Turbulence and structure formation in the interstellar medium

Abstract

The interstellar medium (ISM) structure and dynamics are the result of the interplay between many physical processes such as turbulence, gravity, thermal processes, magnetic fields and cosmic rays which determine the modes of molecular cloud and star formation, and which in turn, define the global galactic properties. In this thesis, we have examined in detail the role played by thermal and gravitational instabilities in the ISM. We have shown that thermal instability (TI) is an effective agent of structure formation in the ISM which can compress diffuse interstellar gas by a factor of ~ 100, leading to the formation of dense molecular structures, which can, if dense enough, become gravitationally bound. However, TI, which operates on small scales (< 60 pc), is unable to drive a self-sustained turbulence in the ISM. The bulk of turbulent motions is most likely injected into the ISM on larger scales by, e.g., supernova explosions or galactic shear. Confirming this idea, we find that turbulence is injected into the ISM of a dwarf irregular galaxy, i.e., Holmberg II, on a scale of ~ 6 kpc. We also show that the interaction of turbulence and TI can explain the observed large scale morphology of the HI gas in Holmberg II. Additionally, we investigate the dynamics of the ISM when the turbulence is driven by supernova explosions occurring at different rates in the medium. It was possible to show that the constancy of the velocity dispersion in the outer parts of galaxies can be explained as the result of the interplay between supernova driving and TI. We also showed that if the gas density and the supernova rates follow a Kennicutt-type law, the resulting velocity dispersion is of the order of ~ 5-6 km s^{-1} at the outer galactic radii, in perfect agreement with the observations.