We ran 3D-MHD kiloparsec-scale numerical simulations of a stratified, vertical column of the ISM, corresponding to a section of a disk Galaxy. We include SN-driven turbulence, magnetic fields, gas heating and cooling, galactic gravitational potential and gas self-gravity. We extract a population of dense clouds formed in the turbulent ISM, and follow their evolution as they evolve and collapse. We find that, when self-gravity is turned off, SN explosions alone appear unable to drive fast turbulent motions inside dense clouds. Once self-gravity is included, dense clouds find themselves in a state of chaotic-hierarchical collapse. We find that self-gravity induces non-thermal motions recovering the observed relations. We then expect clouds to be short lived, quickly evolving towards star formation. In order to agree with observed star formation efficiencies, this process must be terminated by the early destruction of the clouds, presumably from internal stellar feedback.

Figure 1. Animation

6 Myr evolution of the column density after self-gravity is turned on. Self-gravity is turned on at a global evolutionary time of t=230 Myr. The panels show (left) an edge-on projection of the inner 1 kpc × 1.5 kpc of the simulated volume; (bottom right) a face-on projection of the simulated volume, with a 1 kpc2 footprint; and (top right) a close-up of the structured, irregular, dense cloud shown with a dashed box in the bottom right panel.

Figure 2. Animation

Salami slice animation of the number density, temperature, thermal pressure, kinetic energy density, magnetic energy density and total pressure fields through a dense cloud. The slice lies in the x-y plane and covers the range between -50 < z[pc] < 50 for the close-up cloud shown in Figure 1. Left column corresponds to a time tSG = 0. Right column to a time tSG = 3 Myr.