Delayed phase mixing in the self-gravitating Galactic disc

Context . The discovery of the phase spiral in the vertical position–velocity ( z – v z ) distribution of the solar neighbourhood stars revealed the Milky Way (MW) disc disequilibrium. The phase spiral is considered to work as a dynamical clock for dating past perturbations, but some previous studies neglected the disc self-gravity, which might bias estimates of the phase spiral excitation time. Aims . We revisit self-gravitating effects on the evolution of vertical phase spirals and quantify the bias introduced in estimating their excitation time when these effects are ignored. Methods . We analysed a high-resolution self-consistent N -body simulation of the MW interaction with the Sagittarius dwarf galaxy (Sgr), alongside four test-particle simulations in potentials constructed from the N -body snapshots. In the test-particle simulations, we used static and time-dependent potentials that included (or excluded) Sgr and the dark matter (DM) wake. In each case, we estimated the winding time of phase spirals by measuring the slope of the density contrast in the vertical angle–frequency ( θ z –Ω z ) space. Results . In test-particle models, the phase spiral immediately begins to wind after the Sgr pericentric passage, and the winding time closely tracks the true elapsed time since the Sgr pericentric passage. Adding the DM wake yields only a modest (