Spiral shocks, triggering of star formation and the velocity dispersion in giant molecular clouds

We present numerical simulations of the passage of gas through a galactic spiral shock and the subsequent formation of giant molecular clouds (GMCs), and the triggering of star formation. In these simulations, we take account of the observed inhomogeneity, or clumpiness, of the pre-shock interstellar medium. As might be expected, the spiral shock forms dense clouds while dissipating kinetic energy, producing regions that are locally gravitationally bound and collapse to form stars. But the effect of the clumpiness of gas as it passes through the shock is to generate chaotic internal motions in the gas. The kinematics of these motions are found to agree with the observed velocity dispersion/size relation found in star-forming regions. In contrast to the standard picture where continuously driven turbulence generates the density inhomogeneities in star-forming clouds, we find here that it is the clumpiness of the interstellar gas that produces the chaotic motions as it passes through the spiral shock and initiates the star formation process. The velocity dispersion can be understood as being due to the random mass loading of clumps as they converge in the spiral shock. Within these clouds both the time-scale for the decay of these motions, and the time-scale for forming stars, are comparable to the clouds' dynamical lifetimes. In this model there is no need for any internal or external continuous driving mechanism for the 'turbulence'. In addition, the coupling of the clouds' internal kinematics to their externally triggered formation removes the need for the clouds to be self-gravitating. Indeed, while clearly some parts of the clouds are self-gravitating and able to form stars, most of the molecular material remains gravitationally unbound. This can provide a simple explanation for the low efficiency of star formation.