On 25th April 2018, the ESA’s Gaia mission has made its second data release (Gaia DR2). Inspired by our N-body simulations of the Milky Way-like disk in this and our previous projects, Kawata et al. (2014, MNRAS, 443, 2757) and Kawata et al. (2018b, MNRAS, 473, 867), we studied the rotational and vertical velocity structure of the Galactic disk as a function of the Galactocentric radius, across a radial range of 5<Rgal<12 kpc, using the Gaia DR2 data. We find many diagonal ridge features (Fig. 1). We have found also radial wave-like oscillations of the peak of the vertical distribution in the Gaia DR2 data.
Figure 1: Normalized distribution of the rotation velocity for the selected Gaia DR2 stars with the RVS radial velocity data (upper) and our all selected Gaia DR2 stars (lower) as a function of the Galactocentric radius. The vertical dashed lines show the position of the Scutum, Sagittarius, Local, and Perseus spiral arms from left to right, which are calculated from Reid et al. (2014, ApJ, 783, 130). The vertical solid line is the assumed solar radius. The white lines highlight the identified ridge features.
We noticed that the diagonal ridge features found in Gaia DR2 looks qualitatively similar to what we see in our N-body/SPH simulations (an example shown in Fig.2). The simulation shows transient spiral arms and we considered that the transient and winding spiral arms act as perturbers in the disk to make these ridge structures. We followed up this idea in Hunt, Hong, Bovy Kawata & Grand (2019, MNRAS, 481, 3794), and using a toy models of transient and winding spiral arms, we showed that the rotation velocity ridge features can be explained with the transient and winding spiral arms.
