Riding the kinematic waves in the Milky Way disk with Gaia

Abstract

Context. Gaia DR2 has delivered full-sky 6-D measurements for millions of stars, and the quest to understand the dynamics of our Galaxy has entered a new phase. Aims. Our aim is to reveal and characterize the kinematic sub-structure of the different Galactic neighbourhoods, to form a picture of their spatial evolution that can be used to infer the Galactic potential, its evolution and its components. Methods. We take ∼5 million stars in the Galactic disk from the Gaia DR2 catalogue and build the velocity distribution of many different Galactic Neighbourhoods distributed along 5 kpc in Galactic radius and azimuth. We decompose their distribution of stars in the V R -V φ plane with the wavelet transformation and asses the statistical significance of the structures found. Results. We detect many kinematic sub-structures (arches and more rounded groups) that diminish their azimuthal velocity as a function of Galactic radius in a continuous way, connecting volumes up to 3 kpc apart in some cases. The decrease rate is, on average, of ∼23 km s −1 kpc −1 . In azimuth, the kinematic sub-structures present much smaller variations. We also observe a duality in their behaviour: some conserve their vertical angular momentum with radius (e.g., Hercules), while some seem to have nearly constant kinetic energy (e.g., Sirius). These two trends are consistent with the approximate predictions of resonances and of phase mixing, respectively. Besides, the overall spatial evolution of Hercules is consistent with being related to the Outer Lindblad Resonance of the Bar. We also detect structures without apparent counterpart in the vicinity of the Sun. Conclusions. The various trends observed and their continuity with radius and azimuth allows for future work to deeply explore the parameter space of the models. Also, the characterization of extrasolar moving groups opens the opportunity to expand our understanding of the Galaxy beyond the Solar Neighbourhood.

A New Moving Group in the Local Arm

“…Spatial region of this selected sample is similar to region 3 in Liang et al (2017) and it contains the spatial region of V6 from Ramos, Antoja & Figueras (2018). Therefore our contours look very similar to subplot (b) of figure 5 of Liang et al (2017) and subplot (f) of figure 5 of Ramos, Antoja & Figueras (2018). However, the peak around (0, 7) km/s in our figure is much more significant than in theirs which are only in dense regions but not density peaks.…”

“…The median uncertainties values of U, V and W are respectively 3.7, 3.3 and 2.5 km/s of the selected star sample. Spatial region of this selected sample is similar to region 3 in Liang et al (2017) and it contains the spatial region of V6 from Ramos, Antoja & Figueras (2018). Therefore our contours look very similar to subplot (b) of figure 5 of Liang et al (2017) and subplot (f) of figure 5 of Ramos, Antoja & Figueras (2018).…”

“…The difference between our data and Liang et al (2017) lies in that this sample is selected from LAMOST DR5 cross-matched with Gaia DR2 rather than Gaia DR1 TGAS. The difference between our data and Ramos, Antoja & Figueras (2018) mainly lies in that Ramos, Antoja & Figueras (2018) used the Gaia DR2 sample and we used only G,K giants with radial velocity from LAMOST. There are about 1868 stars around the peak in the UV coordinate of our sample.…”

“…Later observations indicate dynamical effects of non-symmetric parts of the Galaxy may be responsible for most of the best known structures, such as the Sirius, Hyades, Pleiades, Hercules and γ Leo moving group (Famaey et al 2005(Famaey et al , 2007Bensby 2007;Famaey, Siebert & Jorissen 2008;Liang et al 2018). With Gaia DR2 data release, many new kinematic sub-structures in the solar neighbourhood were found (Katz et al 2018;Antoja et al 2018;Ramos, Antoja & Figueras 2018) which have been thought to be related to phase mixing. Nonetheless, there are still some moving groups belonging to dispersed stellar clusters.…”

Exploring the Origin of Moving Groups and Diagonal Ridges by Simulations of Stellar Orbits and Birthplaces

“…In the present work, we show that the diagonal ridges could be the result of the stellar resonant orbital structure induced by the spiral perturbation, which creates regions in phase space of stable and unstable orbits. The moving groups and arches in the U -V velocity space of the SN are projections of the diagonal ridges, at Galactic positions close to the solar radius (Ramos et al 2018;Antoja et al 2018).…”