Formation of disks with long-lived spiral arms from violent gravitational dynamics.

By means of simple dynamical experiments we study the combined effect of gravitational and gas dynamics in the evolution of an initially out-of-equilibrium, uniform, and rotating massive overdensity thought of as in isolation. The rapid variation of the system mean-field potential makes the pointlike particles (PPs), which interact only via Newtonian gravity, form a quasistationary thick disk dominated by rotational motions surrounded by far out-of-equilibrium spiral arms. On the other side, the gas component is subjected to compression shocks and radiative cooling so as to develop a much flatter disk, where rotational motions are coherent and the velocity dispersion is smaller than that of PPs.