ABSTRACT
We present a newly found wave-like pattern in mean Galactocentric radial velocity ${\overline{U}_\mathrm{ g}}$ versus guiding centre radius Rg or angular momentum Lz of stars in the RV subsample of Gaia DR2. The short-wave pattern has a wavelength of order $1.2 \, {\rm kpc}$ in Rg or $285 \, {\rm kpc}\, {\rm km}\, {\rm s}^{-1}$ in Lz. The pattern shows only weak changes with Galactocentric radius R and little change in strength in particular with the vertical energy Ez 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 Lz (i.e. trailing) with |z| and Ez. However, we observe a highly significant phase shift in Galactic azimuth ϕ, which is different for different peaks. The peak around $L_{z}\sim 2100 \, {\rm kpc}\, {\rm km}\, {\rm s}^{-1}$ only shows a weak change with ϕ, while the rest of the pattern shows a clearly detectable shift of $\text{d} L_{z}/\text{d} \phi = (200 \pm 22) \, {\rm kpc}\, {\rm km}\, {\rm s}^{-1}\, {\rm rad}^{-1}$. 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 Ug appears to be related to the ${\overline{W}}$ versus Lz pattern detected in Gaia DR1. A comparison of the ${\overline{U}_\mathrm{ g}}\!-\!L_{z}$ wave pattern with changes of ${\overline{U}_\mathrm{ g}}$ versus R, which have been previously discussed, suggests that the latter can be understood as just the ${\overline{U}_\mathrm{ g}}\!-\!L_{z}$ pattern washed out by blurring (i.e. orbital excursions around their guiding centre) of disc stars.
More than just a wrinkle: a wave-like pattern in Ug versus Lz from Gaia data