The different phases of the Galactic ISM can be traced via various radiation mechanisms, in both line and continuum emission. The characteristic radiation is emitted over a very wide range of the electromagnetic spectrum, from the radio to the -ray regime. Therefore, a comprehensive observational investigation of the ISM must be based on multi-frequency data. Since it is necessary for studies of the Galactic ISM to image very wide fields of view, the ideal databases are all-sky surveys.
3.1 The molecular medium (MM)
The most powerful tool for studies of the cold molecular medium (MM) are CO line observations. Dame et al. (1987) conducted a survey of the Galactic plane in the CO(1->0) rotational transition. Many other molecular emission lines are known, which provide valuable information on gas at different temperatures and densities (e.g., other CO transitions, H2, OH, NH3, SiO emission, etc.). For a review of this field see e.g., Henkel et al. (1991).
3.2 The cold neutral medium (CNM)
The cold neutral phase of the ISM (CNM) is not easily visible in emission; instead, H I absorption studies (e.g., Mebold et al. 1982) help to find cold neutral atomic gas. The CNM is primarily associated with molecular gas, existing in compact clouds. At present, the CNM can only be studied in absorption against background continuum sources; thus, no all-sky survey exists.
3.3 The warm neutral medium (WNM)
The best tracer of the warm neutral medium (WNM) is H I line emission, which is ubiquitous in our Galaxy (see e.g., the recent surveys by Hartmann & Burton 1997 and Stark et al. 1992). The WNM is not as dense (n ~ 0.3 cm-3) as molecular clouds and more diffuse. H I gas is usually the (radially) most extended component of the ISM in galaxy disks. Most of our knowledge on the large-scale kinematics of the Milky Way comes from H I line studies (e.g., Burton 1988).
3.4 The warm ionized medium (WIM)
Optical H (and [N II]) line emission is the most easily detectable emission of the WIM in the Galaxy; the WIM is the dominant gas phase in H II regions. The more extended part of the WIM, outside H II regions, is often called ``diffuse ionized gas'' (DIG).
Warm ionized gas emits a wealth of optical emission lines (e.g., H, [N II], [S II], [O III], and others) which can be used as diagnostics of the excitation conditions of the gas (e.g., Mathis 1986). The WIM also emits thermal radio emission, both continuum (free-free radiation) as well as recombination lines (e.g., Westerhout 1958; Mezger 1978).
3.5 The hot ionized medium (HIM)
The million degree hot gaseous component of the Galactic ISM (HIM) emits both a Bremsstrahlung continuum and complex line radiation of electronic transitions in highly excited atoms in the 0.1 to 2 keV energy band. The first all-sky X-ray maps were obtained by McCammon et al. (1983). Major advances in this field have recently been made with data from the ROSAT all-sky survey (Voges et al. 1996).
Another, slightly less energetic part of the hot ionized gas, at temperatures in the 105 K range, cannot be easily detected in emission. Instead, characteristic absorption lines reveal its presence in the Galactic ISM (O VI, N V, C IV; e.g., Jenkins & Meloy 1974; Sembach & Savage 1992).
3.6 Observations of dust
As indicated above, dust is a strong FIR emitter. An all-sky survey was carried out with the IRAS satellite (Neugebauer et al. 1984). IRAS detected emission from a multitude of sources in the Galactic disk, which is strongly correlated with the Galactic H I emission.
The thermal continuum emission of dust that is observable in the FIR extends into the millimeter radio regime. Therefore, another way to image dust in galaxies is to measure its millimeter continuum (e.g., Cox & Mezger 1989).
3.7 Observations of cosmic rays and magnetic fields
The first complete all-sky survey in the radio regime, mapping the distribution of radio synchrotron emission of CR electrons in the Galaxy, was conducted at a frequency of 200 MHz by Dröge & Priester (1956). Much higher resolution and sensitivity was achieved in a 408 MHz survey by Haslam et al. (1982).
Assuming energy equipartition between the CRs and the Galactic magnetic field, the strength of the Galactic magnetic field can be estimated to be about 3-6 µG (Ohno & Shibata 1993; Han & Qiao 1994 and references therein; Beck et al. 1996). Han & Qiao (1994) also present a model of the magnetic field configuration, based on rotation measure analyses, finding a bisymmetric field configuration with a pitch angle of about 8°. For more information on the ISM in the Galactic disk see the reviews listed above (in Section 2) and references therein.