Emergence of theGaiaphase space spirals from bending waves

Abstract

We discuss the physical mechanism by which pure vertical bending waves in a stellar disc evolve to form phase space spirals similar to those discovered by Antoja et al. in Gaia Data Release 2. These spirals are found by projecting Solar Neighbourhood stars onto the z − v z plane. Faint spirals appear in the number density of stars projected onto the z − v z plane, which can be explained by a simple model for phase wrapping. More prominent spirals are seen when bins across the z − v z plane are coloured by median v R or v φ . We use both toy model and fully self-consistent simulations to show that the spirals develop naturally from vertical bending oscillations of a stellar disc. The underlying physics follows from the observation that the vertical energy of a star (essentially, its "radius" in the z − v z plane) correlates with its angular momentum or, alternatively, guiding radius. Moreover, at fixed physical radius, the guiding radius determines the azimuthal velocity. Together, these properties imply a link between in-plane and vertical motion that lead directly to the Gaia spirals. We show that the cubic R − z coupling term in the effective potential is crucial for understanding the morphology of the spirals. This suggests that phase space spirals might be a powerful probe of the Galactic potential. In addition, we argue that self-gravity is necessary to properly model the evolution of the bending waves and their attendant phase space spirals.

Asymmetric Mean Metallicity Distribution of the Milky Way’s Disk

“…The phase overlap between the observed density and metallicity asymmetries can satisfactorily be explained by the phase-space wrapping of stars in the Z-V Z plane, which has recently been discovered by Gaia (Antoja et al 2018;Binney & Schönrich 2018;Bland-Hawthorn et al 2019;Darling & Widrow 2019;Laporte et al 2019).…”

Dissecting the Phase Space Snail Shell

“…Therefore, the V R colorcoded phase space shears into a spiral that differs from the V φ color-coded phase space spiral in the tightness of the winding (Binney & Schönrich 2018). Different simulations also indicate that the coupled motions in horizontal and vertical directions lead to clear snail shells in V R and V φ colorcoded phase spaces (Darling & Widrow 2019;Laporte et al 2019). In particular, the bar buckling perturbation scenario predicts the most clear snail shell in V R color-coded phase space (Khoperskov et al 2019).…”

“…1c in Antoja et al 2018), which was suggested to indicate the tight correlation between the in-plane and vertical motions. The two motions are clearly entangled, but there are still important details to be clarified (Binney & Schönrich 2018;Darling & Widrow 2019). Laporte et al (2019) found snail shell at different stellar age bins, and the shape of the snail changes systematically across the Galactic disk.…”

Hic sunt dracones: Cartography of the Milky Way spiral arms and bar resonances with Gaia Data Release 2

“…Gaia data have already revealed signatures of nonequilibrium and ongoing vertical phase mixing in the Milky Way disc , likely induced by a previous pericentric passage of the Sagittarius dwarf galaxy Bland-Hawthorn et al 2019;Haines et al 2019) or by inter-Here be dragons, a phrase famous in medieval cartography when dragons and sea monsters used to designate uncharted and possibly dangerous regions. nally driven bending waves (Khoperskov et al 2019;Darling & Widrow 2019). A large number of kinematic arches with various morphologies not known prior to Gaia were found (Ramos et al 2018) and large-scale wiggles were discovered in the V φ −R plane Antoja et al 2018).…”