Phase-space and spiral structure
of Milky Way
Even with the arrival of massive data from the Gaia (ESA) space mission mapping a substantial part of our Galaxy, the direct mapping of the spiral structure is almost impossible without making specific selections targeting the youngest stellar populations. The reasons behind such a problem are fundamental: (i) the spatial footprint of the catalogues, shaped by the Gaia scaling law and (ii) the fact that older stellar populations have non-negligible velocity dispersion, blurring the fine structure of the spiral structure.
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To solve these two problems, I have introduced an approach that made it possible to recover the spiral structure of our Galaxy in a region covered by the RVS sample from Gaia DR2 without relying on a specific type of stars.
Stars, while orbiting around the galactic centre, also oscillate around their co-called guiding radii. The latter can be calculated from the angular momentum and reasonable assumption about the rotation curve of the galaxy. Once we replace the instantaneous galactocentric positions of stars with their guiding radii. In this case we can effectively get rid of epicyclic motions and highlight the underlying structure. The animation above shows a test-particle simulation demonstrating the effect of the coordinates (X,Y)->(Xg,Yg) transformation. As one can see, the spiral arms are much more prominent in the guiding space.
The figure above demonstrates the identification of spiral arms in the Milky Way. The left panel shows the distribution of star counts from the Gaia DR2 dataset. To address density variations caused by the Gaia footprint, we downsample specific regions to achieve a flat density distribution, minimizing contamination from observational biases. Physical coordinates are then transformed into the guiding space, revealing several diagonal overdensities. It is important to note that these overdensities, appearing in the pseudo-space of guiding coordinates, do not directly correspond to spiral arms.
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By selecting stars within these overdensities and mapping their locations back to the physical XY space, we identify corresponding structures, as shown in the rightmost panel. In the Gaia DR2 dataset, we detected six such overdensities. By incorporating additional kinematic information, we associated four of these structures with spiral arms and the remaining two with resonances of the Galactic bar. This analysis yields an estimated bar pattern speed of approximately 37–39 km/s/kpc, offering critical insights into the dynamics of the Milky Way.
The left figure features a modified artist's sketch of the Milky Way's structure, adjusted to align with the recovered morphology of its spiral arms.
In the follow-up work, we demonstrated that the Milky Way spiral arms show a distinct metallicity pattern and find their physical connection with other kinematic features across the Milky Way disc.
Currently, I study the chemical abundances of stars and the ISM in and around spiral arms, using both simulations and observational data, aiming to get close to understanding the origin of galactic spiral structures.
The discovery of the Gaia phase-space spiral often called the "phase-snail" (see figure above), was a groundbreaking revelation in galactic dynamics, made possible by data from the Gaia mission (ESA). First identified in 2018, the spiral appeared as a striking pattern when plotting the vertical positions and velocities of stars near the Sun in phase space. This structure is thought to have formed due to a perturbation in the Milky Way's disk caused by the gravitational influence of the Sagittarius (Sgr) Dwarf Galaxy during its close encounter with the Milky Way.
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However, soon after the discovery, we proposed an alternative explanation for the formation of the Gaia phase-space spiral, suggesting its origin from waves triggered by the buckling of the Galactic bar, propagating outward through the disk. Bar buckling is a dynamic instability where the central bar structure of the Milky Way temporarily bends and distorts vertically, generating ripples that spread across the galaxy (watch an animation below).