Physical Processes of Interstellar Turbulence

This review discusses the role of radiative heating and cooling, as well as self-gravity, in shaping the nature of the turbulence in the interstellar medium (ISM) of our galaxy. The ability of the gas to radiatively cool, while simultaneously being immersed in a radiative heat bath, causes it to be much more compressible than if it were adiabatic, and, in some regimes of density and temperature, to become thermally unstable, and thus tend to spontaneously segregate into separate phases, one warm and diffuse, the other dense and cold. On the other hand, turbulence is an inherently mixing process, thus tending to replenish the density and temperature ranges that would be forbidden under thermal processes alone. The turbulence in the ionized ISM appears to be transonic (i.e, with Mach numbers \(M_{\text{s}}\,{\sim }\,1\)), and thus to behave essentially incompressibly. However, in the neutral medium, thermal instability causes the sound speed of the gas to fluctuate by up to factors of \({\sim}30\), and thus the flow can be highly supersonic with respect to the dense, cold gas. However, numerical simulations suggest that the supersonic velocity dispersion corresponds more to the ensemble of cold clumps than to the clumps’ internal velocity dispersion. Finally, coherent large-scale compressions in the warm neutral medium (induced by, say, the passage of spiral arms or by supernova shock waves) can produce large, dense, and turbulent clouds that are affected by their own self-gravity, and begin to contract gravitationally. Because they are populated by the nonlinear turbulent density fluctuations, whose local free-fall times can be significantly smaller than that of the whole cloud, the fluctuations terminate their collapse earlier, giving rise to a regime of hierarchical gravitational fragmentation, with small-scale collapses occurring within larger-scale ones. Thus, the “turbulence” in the cold, dense clouds may actually consist primarily of gravitationally contracting motions at all scales within them.