The Galactic Disk in Action Space as seen by Gaia

Wilma Trick

MPA Garching

The second Gaia data release from April 2018 has revealed an astounding amount of non-axisymmetric stellar substructure in the Milky Way. In this talk, I will present the orbit structure of the Galactic disk that consistently reaches out to 1.5 kpc from the Sun, revealed in the space of orbital actions (JR,Lz,Jz) of Gaia DR2 stars as at least seven overdensities along lines of constant slope JR vs. Lz. The known moving groups in the Solar neighbourhood's velocity space are the local manifestation of this extended orbit structure. This kind of substructure is expected to originate from dynamical resonance effects with the spiral arms and the central bar in the Milky Way. I present some very recent, preliminary results based on a numerical simulation that show that actions, angles, and frequencies estimated in an axisymmetric gravitational potential model can still be informative about the resonance mechanism even if our Galaxy is clearly not axisymmetric. I show that the Gaia action data favours two models for the bar pattern speed ("slow" 33 km/s/kpc and "fast" 51 km/s/kpc) that are consistent with existing bar models from modeling the inner Galaxy and the local Hercules stream, respectively. The orbital angles will help in the future to discriminate between these two models.