The origin and fate of the Gaia phase-space snail

The Gaia snail is a spiral feature in the distribution of solar-neighbourhood stars in position and velocity normal to the Galactic midplane. The snail probably arises from phase mixing of gravitational disturbances that perturbed the disc in the distant past. The most common hypothesis is that the primary disturbance resulted from a passage of the Sagittarius dwarf galaxy close to the solar neighbourhood. In this paper, we investigate the alternative hypothesis that the snail is created by many small disturbances rather than one large one, that is, by Gaussian noise in the gravitational potential, probably due mostly to substructures in the dark-matter halo. We show that this hypothesis naturally reproduces most of the properties of the snail. In particular, it predicts correctly, with no free parameters, that the apparent age of the snail is $\sim 0.5{\rm\ Gyr}$. An important ingredient of this model is that any snail-like feature in the solar neighbourhood, whatever its cause, is erased by scattering from giant molecular clouds or other small-scale structure on a time-scale $\lesssim 1{\rm\ Gyr}$.