More than just a wrinkle: a wave-like pattern in U_g versus L_z from Gaia data

Abstract

We present a newly found wave-like pattern in mean Galactocentric radial velocity {\overline{U}_ 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 ϕ, which is different for different peaks. The peak around L_z∼ 2100 kpc km s^{-1} only shows a weak change with ϕ, 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 {\overline{W}} versus L_z pattern detected in Gaia DR1. A comparison of the {\overline{U}_ g}-L_z wave pattern with changes of {\overline{U}_ g} versus R, which have been previously discussed, suggests that the latter can be understood as just the {\overline{U}_ g}-L_z pattern washed out by blurring (i.e. orbital excursions around their guiding centre) of disc stars.