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The echo of the stellar bar formation in our Galaxy captured by Gaia

24 février 2019

New numerical simulations of the bar formation in the Milky Way, carried out by astronomers of Paris Observatory, show that it is possible to explain the phase space spirals recently discovered by Gaia. These spirals develop spontaneously from vertical oscillations caused by the bar buckling and formation of a peanut bulge. When vertical resonances raise the orbits of the stars in the bar, to form the pseudo-bulge, the vertical oscillations propagate outwards, and are at the origin of the winding ad wrapping in the phase space. Thus, it is not necessary to invoke the disruption of a massive satellite to explain the incomplete mixing process in phase space. The internal disturbance of the bar is sufficient, and can last up to 3 billion years.

Since the publication of its second catalog, ESA’s Gaia mission does not stop making new discoveries.

Figure 1 : Mouvements verticaux induits dans le disque, à l’époque de formation de la barre. Le panneau du haut montre la force de la barre dans la simulation, en fonction du temps. L’époque de formation de la barre est indiquée par la flèche verte, celle de formation du bulbe à cacahuète par la flèche rouge. Quand la force de le barre est maximale, on voit des perturbations verticales apparaître dans le disque. Elles sont visibles autant dans la hauteur moyenne des étoiles par rapport au disque galactique (panneau du milieu) que dans leur vitesse verticale (panneau du bas). Ces perturbations se propagent pendant des milliards d’années dans le disque. Les différentes courbes dans chaque panneau dépendent de la distance du centre de la galaxie simulée, de 0.5 à 20 kpc.

One of the most striking is the existence of particular structures in the phase space, that is to say in the space that combines positions and velocities of the stars to describe their motion. If we trace the distance to the galactic plane of the stars in the solar neighborhood as a function of their speed in the same direction, their distribution shows a spiral structure. This has so far been interpreted as due to a non-equilibrium state of our galaxy, possibly caused by the last passage through the galaxy disk of Sagittarius, the satellite currently closest to the Milky Way. The current motion of the stars would keep the memory of this recent disturbance.

Figure 2 : Spirales dans l’espace des phases révélées par Gaia (à gauche) et détectées dans la simulation (à droite, voir Khoperskov et al 2019). Dans les deux panneaux, la couleur indique la vitesse azimutale moyenne.

In a work published in A&A Letters, researchers from the Paris Observatory and MPE, Garching, proposed a different scenario to explain the origin of these structures. They would constitute the fossil tracer of the formation phase of the Galactic bar, when it acquired its characteristic peanut-shape structure observed today. The radial and vertical motions generated by its formation would have spread for billions of years in the disk (see Fig. 1) and would be measurable today thanks to Gaia (see Fig 2). To show this, the team has made one of the highest resolution simulations ever made of the Galaxy, modeling the evolution of more than 100 million particles in the disc, for several billion years. This work opens many questions and perspectives : is it possible to use these spirals to date the time of bar formation, which is still unknown in our Galaxy ? And how to disentangle the signatures left by the formation of the bar from those induced by the passage of the Sagittarius galaxy ? Questions that this team plans to work on in the coming months.

Animation (MP4)
Khoperskov et al (2019)

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