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By means of simple dynamical experiments we study the combined effect ofgravitational and gas dynamics in the evolution of an initiallyout-of-equilibrium, uniform and rotating massive over-density thought of as inisolation. The rapid variation of the system mean-field potential makes thepoint like particles (PPs), which interact only via Newtonian gravity, form aquasistationary thick disk dominated by rotational motions surrounded by farout-of-equilibrium spiral arms. On the other side, the gas component issubjected to compression shocks and radiative cooling so as to develop a muchflatter disk, where rotational motions are coherent and the velocity dispersionis smaller than that of PPs. Around such gaseous disk long-lived, butnonstationary, spiral arms form: these are made of gaseous particles that movecoherently because have acquired a specific phase-space correlation during thegravitational collapse phase. Such a phase-space correlation represents asignature of the violent origin of the arms and implies both the motion ofmatter and the transfer of energy. On larger scales, where the radial velocitycomponent is significantly larger than the rotational one, the gas follows thesame out-of-equilibrium spiral arms traced by PPs. We finally outline theastrophysical and cosmological implications of our results.

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