Probing ISM Models with H observations

Relevance of H observations for the ISM

In spite of the impressive observational and theoretical advances that have occurred in the last decade, there are still a number of important issues concerning the structure and evolution of the Galactic interstellar medium (ISM) which require more study. Among these, three particularly relevant issues are discussed below. First, one would like to know to what extent the energy budget of the ISM is regulated by turbulence. Energy in this form is certainly supplied to the gas by the concurrent action of supernova explosions, winds from massive stars and HII regions. However, it is a difficult problem to ascertain the relative power of these sources and the total amount of energy stored in different forms (i.e., thermal, radiative, magnetic and cosmic ray energy). Next, we understand that the disk of the Milky Way is not a closed system but is connected to the Galactic halo by different forms of energy transfer involving mass entrainment (particularly hot gas but also cold gas and dust) and photons leaking from the production regions. It it therefore important, both for energetic and evolutionary (chemical and dynamical) considerations to assess the relevance of these disk/halo interactions, which, in brief, are often referred to as the Galactic Fountain. Finally, after more than 10 years from its firm discovery, the mystery of the existence of a vertically extended, ionized gas distribution, the so-called "Reynolds layer'' has not yet been dispelled. The aim of this work is to show how observations can help us in making progresses in each of these areas.

  
Figure 1: Thermal pressure contributions to the total pressure (determined from hydrostatic equilibrium) from the known gaseous phases of the ISM, where i=wn,wi,h indicate warm neutral, warm ionized, hot components, respectively. The difference shows the importance of non-thermal (turbulent, magnetic, cosmic ray) forms of energy density.

© Copyright Astronomical Society of Australia 1997