Clouds as turbulent density fluctuations: Implications for pressure confinement and spectral line data interpretation

We examine the idea that diffuse H I and giant molecular clouds and their s ubstructure form as density fluctuations induced by large-scale interstella r turbulence. We do this by closely investigating the topology of the veloc ity, density, and magnetic fields within and at the boundaries of the cloud s emerging in high-resolution two-dimensional simulations of the interstell ar medium (ISM) including self-gravity, magnetic fields, parameterized heat ing and cooling, and a simple model for star formation. We find that the ve locity field is continuous across cloud boundaries for a hierarchy of cloud s of progressively smaller sizes. Cloud boundaries defined by a density-thr eshold criterion are found to be quite arbitrary, with no correspondence to any actual physical boundary, such as a density discontinuity. Abrupt velo city jumps are coincident with the density maxima, which indicates that the clouds are formed by colliding gas streams. This conclusion is also suppor ted by the fact that the volume and surface kinetic terms in the Eulerian v irial theorem for a cloud ensemble are comparable in general and by the top ology of the magnetic field, which exhibits bends and reversals where the g as streams collide. However, no unique trend of alignment between density a nd magnetic features is observed. Both sub- and super-Alfvenic motions are observed within the clouds. In light of these results, we argue that therma l pressure equilibrium is irrelevant for cloud confinement in a turbulent m edium, since inertial motions can still distort or disrupt a cloud, unless it is strongly gravitationally bound. Turbulent pressure confinement appear s self-defeating because turbulence contains large-scale motions that neces sarily distort Lagrangian cloud boundaries or equivalently cause flux throu gh Eulerian boundaries. We then discuss the compatibility of the present sc enario with observational data. We find that density-weighted velocity hist ograms are consistent with observational line profiles of comparable spatia l and velocity resolution, exhibiting similar FWHMs and similar multicompon ent structure. An analysis of the regions contributing to each velocity int erval indicates that the histogram "features" do not come from isolated "cl umps" but rather from extended regions throughout a cloud, which often have very different total velocity vectors. Finally, we argue that the scenario presented here may also be applicable to small scales with larger densitie s (molecular clouds and their substructure, up to at least n similar to 10( 3)-10(5) cm(-3)) and conjecture that quasihydrostatic configurations cannot be produced from turbulent fluctuations unless the thermodynamic behavior of the flow becomes nearly adiabatic. We demonstrate, using appropriate coo ling rates, that this will not occur except for very small compressions (le ss than or similar to 10(-2) pc) or until protostellar densities are reache d for collapse.