Galactic seismology: can the Gaia ‘phase spiral’ co-exist with a clumpy, turbulent interstellar medium?

The Gaia satellite revealed a remarkable spiral pattern (‘phase spiral’, PS) in the z–V_z phase–plane throughout the solar neighbourhood, where z and V_z are the displacement and velocity of a star perpendicular to the Galactic plane. As demonstrated by many groups, the kinematic signature reflects the Galactic stellar disc’s response to a dynamical disturbance some 0.3–3 Gyr ago. However, previous controlled simulations did not consider the impact of the multiphase interstellar medium (ISM) on the existence of the PS. This is crucial because it has been suggested that this weak signal is highly susceptible to scattering by small-scale density fluctuations typical of the ISM. This has motivated us to explore the formation and fate of the PS in a suite of high-resolution, N-body/hydrodynamical simulations of an idealized Galaxy analogue bearing a realistic ISM that interacts impulsively with a massive perturber. In our models, high-gas surface densities within the disc encourage vigorous star formation, which in turn couples with the gas via feedback to drive turbulence. We find that the PS is almost non-existent if the disc potential is too strong or the ISM is highly structured on sub-kiloparsec scales. This can happen in the absence of stellar feedback when the gas is allowed to cool. In the presence of turbulent gas maintained by stellar feedback, the PS has a patchy spatial distribution and a high degree of intermittency on kiloparsec scales. We anticipate that future studies of the phase-spiral behaviour on all scales will provide crucial information on star-gas dynamics.