Turbulence in Atomic Hydrogen

Summary

Understanding the properties of interstellar turbulence is a great intellectual challenge and the urge to solve this problem is partially motivated by a necessity to explain the star formation mystery. This review deals with a recently suggested inversion technique as applied to atomic hydrogen. This technique allows to determine 3D turbulence statistics through the variations of 21 cm intensity. We claim that a radio interferometer is an ideal tool for such a study as its visibility function is directly related to the statistics of galactic HI. Next, we show how galactic rotation curve can be used to study the turbulence slice by slice and relate the statistics given in galactic coordinates and in the velocity space. The application of the technique to HI data reveals a shallow spectrum of the underlying HI density that is not compatible with a naive Kolmogorov picture. We show that the random density corresponding to the found spectrum tends to form low contrast filaments that are elongated towards the observer.

Introduction

The properties of the interstellar medium strongly suggest that it is turbulent. Here turbulence is understood as unpredictable spatial and temporal behavior of nonlinear systems as preached by Scalo (1985, 1987).

The importance of turbulence in molecular clouds and its relation to star formation has long been appreciated (Dickman 1985). Recent progress in numerical simulations of molecular cloud dynamics (see Ostriker, this volume) indicates the intrinsic connection between the turbulence in different phases of the interstellar medium (McKee & Ostriker 1977).