  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 over-density thought of as in
isolation. The rapid variation of the system mean-field potential makes the
point like 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. Around such gaseous disk long-lived, but
nonstationary, spiral arms form: these are made of gaseous particles that move
coherently because have acquired a specific phase-space correlation during the
gravitational collapse phase. Such a phase-space correlation represents a
signature of the violent origin of the arms and implies both the motion of
matter and the transfer of energy. On larger scales, where the radial velocity
component is significantly larger than the rotational one, the gas follows the
same out-of-equilibrium spiral arms traced by PPs. We finally outline the
astrophysical and cosmological implications of our results.
