The angular momentum spiral of the Milky Way disc in <i>Gaia</i>

ABSTRACT Data from the Gaia mission shows prominent phase-space spirals that are the signatures of disequilibrium in the Milky Way (MW) disc. In this work, we present a novel perspective on the phase-space spiral in angular momentum (AM) space. Using Gaia DR3, we detect a prominent AM spiral in the solar neighbourhood. We demonstrate that the spiral detected in the $z-v_z$ phase-space projection can be straightforwardly derived from the AM spiral. We also find that MW stars on orbits with low or even moderate eccentricity follow close-to-elliptical trajectories in AM, which allows us to develop generative models for the spiral analytically, for example by tilting the MW disc at an earlier time. A ‘chi-by-eye’ fit of this simple model successfully produces a winding spiral that varies with $L_z$ that generally matches the winding and amplitude of the data across most $L_z$ bins. Modelling the phase spiral in AM is a promising method to constrain both past perturbations and the MW potential simultaneously. Our AM framework simplifies the interpretation of the spiral and offers a robust approach to modelling disequilibrium in the MW disc using all six dimensions of phase space.