centre) of disc stars.