Tidally induced spiral arm wraps encoded in phase space

Abstract

We study the dynamics of tidally induced spiral arms in our Galaxy in the context of its encounters with Sagittarius. We built toy models of the interaction between a host and a satellite galaxy using orbital integrations after a tidal encounter. We derived analytically the shape of the structures in phase space as a function of time for simple power-law potential models. We compared these models to a more realistic N-body simulation of the MW Sagittarius-like interaction and to data from Gaia DR3. As found before, an impulsive distant tidal approach of a galactic satellite leads to a two-armed spiral structure made of orbits in between their apocentres and pericentres, thus, with average negative galactocentric radial velocity. The two-arm pattern rotates at an angular speed of $\omega-1/2\kappa$ which depends on radius, thus causing a wind-up with time. This produces ridges in the $R$-$V_\phi$ projection with alternating signs of $V_R$ and oscillations of $V_R$ in the $L_Z$-$\phi$ space, like those observed in the Gaia data. The frequency of these kinematic features increases with time, offering a powerful means to infer the potential and the perturbation's onset time and azimuthal phase. Fourier analysis allows us to date the impact times of simple models and even to date perturbations from various pericentric passages. For the MW, the Fourier analysis indicates a superposition of two different frequencies, as in previous studies. Assuming that both are due to impulsive and distant pericentre passages, we find perturbation times <0.6 Gyr and in the range of 0.8-2.1 Gyr. The latter is compatible with a previous pericentre of Sagittarius and would be related to four wraps of the spiral arms in the observed radial range. Further work on the self-gravitating response of galactic discs and possible degeneracies with secular processes by the bar is necessary. (ABR)