2. THE COMPLEX STRUCTURE OF THE ISM IN GALAXY DISKS

Observational studies of interstellar matter naturally started in our local vicinity - within the Milky Way. Here we have a close-up view of objects and can obtain the most detailed structural and kinematic information. Thus, a compilation of our knowledge on the Galactic ISM is a natural starting point.

2.1 Five gas phases

Numerous reviews have been given on the composition of interstellar gas, e.g., by Dettmar (1992), Brinks (1989); and Kulkarni and Heiles (1988). Most authors list five gas phases (see Table 1 by Dettmar 1992): the molecular medium (MM), the cold neutral medium (CNM), the warm neutral medium (WNM), and the hot ionized medium (HIM).

2.2 Additional components of the ISM

Here, I adopt a more general view of the ISM and include in its description three additional components: interstellar dust, cosmic rays (CRs), and magnetic fields (B-fields).

Since these three components are not commonly included in descriptions of the ISM, some basic properties are listed below.

2.2.1 Dust

Not being a gaseous medium, dust is often excluded from discussions of the ISM. However, it plays a crucial role, especially as a coolant and repository of metals. Therefore, a comprehensive description must include it.

Interstellar dust consists mostly of polycyclic aromatic hydrocarbons (PAHs), graphite, and silicate (e.g., Dwek et al. 1997; in particular their Fig. 3). When heated by stellar UV radiation, dust emits copious far-infrared (FIR) continuum emission (e.g., Telesco 1988). The general importance of FIR emission from dust can be judged from the fact that the total bolometric luminosity of galaxies, in particular those with high star formation rates (SFRs), can be well approximated by their FIR luminosity (Soifer et al. 1987a). This implies that galaxies are opaque to most of the ionizing stellar UV radiation (Leitherer et al. 1995), which does not escape freely into intergalactic space, but is mostly absorbed by dust and re-emitted in the FIR.

FIR colors, like e.g., the 60µm/100µm flux density ratios measured by the IRAS satellite, provide an estimate of the temperature of the warm (T_d 17 K) dust, from which in turn several quantities of the heating sources can be deduced. For a review of FIR emission from Galactic sources see Beichman (1987).

2.2.2 Cosmic rays and magnetic fields

CRs are relativistic particles, i.e. protons, atomic nuclei, and electrons, forming an ionized plasma of highly energetic particles. All ionized particles in motion carry (or are carried by) magnetic fields (B-fields). Ionized matter, starting at very low ionization levels, and B-fields are coupled to each other; the motion of ionized particles is ``frozen in'' to the B-field. Two primary sources of CRs are believed to be SNe and SNRs. Strong magnetic fields are also created in SNRs, when they interact with the ambient ISM. Together with the CRs, these fields can spread quickly over large distances, creating large-scale B-fields. The mean strength of the Galactic B-field is of order 3-6 µG; a model of the Galactic disk field was published by Han & Qiao (1994).

Emission of protons/nuclei (in the MeV to TeV range) is currently easily detectable only in our Galaxy (e.g., Bloemen 1989). More easily observable radiation, namely synchrotron radio continuum, which is also detectable in external galaxies (see below), comes from CR electrons ⁽²⁾. More details on radio emission from our Galaxy can be found in Salter & Brown (1988).