We considered the impact of dissipative non-gravitational physics such as gas dynamics on the purely gravitational dynamics of an isolated system . We showed that long-lived, but non-stationary, spiral arms can formed as consequence of the violent collapse of an isolated over-density, characterized by a mixture of purely self-gravitating matter and gas that may dissipate energy via radiative cooling. Spiral arms are made of gaseous particles that move coherently because they have acquired a specific phase-space correlation during the gravitational collapse phase: this represents a signature of the violent origin of the arms and implies both the motion of matter and the transfer of energy. In this violent scenario the galaxy consists in a tick and think disk which are  not embedded in an halo structure, that is instead typically formed in a slow and mild bottom-up dynamics.