The Intermittency of ISM Turbulence: What Do the Observations Tell Us?

Abstract

The interstellar medium is highly turbulent, but this medium, a partially ionized plasma, is also multi-phase, compressible and magnetized, hence the complexity of its turbulence extends beyond theoretical grasp. Turbulence being with gravity a key player in the star formation process, it is anticipated that its dissipation is a key process too. A fundamental property of turbulent dissipation is its space-time intermittency, studied mostly in hydrodynamical turbulence. After an overview of our limited knowledge of intermittency based on laboratory experiments, theory and numerical simulations, this chapter gathers the set of observations, now made possible by the new capabilities of molecular spectroscopy and polarimetry, that may be seen as signatures of intermittency in the magnetized turbulent interstellar medium. It includes powerful statistical approaches of the interstellar velocity field, the detection of large velocity-shears at very small scales, and chemical and radiative diagnostics of intermittent dissipation. Models of magnetized dissipation bursts, either in the form of coherent vortices or low velocity C-shocks are also presented and confronted to observations, as well as results on the regions of most intense dissipation in spectral simulations of magneto-hydrodynamical (MHD) and non-ideal turbulence. 9.1 The Turbulent Interstellar Medium The interstellar medium (ISM) contributes to less that 1 % to the mass of our Galaxy (and in most galaxies of the present day universe) and yet it is the reservoir of gas that still allows the formation of new stars. It is paradoxical that this medium plays such a critical role in the cycle of cosmic matter and yet contributes such a small fraction of the galaxy masses. It is part of an open cycle that drives baryons from dying stars to new stars, at a rate of a few M ˇ year 1 in our Galaxy, comparable to the rate of gas infall from the extragalatic environment. The warmest phases of the ISM are fully ionized. Its neutral and coldest phases, which comprise the bulk of its E. Falgarone () • G. Momferratos • P. Lesaffre Ecole Normale Supérieure,