Kiloparsec-scale simulations of star formation in disk galaxies : III. structure and dynamics of filaments and clumps in giant molecular clouds

We present hydrodynamic simulations of self-gravitating dense gas in a galactic disk, exploring scales ranging from 1 kpc down to similar to 0.1 pc. Our primary goal is to understand how dense filaments form in giant molecular clouds (GMCs). These structures, often observed as infrared dark clouds (IRDCs) in the Galactic plane, are thought to be the precursors to massive stars and star clusters, so their formation may be the rate-limiting step controlling global star formation rates in galactic systems as described by the Kennicutt-Schmidt relation. Our study follows on from Van Loo et al., which carried out simulations to 0.5 pc resolution and examined global aspects of the formation of dense gas clumps and the resulting star formation rate. Here, using our higher resolution, we examine the detailed structural, kinematic, and dynamical properties of dense filaments and clumps, including mass surface density (Sigma) probability distribution functions, filament mass per unit length and its dispersion, lateral Sigma profiles, filament fragmentation, filament velocity gradients and infall, and degree of filament and clump virialization. Where possible, these properties are compared to observations of IRDCs. By many metrics, especially too large mass fractions of high Sigma > 1 g cm(-2) material, too high mass per unit length dispersion due to dense clump formation, too high velocity gradients, and too high velocity dispersion for a given mass per unit length, the simulated filaments differ from observed IRDCs. We thus conclude that IRDCs do not form from global fast collapse of GMCs. Rather, we expect that IRDC formation and collapse are slowed significantly by the influence of dynamically important magnetic fields, which may thus play a crucial role in regulating galactic star formation rates