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 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 _ { 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 , although 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 clouds that are affected by their own self-gravity , and begin to contract gravitationally .
Because they are populated by large-amplitude 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 be dominated by a gravitationally contracting component at all scales .